When was the first thermonuclear weapon developed?

The first full-scale thermonuclear test was carried out by the United States in 1952; the concept has since been employed by most of the world's nuclear powers in the design of their weapons. The design of all modern thermonuclear weapons in the United States is known as the "Teller–Ulam configuration" for its two chief contributors, Edward Teller and Stanislaw Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of physicist John von Neumann. Similar devices were developed by the Soviet Union, United Kingdom, France, and China.

Thermonuclear weapon

A thermonuclear weapon, or fusion weapon, is a second-generation nuclear weapon design which affords vastly greater destructive power than first-generation atomic bombs. Modern fusion weapons consist essentially of two main components: a nuclear fission primary stage (fueled by Uranium-235 or Plutonium-239) and a separate nuclear fusion secondary stage containing thermonuclear fuel: the heavy hydrogen isotopes deuterium and tritium, or in modern weapons lithium deuteride. For this reason thermonuclear weapons are often colloquially called hydrogen bombs, or H-bombs.

A fusion explosion begins with the detonation of the fission primary stage. Its temperature soars past approximately one hundred million Kelvins, causing it to glow intensely with thermal x-radiation. These x-rays flood the void (the "radiation channel" often filled with polystyrene foam) between the primary and secondary assemblies placed within an enclosure called a radiation case, which confines the x-ray energy and resists getting pressed outwards. The distance separating the two assemblies ensures that debris fragments from the fission primary (which move much slower than x-ray photons) cannot disassemble the secondary before the fusion explosion runs to completion.

The secondary fusion stage–consisting of pusher/tamper, fusion fuel, and plutonium spark plug–is imploded by the x-ray energy pressing it inward. This compresses it and drives up the density of the plutonium spark plug at its center. The density of the plutonium fuel rises to such an extent that the spark plug is driven into a supercritical state, and it begins a nuclear fission chain reaction. The fission products so produced heat the highly compressed, and thus superdense, thermonuclear fuel surrounding the spark plug to the region of some three hundred million Kelvins, igniting fusion reactions between fusion fuel nuclei. In modern weapons fueled by lithium deuteride, the fissioning plutonium spark plug also emits free neutrons which collide with lithium nuclei and supply the tritium component of the thermonuclear fuel.

The radiation implosion mechanism exploits the temperature difference between the secondary stage's hot, surrounding radiation channel and its relatively cool interior. This temperature difference is briefly maintained by a massive heat barrier called the "pusher"/"tamper", which also serves as an implosion tamper, increasing and prolonging the compression of the secondary. If made of uranium, enriched uranium or plutonium, it can capture fusion neutrons produced by the fusion reaction and undergo fission itself, increasing the overall explosive yield. In addition to that, some designs also make the radiation case out of a fissile material that undergoes fission. As a result, such bombs get a third tertiary fission stage, and the majority of current Teller–Ulam are fission-fusion-fission weapons. Fission of the tamper or radiation case is the main contribution to the total yield and produces radioactive fission product fallout.

As thermonuclear weapons represent the most efficient design for weapon energy yield in weapons with yields above, virtually all the nuclear weapons of this size deployed by the five nuclear-weapon states under the Non-Proliferation Treaty today are thermonuclear weapons using the Teller–Ulam design.

Detailed knowledge of fission and fusion weapons is classified to some degree in virtually every industrialized nation. In the United States, such knowledge can by default be classified as "Restricted Data", even if it is created by persons who are not government employees or associated with weapons programs, in a legal doctrine known as "born secret" (though the constitutional standing of the doctrine has been at times called into question; see "United States v. Progressive, Inc."). Born secret is rarely invoked for cases of private speculation. The official policy of the United States Department of Energy has been not to acknowledge the leaking of design information, as such acknowledgment would potentially validate the information as accurate. In a small number of prior cases, the U.S. government has attempted to censor weapons information in the public press, with limited success. According to the "New York Times", physicist Kenneth W. Ford defied government orders to remove classified information from his book, "Building the H Bomb: A Personal History". Ford claims he used only pre-existing information and even submitted a manuscript to the government, which wanted to remove entire sections of the book for concern that foreign nations could use the information.

Though large quantities of vague data have been officially released, and larger quantities of vague data have been unofficially leaked by former bomb designers, most public descriptions of nuclear weapon design details rely to some degree on speculation, reverse engineering from known information, or comparison with similar fields of physics (inertial confinement fusion is the primary example). Such processes have resulted in a body of unclassified knowledge about nuclear bombs that is generally consistent with official unclassified information releases, related physics, and is thought to be internally consistent, though there are some points of interpretation that are still considered open. The state of public knowledge about the Teller–Ulam design has been mostly shaped from a few specific incidents outlined in a section below.

The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in "stages", with the detonation of each stage providing the energy to ignite the next stage. At a bare minimum, this implies a "primary" section that consists of an implosion-type fission bomb (a "trigger"), and a "secondary" section that consists of fusion fuel. The energy released by the "primary" compresses the "secondary" through a process called "radiation implosion", at which point it is heated and undergoes nuclear fusion. This process could be continued, with energy from the secondary igniting a third fusion stage; Russia's AN602 "Tsar Bomba" is thought to have been a three-stage fission-fusion-fusion device. Theoretically by continuing this process thermonuclear weapons with arbitrarily high yield could be constructed. This contrasts with fission weapons which are limited in yield.

Surrounding the other components is a hohlraum or "radiation case", a container that traps the first stage or primary's energy inside temporarily. The outside of this radiation case, which is also normally the outside casing of the bomb, is the only direct visual evidence publicly available of any thermonuclear bomb component's configuration. Numerous photographs of various thermonuclear bomb exteriors have been declassified.

The primary is thought to be a standard implosion method fission bomb, though likely with a core boosted by small amounts of fusion fuel (usually 50/50% deuterium/tritium gas) for extra efficiency; the fusion fuel releases excess neutrons when heated and compressed, inducing additional fission. When fired, the plutonium-239 (Pu-239) or uranium-235 (U-235) core would be compressed to a smaller sphere by special layers of conventional high explosives arranged around it in an explosive lens pattern, initiating the nuclear chain reaction that powers the conventional "atomic bomb".

The secondary is usually shown as a column of fusion fuel and other components wrapped in many layers. Around the column is first a "pusher-tamper", a heavy layer of uranium-238 (U-238) or lead that helps compress the fusion fuel (and, in the case of uranium, may eventually undergo fission itself). Inside this is the fusion fuel itself, usually a form of lithium deuteride, which is used because it is easier to weaponize than liquefied tritium/deuterium gas. This dry fuel, when bombarded by neutrons, produces tritium, a heavy isotope of hydrogen which can undergo nuclear fusion, along with the deuterium present in the mixture. (See the article on nuclear fusion for a more detailed technical discussion of fusion reactions.) Inside the layer of fuel is the "spark plug", a hollow column of fissile material (plutonium-239 or uranium-235) often boosted by deuterium gas. The spark plug, when compressed, can itself undergo nuclear fission (because of the shape, it is not a critical mass without compression). The tertiary, if one is present, would be set below the secondary and probably be made up of the same materials.

Separating the secondary from the primary is the interstage. The fissioning primary produces four types of energy: 1) expanding hot gases from high explosive charges that implode the primary; 2) superheated plasma that was originally the bomb's fissile material and its tamper; 3) the electromagnetic radiation; and 4) the neutrons from the primary's nuclear detonation. The interstage is responsible for accurately modulating the transfer of energy from the primary to the secondary. It must direct the hot gases, plasma, electromagnetic radiation and neutrons toward the right place at the right time. Less than optimal interstage designs have resulted in the secondary failing to work entirely on multiple shots, known as a "fissile fizzle". The Koon shot of Operation Castle is a good example; a small flaw allowed the neutron flux from the primary to prematurely begin heating the secondary, weakening the compression enough to prevent any fusion.

There is very little detailed information in the open literature about the mechanism of the interstage. One of the best sources is a simplified diagram of a British thermonuclear weapon similar to the American W80 warhead. It was released by Greenpeace in a report titled ""Dual Use Nuclear Technology"". The major components and their arrangement are in the diagram, though details are almost absent; what scattered details it does include likely have intentional omissions or inaccuracies. They are labeled "End-cap and Neutron Focus Lens" and "Reflector Wrap"; the former channels neutrons to the U-235/Pu-239 Spark Plug while the latter refers to an X-ray reflector; typically a cylinder made out of an X-ray opaque material such as uranium with the primary and secondary at either end. It does not reflect like a mirror; instead, it gets heated to a high temperature by the X-ray flux from the primary, then it emits more evenly spread X-rays that travel to the secondary, causing what is known as radiation implosion. In Ivy Mike, gold was used as a coating over the uranium to enhance the blackbody effect. Next comes the "Reflector/Neutron Gun Carriage". The reflector seals the gap between the Neutron Focus Lens (in the center) and the outer casing near the primary. It separates the primary from the secondary and performs the same function as the previous reflector. There are about six neutron guns (seen here from Sandia National Laboratories) each poking through the outer edge of the reflector with one end in each section; all are clamped to the carriage and arranged more or less evenly around the casing's circumference. The neutron guns are tilted so the neutron emitting end of each gun end is pointed towards the central axis of the bomb. Neutrons from each neutron gun pass through and are focused by the neutron focus lens towards the centre of primary in order to boost the initial fissioning of the plutonium. A "Polystyrene Polarizer/Plasma Source" is also shown (see below).

The first U.S. government document to mention the interstage was only recently released to the public promoting the 2004 initiation of the Reliable Replacement Warhead Program. A graphic includes blurbs describing the potential advantage of a RRW on a part by part level, with the interstage blurb saying a new design would replace "toxic, brittle material" and "expensive'special' material... [which require] unique facilities". The "toxic, brittle material" is widely assumed to be beryllium which fits that description and would also moderate the neutron flux from the primary. Some material to absorb and re-radiate the X-rays in a particular manner may also be used.

Candidates for the "special material" are polystyrene and a substance called "FOGBANK", an unclassified codename. FOGBANK's composition is classified, though aerogel has been suggested as a possibility. It was first used in thermonuclear weapons with the W-76 thermonuclear warhead, and produced at a plant in the Y-12 Complex at Oak Ridge, Tennessee for use in the W-76. Production of FOGBANK lapsed after the W-76 production run ended. The W-76 Life Extension Program required more FOGBANK to be made. This was complicated by the fact that the original FOGBANK's properties weren't fully documented, so a massive effort was mounted to re-invent the process. An impurity crucial to the properties of the old FOGBANK was omitted during the new process. Only close analysis of new and old batches revealed the nature of that impurity. The manufacturing process used acetonitrile as a solvent, which led to at least three evacuations of the FOGBANK plant in 2006. Widely used in the petroleum and pharmaceutical industries, acetonitrile is flammable and toxic. Y-12 is the sole producer of FOGBANK.

A simplified summary of the above explanation is:Thermonuclear weapons may or may not use a boosted primary stage, use different types of fusion fuel, and may surround the fusion fuel with beryllium (or another neutron reflecting material) instead of depleted uranium to prevent early premature fission from occurring before the secondary is optimally compressed.

The basic idea of the Teller–Ulam configuration is that each "stage" would undergo fission or fusion (or both) and release energy, much of which would be transferred to another stage to trigger it. How exactly the energy is "transported" from the "primary" to the "secondary" has been the subject of some disagreement in the open press, but is thought to be transmitted through the X-rays and Gamma rays that are emitted from the fissioning "primary". This energy is then used to compress the "secondary". The crucial detail of "how" the X-rays create the pressure is the main remaining disputed point in the unclassified press. There are three proposed theories:

The radiation pressure exerted by the large quantity of X-ray photons inside the closed casing might be enough to compress the secondary. Electromagnetic radiation such as X-rays or light carries momentum and exerts a force on any surface it strikes. The pressure of radiation at the intensities seen in everyday life, such as sunlight striking a surface, is usually imperceptible, but at the extreme intensities found in a thermonuclear bomb the pressure is enormous.

For two thermonuclear bombs for which the general size and primary characteristics are well understood, the Ivy Mike test bomb and the modern W-80 cruise missile warhead variant of the W-61 design, the radiation pressure was calculated to be 73 million bar (atmospheres) (7.3 T Pa) for the Ivy Mike design and 1,400 million bar (140 TPa) for the W-80.

Foam plasma pressure is the concept that Chuck Hansen introduced during the Progressive case, based on research that located declassified documents listing special foams as liner components within the radiation case of thermonuclear weapons.

The sequence of firing the weapon (with the foam) would be as follows:

This would complete the fission-fusion-fission sequence. Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive products or large amounts of nuclear fallout. The fission reactions though, especially the last fission reactions, release a tremendous amount of fission products and fallout. If the last fission stage is omitted, by replacing the uranium tamper with one made of lead, for example, the overall explosive force is reduced by approximately half but the amount of fallout is relatively low. The neutron bomb is a hydrogen bomb with an intentionally thin tamper, allowing most of the fast fusion neutrons as possible to escape.

Current technical criticisms of the idea of "foam plasma pressure" focus on unclassified analysis from similar high energy physics fields that indicate that the pressure produced by such a plasma would only be a "small multiplier" of the basic photon pressure within the radiation case, and also that the known foam materials intrinsically have a very low absorption efficiency of the gamma ray and X-ray radiation from the primary. Most of the energy produced would be absorbed by either the walls of the radiation case or the tamper around the secondary. Analyzing the effects of that absorbed energy led to the third mechanism: ablation.

The outer casing of the secondary assembly is called the "tamper-pusher". The purpose of a tamper in an implosion bomb is to delay the expansion of the reacting fuel supply (which is very hot dense plasma) until the fuel is fully consumed and the explosion runs to completion. The same tamper material serves also as a pusher in that it is the medium by which the outside pressure (force acting on the surface area of the secondary) is transferred to the mass of fusion fuel.
The proposed tamper-pusher ablation mechanism posits that the outer layers of the thermonuclear secondary's tamper-pusher are heated so extremely by the primary's X-ray flux that they expand violently and ablate away (fly off). Because total momentum is conserved, this mass of high speed ejecta impels the rest of the tamper-pusher to recoil inwards at tremendous velocity, crushing the fusion fuel and the spark plug. The tamper-pusher is built robustly enough to insulate the fusion fuel from the extreme heat outside, which otherwise would spoil the compression.

Rough calculations for the basic ablation effect are relatively simple: the energy from the primary is distributed evenly onto all of the surfaces within the outer radiation case, with the components coming to a thermal equilibrium, and the effects of that thermal energy are then analyzed. The energy is mostly deposited within about one X-ray optical thickness of the tamper/pusher outer surface, and the temperature of that layer can then be calculated. The velocity at which the surface then expands outwards is calculated and, from a basic Newtonian momentum balance, the velocity at which the rest of the tamper implodes inwards.

Applying the more detailed form of those calculations to the Ivy Mike device yields vaporized pusher gas expansion velocity of 290 kilometers per second and an implosion velocity of perhaps 400 kilometers per second if 3/4 of the total tamper/pusher mass is ablated off, the most energy efficient proportion. For the W-80 the gas expansion velocity is roughly 410 kilometers per second and the implosion velocity 570 kilometers per second. The pressure due to the ablating material is calculated to be 5.3 billion bar (530 T Pa) in the Ivy Mike device and 64 billion bar (6.4 P Pa) in the W-80 device.

Comparing the three mechanisms proposed, it can be seen that:

The calculated ablation pressure is one order of magnitude greater than the higher proposed plasma pressures and nearly two orders of magnitude greater than calculated radiation pressure. No mechanism to avoid the absorption of energy into the radiation case wall and the secondary tamper has been suggested, making ablation apparently unavoidable. The other mechanisms appear to be unneeded.

United States Department of Defense official declassification reports indicate that foamed plastic materials are or may be used in radiation case liners, and despite the low direct plasma pressure they may be of use in delaying the ablation until energy has distributed evenly and a sufficient fraction has reached the secondary's tamper/pusher.

Richard Rhodes' book "Dark Sun" stated that a layer of plastic foam was fixed to the lead liner of the inside of the Ivy Mike steel casing using copper nails. Rhodes quotes several designers of that bomb explaining that the plastic foam layer inside the outer case is to delay ablation and thus recoil of the outer case: if the foam were not there, metal would ablate from the inside of the outer case with a large impulse, causing the casing to recoil outwards rapidly. The purpose of the casing is to contain the explosion for as long as possible, allowing as much X-ray ablation of the metallic surface of the secondary stage as possible, so it compresses the secondary efficiently, maximizing the fusion yield. Plastic foam has a low density, so causes a smaller impulse when it ablates than metal does.

A number of possible variations to the weapon design have been proposed:

Two special variations exist that will be discussed in a subsequent section: the cryogenically cooled liquid deuterium device used for the Ivy Mike test, and the putative design of the W88 nuclear warhead—a small, MIRVed version of the Teller–Ulam configuration with a prolate (egg or watermelon shaped) primary and an elliptical secondary.

Most bombs do not apparently have tertiary "stages"—that is, third compression stage(s), which are additional fusion stages compressed by a previous fusion stage. (The fissioning of the last blanket of uranium, which provides about half the yield in large bombs, does not count as a "stage" in this terminology.)

The U.S. tested three-stage bombs in several explosions (see Operation Redwing) but is thought to have fielded only one such tertiary model, i.e., a bomb in which a fission stage, followed by a fusion stage, finally compresses yet another fusion stage. This U.S. design was the heavy but highly efficient (i.e., nuclear weapon yield per unit bomb weight) 25 Mt B41 nuclear bomb. The Soviet Union is thought to have used multiple stages (including more than one tertiary fusion stage) in their 50 megaton (100 Mt in intended use) Tsar Bomba (however, as with other bombs, the fissionable jacket could be replaced with lead in such a bomb, and in this one, for demonstration, it was). If any hydrogen bombs have been made from configurations other than those based on the Teller–Ulam design, the fact of it is not publicly known. (A possible exception to this is the Soviet early "Sloika" design).

In essence, the Teller–Ulam configuration relies on at least two instances of implosion occurring: first, the conventional (chemical) explosives in the primary would compress the fissile core, resulting in a fission explosion many times more powerful than that which chemical explosives could achieve alone (first stage). Second, the radiation from the fissioning of the primary would be used to compress and ignite the secondary fusion stage, resulting in a fusion explosion many times more powerful than the fission explosion alone. This chain of compression could conceivably be continued with an arbitrary number of tertiary fusion stages, each igniting more fusion fuel in the next stage although this is debated (see more: Arbitrarily large yield debate). Finally, efficient bombs (but not so-called neutron bombs) end with the fissioning of the final natural uranium tamper, something that could not normally be achieved without the neutron flux provided by the fusion reactions in secondary or tertiary stages. Such designs are suggested to be capable of being scaled up to an arbitrary large yield (with apparently as many fusion stages as desired), potentially to the level of a "doomsday device." However, usually such weapons were not more than a dozen megatons, which was generally considered enough to destroy even most hardened practical targets (for example, a control facility such as the Cheyenne Mountain Complex). Even such large bombs have been replaced by smaller-yield bunker buster type nuclear bombs (see more: nuclear bunker buster).

As discussed above, for destruction of cities and non-hardened targets, breaking the mass of a single missile payload down into smaller MIRV bombs, in order to spread the energy of the explosions into a "pancake" area, is far more efficient in terms of area-destruction per unit of bomb energy. This also applies to single bombs deliverable by cruise missile or other system, such as a bomber, resulting in most operational warheads in the U.S. program having yields of less than 500 kilotons.

The idea of a thermonuclear fusion bomb ignited by a smaller fission bomb was first proposed by Enrico Fermi to his colleague Edward Teller in 1941 at the start of what would become the Manhattan Project. Teller spent most of the Manhattan Project attempting to figure out how to make the design work, to some degree neglecting his assigned work on the fission bomb program. His difficult and devil's advocate attitude in discussions led Robert Oppenheimer to sidetrack him and other "problem" physicists into the super program to smooth his way.

Stanislaw Ulam, a co-worker of Teller, made the first key conceptual leaps towards a workable fusion design. Ulam's two innovations that rendered the fusion bomb practical were that compression of the thermonuclear fuel before extreme heating was a practical path towards the conditions needed for fusion, and the idea of staging or placing a separate thermonuclear component outside a fission primary component, and somehow using the primary to compress the secondary. Teller then realized that the gamma and X-ray radiation produced in the primary could transfer enough energy into the secondary to create a successful implosion and fusion burn, if the whole assembly was wrapped in a "hohlraum" or radiation case. Teller and his various proponents and detractors later disputed the degree to which Ulam had contributed to the theories underlying this mechanism. Indeed, shortly before his death, and in a last-ditch effort to discredit Ulam's contributions, Teller claimed that one of his own "graduate students" had proposed the mechanism.

The "George" shot of Operation Greenhouse of 9 May 1951 tested the basic concept for the first time on a very small scale. As the first successful (uncontrolled) release of nuclear fusion energy, which made up a small fraction of the 225 kt total yield, it raised expectations to a near certainty that the concept would work.

On November 1, 1952, the Teller–Ulam configuration was tested at full scale in the "Ivy Mike" shot at an island in the Enewetak Atoll, with a yield of 10.4 megatons (over 450 times more powerful than the bomb dropped on Nagasaki during World War II). The device, dubbed the "Sausage", used an extra-large fission bomb as a "trigger" and liquid deuterium—kept in its liquid state by 20 short tons (18 metric tons) of cryogenic equipment—as its fusion fuel, and weighed around 80 short tons (70 metric tons) altogether.

The liquid deuterium fuel of Ivy Mike was impractical for a deployable weapon, and the next advance was to use a solid lithium deuteride fusion fuel instead. In 1954 this was tested in the "Castle Bravo" shot (the device was code-named "Shrimp"), which had a yield of 15 megatons (2.5 times expected) and is the largest U.S. bomb ever tested.

Efforts in the United States soon shifted towards developing miniaturized Teller–Ulam weapons that could fit into intercontinental ballistic missiles and submarine-launched ballistic missiles. By 1960, with the W47 warhead deployed on Polaris ballistic missile submarines, megaton-class warheads were as small as 18 inches (0.5 m) in diameter and 720 pounds (320 kg) in weight. It was later found in live testing that the Polaris warhead did not work reliably and had to be redesigned. Further innovation in miniaturizing warheads was accomplished by the mid-1970s, when versions of the Teller–Ulam design were created that could fit ten or more warheads on the end of a small MIRVed missile (see the section on the W88 below).

The Soviet thermonuclear weapons program was aided heavily by Klaus Fuchs. Fuchs’ most valuable contribution to the Soviet weapons program concerned the hydrogen bomb. The idea of a hydrogen bomb arose from discussions between Enrico Fermi and Edward Teller in 1941. From 1943 Teller lectured at Los Alamos on what he called the "super". Following their meeting, Fermi was convinced by Teller to present a series of lectures detailing the current state of research into thermonuclear weapons. In September 1945 Fuchs passed a synopsis of these lectures to the Soviets. This information was important to the Soviets, but not solely for the information about the US bomb project. The importance of this material was in that it confirmed that the United States were working on their own thermonuclear weapon research. Although the information provided by Fuchs regarding the thermonuclear weapons research was not seen as entirely beneficial, it still provided the Soviet Union with knowledge such as the properties of tritium. Tritium is an isotope of hydrogen with two neutrons, which allows for more efficient fusion reactions to occur during the detonation of a nuclear weapon. Discovering the properties of this radioactive material would allow the Soviet Union to develop a more powerful weapon that requires less fuel. Following Fuchs's return, experts from the Soviet Union spent a great deal of time researching his findings for themselves. Even though the Soviets did obtain some original ideas, the findings of this research served to confirm Fuchs's notes from the American lectures on the matter. After his return to England in mid-1946, Fuchs was not again in touch with Soviet intelligence until September 1947, when his controller confirmed the Soviet interest in thermonuclear weapons. In response Fuchs provided details of the "ongoing theoretical superbomb studies in the U.S. under the direction of Teller and Enrico Fermi at the University of Chicago." Fuchs obtained information regardless of the American McMahon Act, which prevented Anglo-American cooperation on nuclear weapons research. Under this act, Fuchs did not have routine access to American collaborators like Fermi and Teller. Fuchs was very close to Teller at Los Alamos, and while there Fuchs had worked on thermonuclear weapons. As Teller later recalled, "he [Fuchs] talked with me and others frequently in depth about our intensive efforts… it was easy and pleasant to discuss my work with him. He also made impressive contributions, and I learned many technical facts from him." Fuchs obtained the information, it energized the Soviets to direct new intelligence activities against research in Chicago. In February 1948 the Soviet Union formally began its hydrogen bomb program. A month later Fuchs again met with Feklisov, an event which "played an exceptional role in the subsequent course of the Soviet thermonuclear bomb program." A report of June 1953 warned that, although no indication of Soviet development of hydrogen bombs had been found, "Soviet research, development and even field testing of thermonuclear reactions based on the disclosures of Fuchs may take place by mid-1953." U.S. intelligence thus recognized for the first time that Fuchs' material held invaluable information for the Soviet thermonuclear weapons program.

The first Soviet fusion design, developed by Andrei Sakharov and Vitaly Ginzburg in 1949 (before the Soviets had a working fission bomb), was dubbed the "Sloika", after a Russian layer cake, and was not of the Teller–Ulam configuration. It used alternating layers of fissile material and lithium deuteride fusion fuel spiked with tritium (this was later dubbed Sakharov's "First Idea"). Though nuclear fusion might have been technically achievable, it did not have the scaling property of a "staged" weapon. Thus, such a design could not produce thermonuclear weapons whose explosive yields could be made arbitrarily large (unlike U.S. designs at that time). The fusion layer wrapped around the fission core could only moderately multiply the fission energy (modern Teller–Ulam designs can multiply it 30-fold). Additionally, the whole fusion stage had to be imploded by conventional explosives, along with the fission core, substantially multiplying the amount of chemical explosives needed.

The first Sloika design test, RDS-6s, was detonated in 1953 with a yield equivalent to 400 kilotons of TNT (15–20% from fusion). Attempts to use a "Sloika" design to achieve megaton-range results proved unfeasible. After the United States tested the "Ivy Mike" bomb in November 1952, proving that a multimegaton bomb could be created, the Soviets searched for an additional design. The "Second Idea", as Sakharov referred to it in his memoirs, was a previous proposal by Ginzburg in November 1948 to use lithium deuteride in the bomb, which would, in the course of being bombarded by neutrons, produce tritium and free deuterium. In late 1953 physicist Viktor Davidenko achieved the first breakthrough, that of keeping the "primary" and "secondary" parts of the bombs in separate pieces ("staging"). The next breakthrough was discovered and developed by Sakharov and Yakov Zel'dovich, that of using the X-rays from the fission bomb to compress the "secondary" before fusion ("radiation implosion"), in early 1954. Sakharov's "Third Idea", as the Teller–Ulam design was known in the USSR, was tested in the shot "RDS-37" in November 1955 with a yield of 1.6 megatons.

The Soviets demonstrated the power of the "staging" concept in October 1961, when they detonated the massive and unwieldy "Tsar Bomba", a 50 megaton hydrogen bomb that derived almost 97% of its energy from fusion. It was the largest nuclear weapon developed and tested by any country.

In 1954 work began at Aldermaston to develop the British fusion bomb, with Sir William Penney in charge of the project. British knowledge on how to make a thermonuclear fusion bomb was rudimentary, and at the time the United States was not exchanging any nuclear knowledge because of the Atomic Energy Act of 1946. However, the British were allowed to observe the American Castle tests and used sampling aircraft in the mushroom clouds, providing them with clear, direct evidence of the compression produced in the secondary stages by radiation implosion.

Because of these difficulties, in 1955 British prime minister Anthony Eden agreed to a secret plan, whereby if the Aldermaston scientists failed or were greatly delayed in developing the fusion bomb, it would be replaced by an extremely large fission bomb.

In 1957 the Operation Grapple tests were carried out. The first test, Green Granite was a prototype fusion bomb, but failed to produce equivalent yields compared to the Americans and Soviets, achieving only approximately 300 kilotons. The second test Orange Herald was the modified fission bomb and produced 720 kilotons—making it the largest fission explosion ever. At the time almost everyone (including the pilots of the plane that dropped it) thought that this was a fusion bomb. This bomb was put into service in 1958. A second prototype fusion bomb Purple Granite was used in the third test, but only produced approximately 150 kilotons.

A second set of tests was scheduled, with testing recommencing in September 1957. The first test was based on a "… new simpler design. A two stage thermonuclear bomb that had a much more powerful trigger". This test Grapple X Round C was exploded on November 8 and yielded approximately 1.8 megatons. On April 28, 1958 a bomb was dropped that yielded 3 megatons—Britain's most powerful test. Two final air burst tests on September 2 and September 11, 1958, dropped smaller bombs that yielded around 1 megaton each.

American observers had been invited to these kinds of tests. After Britain's successful detonation of a megaton-range device (and thus demonstrating a practical understanding of the Teller–Ulam design "secret"), the United States agreed to exchange some of its nuclear designs with the United Kingdom, leading to the 1958 US–UK Mutual Defence Agreement. Instead of continuing with its own design, the British were given access to the design of the smaller American Mk 28 warhead and were able to manufacture copies.

The United Kingdom had worked closely with the Americans on the Manhattan Project. British access to nuclear weapons information was cut-off by the United States at one point due to concerns about Soviet espionage. Full cooperation was not reestablished until an agreement governing the handling of secret information and other issues was signed.

Mao Zedong decided to begin a Chinese nuclear-weapons program during the First Taiwan Strait Crisis of 1954–1955. The People's Republic of China detonated its first hydrogen (thermonuclear) bomb on June 17, 1967, 32 months after detonating its first fission weapon, with a yield of 3.31 Mt. It took place in the Lop Nor Test Site, in northwest China. China had received extensive technical help from the Soviet Union to jump-start their nuclear program, but by 1960, the rift between the Soviet Union and China had become so great that the Soviet Union ceased all assistance to China.

A story in "The New York Times" by William Broad reported that in 1995, a supposed Chinese double agent delivered information indicating that China knew secret details of the U.S. W88 warhead, supposedly through espionage. (This line of investigation eventually resulted in the abortive trial of Wen Ho Lee.)

France's journey in building nuclear weapons began prior to World War II in 1939. The development of nuclear weapons was slowed during the country's German invasion. The United States did not want France to acquire expert knowledge about nuclear weaponry, which ultimately led to the Alsos Mission. The missions followed closely behind the advancing forward-front to obtain information about how close Germany was to building an atomic weapon. Following the surrender of the Nazis, Germany was divided into "zones of occupation". The "zone" given to the French was suspected to contain several nuclear research facilities. The United States conducted Operation Harborage to seize any and all information about nuclear weaponry from the French. The Operation strategized to have American troops intercede advancing French army, allowing the Americans to seize any German scientists or records as well as destroy the remaining functional facilities.

In 1945, the French Atomic Energy Commission (Commissariat à l’Énergie Atomique, CEA) was founded under Charles de Gaulle; the CEA served as the country's atomic energy authority, overseeing commercial, military, and scientific uses of atomic power. However it was not until 1952 that a tangible goal of building plutonium reactors progressed. Two years later, a reactor was being built and a plutonium separating plant began construction shortly after. In 1954 the question about continuing to explore building an atomic bomb was raised. The French cabinet seemed to be favoring less the building of an atomic bomb. Ultimately, the Prime Minister decided to continue efforts developing an atomic bomb in secret. In late 1956, tasks were delegated between the CEA and Defense Ministry to propel atomic development such as finding a test site, providing the necessary uranium, and physical device assembly.

Charles de Gaulle returned to power and was elected France's Fifth Republic's first president in 1958. De Gaulle, a strong believer in the nuclear weapons program, approved the country's first nuclear test to take place in one of the early months of 1960. The country's first nuclear explosion took place on 13 February at Reggane Oasis in the Sahara Desert in French Algeria of the time. It was called "Gerboise Bleue", translating to "Blue jerboa". The first explosion was detonated at a tower height of 105 meters. The bomb used a plutonium implosion design with a yield of 70 kilotons. The Reggane Oasis test site was used for three more atmospheric tests before testing activity moved to a second site, Ecker, to carry out a total of 13 underground tests into 1967.

The French nuclear testing site was moved to the unpopulated French atolls in the Pacific Ocean. The first test conducted at these new sites was the "Canopus" test in the Fangataufa atoll in French Polynesia on 24 August 1968, the country's first multistage thermonuclear weapon test. The bomb was detonated from a balloon at a height of 520 meters. The result of this test was significant atmospheric contamination. Very little is known about France's development of the Teller–Ulam design, beyond the fact that France detonated a 2.6 Mt device in the 'Canopus" test. France reportedly had great difficulty with its initial development of the Teller-Ulam design, but it later overcame these, and is believed to have nuclear weapons equal in sophistication to the other major nuclear powers.

France and China did not sign or ratify the Partial Nuclear Test Ban Treaty of 1963, which banned nuclear test explosions in the atmosphere, underwater, or in outer space. Between 1966 and 1996 France carried out more than 190 nuclear tests. France's final nuclear test took place on January 27, 1996, and then the country dismantled its Polynesian test sites. France signed the Comprehensive Nuclear-Test-Ban Treaty that same year, and then ratified the Treaty within two years.

France confirmed that its nuclear arsenal contains about 300 warheads, carried by submarine-launched ballistic missiles (SLBMs) and fighter-bombers in 2015. France has four Triomphant-class ballistic missile submarines. One ballistic missile submarine is deployed in the deep ocean, but a total of three must be in operational use at all times. The three older submarines are armed with 16 M45 missiles. The newest submarine, "Le Terrible", was commissioned in 2010, and it has M51 missiles capable of carrying TN 75 thermonuclear warheads. The air fleet is four squadrons at four different bases. In total, there are 23 Mirage 2000N aircraft and 20 Rafales capable of carrying nuclear warheads. The M51.1 missiles are intended to be replaced with the new M51.2 warhead beginning in 2016, which has a 3,000 km greater range than the M51.1.

President François Hollande announced 180 billion euros would be used from the annual defense budget to improve the country's nuclear deterrence. France contains 13 International Monitoring System facilities that monitor for nuclear explosive activity on Earth through the use of seismic, infrasound, and hydroacoustic monitors.

France also has about 60 air-launched missiles tipped with TN 80/TN 81 warheads with a yield of about 300 kilotons each. France's nuclear program has been carefully designed to ensure that these weapons remain usable decades into the future. Currently, France is no longer deliberately producing critical mass materials such as plutonium and enriched uranium, but it still relies on nuclear energy for electricity, with Pu-239 as a byproduct.

On May 11, 1998, India reportedly detonated a thermonuclear bomb in its Operation Shakti tests ("Shakti-I", specifically). Dr. Samar Mubarakmand, a Pakistani nuclear physicist, asserted that Shakti-1 was a successful thermonuclear test. The yield of India's hydrogen bomb remains highly debatable among the Indian science community and the international scholars. The question of politicisation and disputes between Indian scientists further complicated the matter.

In an interview in August 2009, the director for the 1998 test site preparations, Dr. K. Santhanam claimed that the yield of the thermonuclear explosion was lower than expected and that India should therefore not rush into signing the CTBT. Other Indian scientists involved in the test have disputed Dr. K. Santhanam's claim. International sources, using local data and citing a United States Geological Survey report compiling seismic data from 125 IRIS stations across the world, argue that the magnitudes suggested a combined yield of up to 60 kilotonnes, consistent with the Indian announced total yield of 56 kilotonnes.

Israel is alleged to possess thermonuclear weapons of the Teller–Ulam design, but it is not known to have tested any nuclear devices, although it is widely speculated that the Vela Incident of 1979 may have been a joint Israeli–South African nuclear test.

It is well established that Edward Teller advised and guided the Israeli establishment on general nuclear matters for some twenty years. Between 1964 and 1967, Teller made six visits to Israel where he lectured at the Tel Aviv University on general topics in theoretical physics. It took him a year to convince the CIA about Israel's capability and finally in 1976, Carl Duckett of the CIA testified to the U.S. Congress, after receiving credible information from an "American scientist" (Teller), on Israel's nuclear capability. During the 1990s, Teller eventually confirmed speculations in the media that it was during his visits in the 1960s that he concluded that Israel was in possession of nuclear weapons. After he conveyed the matter to the higher level of the U.S. government, Teller reportedly said: "They [Israel] have it, and they were clever enough to trust their research and not to test, they know that to test would get them into trouble."

According to the scientific data received and published by PAEC, the Corps of Engineers, and Kahuta Research Laboratories (KRL), in May 1998, Pakistan carried out six underground nuclear tests in Chagai Hills and Kharan Desert in Balochistan Province (see the code-names of the tests, "Chagai-I" and "Chagai-II"). None of these boosted fission devices was the thermonuclear weapon design, according to KRL and PAEC.

North Korea claimed to have tested its miniaturised thermonuclear bomb on 6 January 2016. North Korea's first three nuclear tests (2006, 2009 and 2013) were relatively low yield and do not appear to have been of a thermonuclear weapon design. In 2013, the South Korean Defense Ministry speculated that North Korea may be trying to develop a "hydrogen bomb" and such a device may be North Korea's next weapons test. In January 2016, North Korea claimed to have successfully tested a hydrogen bomb, although only a magnitude 5.1 seismic event was detected at the time of the test, a similar magnitude to the 2013 test of a 6–9 kt atomic bomb. These seismic recordings cast doubt upon North Korea's claim that a hydrogen bomb was tested and suggest it was a non-fusion nuclear test.

On 3 September 2017, the country's state media reported that a hydrogen bomb test was conducted which resulted in "perfect success". According to the U.S. Geological Survey (USGS), the blast resulted in an earthquake with a magnitude of 6.3, 10 times more powerful than previous nuclear tests conducted by North Korea. U.S. Intelligence released an early assessment that the yield estimate was 140 kilotons, with an uncertainty range of 70 to 280 kilotons.

On 12 September, NORSAR revised its estimate of the earthquake magnitude upward to 6.1, matching that of the CTBTO, but less powerful than the USGS estimate of 6.3. Its yield estimate was revised to 250 kilotons, while noting the estimate had some uncertainty and an undisclosed margin of error.

On 13 September, an analysis of before and after synthetic-aperture radar satellite imagery of the test site was published suggesting the test occurred under of rock and the yield "could have been in excess of 300 kilotons".

The Teller–Ulam design was for many years considered one of the top nuclear secrets, and even today it is not discussed in any detail by official publications with origins "behind the fence" of classification. United States Department of Energy (DOE) policy has been, and continues to be, that they do not acknowledge when "leaks" occur, because doing so would acknowledge the accuracy of the supposed leaked information. Aside from images of the warhead casing, most information in the public domain about this design is relegated to a few terse statements by the DOE and the work of a few individual investigators.

In 1972 the United States government declassified a document stating "[I]n thermonuclear (TN) weapons, a fission 'primary' is used to trigger a TN reaction in thermonuclear fuel referred to as a'secondary'", and in 1979 added, "[I]n thermonuclear weapons, radiation from a fission explosive can be contained and used to transfer energy to compress and ignite a physically separate component containing thermonuclear fuel." To this latter sentence the US government specified that ""Any elaboration of this statement will be classified"." The only information that may pertain to the "spark plug" was declassified in 1991: "Fact that fissile or fissionable materials are present in some secondaries, material unidentified, location unspecified, use unspecified, and weapons undesignated." In 1998 the DOE declassified the statement that "The fact that materials may be present in channels and the term 'channel filler,' with no elaboration", which may refer to the polystyrene foam (or an analogous substance).

Whether these statements vindicate some or all of the models presented above is up for interpretation, and official U.S. government releases about the technical details of nuclear weapons have been purposely equivocating in the past (see, e.g., Smyth Report). Other information, such as the types of fuel used in some of the early weapons, has been declassified, though precise technical information has not been.

Most of the current ideas on the workings of the Teller–Ulam design came into public awareness after the Department of Energy (DOE) attempted to censor a magazine article by U.S. antiweapons activist Howard Morland in 1979 on the "secret of the hydrogen bomb". In 1978, Morland had decided that discovering and exposing this "last remaining secret" would focus attention onto the arms race and allow citizens to feel empowered to question official statements on the importance of nuclear weapons and nuclear secrecy. Most of Morland's ideas about how the weapon worked were compiled from highly accessible sources—the drawings that most inspired his approach came from none other than the "Encyclopedia Americana". Morland also interviewed (often informally) many former Los Alamos scientists (including Teller and Ulam, though neither gave him any useful information), and used a variety of interpersonal strategies to encourage informative responses from them (i.e., asking questions such as "Do they still use spark plugs?" even if he was not aware what the latter term specifically referred to).

Morland eventually concluded that the "secret" was that the "primary" and "secondary" were kept separate and that radiation pressure from the "primary" compressed the "secondary" before igniting it. When an early draft of the article, to be published in "The Progressive" magazine, was sent to the DOE after falling into the hands of a professor who was opposed to Morland's goal, the DOE requested that the article not be published, and pressed for a temporary injunction. The DOE argued that Morland's information was (1) likely derived from classified sources, (2) if not derived from classified sources, itself counted as "secret" information under the "born secret" clause of the 1954 Atomic Energy Act, and (3) was dangerous and would encourage nuclear proliferation.

Morland and his lawyers disagreed on all points, but the injunction was granted, as the judge in the case felt that it was safer to grant the injunction and allow Morland, et al., to appeal, which they did in "United States v. The Progressive" (1979).

Through a variety of more complicated circumstances, the DOE case began to wane as it became clear that some of the data they were attempting to claim as "secret" had been published in a students' encyclopedia a few years earlier. After another H-bomb speculator, Chuck Hansen, had his own ideas about the "secret" (quite different from Morland's) published in a Wisconsin newspaper, the DOE claimed that "The Progressive" case was moot, dropped its suit, and allowed the magazine to publish its article, which it did in November 1979. Morland had by then, however, changed his opinion of how the bomb worked, suggesting that a foam medium (the polystyrene) rather than radiation pressure was used to compress the "secondary", and that in the "secondary" there was a "spark plug" of fissile material as well. He published these changes, based in part on the proceedings of the appeals trial, as a short erratum in "The Progressive" a month later. In 1981, Morland published a book about his experience, describing in detail the train of thought that led him to his conclusions about the "secret".

Morland's work is interpreted as being at least partially correct because the DOE had sought to censor it, one of the few times they violated their usual approach of not acknowledging "secret" material that had been released; however, to what degree it lacks information, or has incorrect information, is not known with any confidence. The difficulty that a number of nations had in developing the Teller–Ulam design (even when they apparently understood the design, such as with the United Kingdom), makes it somewhat unlikely that this simple information alone is what provides the ability to manufacture thermonuclear weapons. Nevertheless, the ideas put forward by Morland in 1979 have been the basis for all the current speculation on the Teller–Ulam design.

Two years before his death in 1989, Andrei Sakharov's comments at a scientists’ forum helped begin the process for the elimination of thousands of nuclear ballistic missiles from the US and Soviet arsenals. Sakharov (1921–89) was recruited into the Soviet Union's nuclear weapons program in 1948, a year after he completed his doctorate. In 1949 the US detected the first Soviet test of a fission bomb, and the two countries embarked on a desperate race to design a thermonuclear hydrogen bomb that was a thousand times more powerful. Like his US counterparts, Sakharov justified his H-bomb work by pointing to the danger of the other country's achieving a monopoly. But also like some of the US scientists who had worked on the Manhattan Project, he felt a responsibility to inform his nation's leadership and then the world about the dangers from nuclear weapons. Sakharov's first attempt to influence policy was brought about by his concern about possible genetic damage from long-lived radioactive carbon-14 created in the atmosphere from nitrogen-14 by the enormous fluxes of neutrons released in H-bomb tests. In 1968 a friend suggested that Sakharov write an essay about the role of the intelligentsia in world affairs. Self-publishing was the method at the time for spreading unapproved manuscripts in the Soviet Union. Many readers would create multiple copies by typing with multiple sheets of paper interleaved with carbon paper. One copy of Sakharov's essay, "Reflections on Progress, Peaceful Coexistence, and Intellectual Freedom," was smuggled out of the Soviet Union and published by the New York Times. More than 18 million reprints were produced during 1968–69. After the essay was published, Sakharov was barred from returning to work in the nuclear weapons program and took a research position in Moscow. In 1980, after an interview with the New York Times in which he denounced the Soviet invasion of Afghanistan the government put him beyond the reach of Western media by exiling him and his wife to Gorky. In March 1985 Gorbachev became general secretary of the Soviet Communist Party. More than a year and a half later, he persuaded the Politburo, the party's executive committee, to allow Sakharov and Bonner to return to Moscow. Sakharov was elected as an opposition member to the Soviet Congress of People's Deputies in 1989. Later that year he had a cardiac arrhythmia and died in his apartment. He left behind a draft of a new Soviet constitution that emphasized democracy and human rights.

In his 1995 book "Dark Sun: The Making of the Hydrogen Bomb", author Richard Rhodes describes in detail the internal components of the "Ivy Mike" "Sausage" device, based on information obtained from extensive interviews with the scientists and engineers who assembled it. According to Rhodes, the actual mechanism for the compression of the secondary was a combination of the radiation pressure, foam plasma pressure, and tamper-pusher ablation theories described above; the radiation from the primary heated the polyethylene foam lining the casing to a plasma, which then re-radiated radiation into the secondary's pusher, causing its surface to ablate and driving it inwards, compressing the secondary, igniting the sparkplug, and causing the fusion reaction. The general applicability of this principle is unclear.

In 1999 a reporter for the "San Jose Mercury News" reported that the U.S. W88 nuclear warhead, a small MIRVed warhead used on the Trident II SLBM, had a prolate (egg or watermelon shaped) "primary" (code-named "Komodo") and a spherical "secondary" (code-named "Cursa") inside a specially shaped radiation case (known as the "peanut" for its shape).

The reentry cones for the W88 and W87 are the same size, 1.75 meters (69 in) long, with a maximum diameter of 55 cm. (22 in). The higher yield of the W88 implies a larger secondary, which produces most of the yield. Putting the secondary, which is heavier than the primary, in the wider part of the cone allows it to be larger, but it also moves the center of mass aft, potentially causing aerodynamic stability problems during reentry. Dead-weight ballast must be added to the nose to move the center of mass forward.

To make the primary small enough to fit into the narrow part of the cone, its bulky insensitive high explosive charges must be replaced with more compact "non-insensitive" high explosives that are more hazardous to handle. The higher yield of the W88, which is the last new warhead produced by the United States, thus comes at a price of higher warhead weight and higher workplace hazard. The W88 also contains tritium, which has a half life of only 12.32 years and must be repeatedly replaced. If these stories are true, it would explain the reported higher yield of the W88, 475 kilotons, compared with only 300 kilotons for the earlier W87 warhead.

The United States Atomic Energy Commission, directed by David E. Lilienthal until 1950, was in charge of designing and building nuclear weapons. The policy was 100% civilian control. The U.S. had only 9 atomic bombs in 1946, but the stockpile grew to 650 by 1951. Lilienthal wanted to give high priority to peaceful uses, especially nuclear power plants, but coal was cheap and the power industry was not interested. Construction of the first nuclear plant would not begin until 1954. In early 1950, Truman authorized the development of thermonuclear weapons, a more powerful version of atomic bombs. Truman's decision to develop thermonuclear weapons faced opposition from many liberals and some government officials, but Truman believed that the Soviet Union would likely develop the weapons, and he was unwilling to allow the Soviets to have such an advantage. The first test of thermonuclear weaponry was conducted by the United States in 1952, and the Soviet Union performed its own thermonuclear test in August 1953.

Presidency of Harry S. Truman

The presidency of Harry S. Truman began on April 12, 1945, when Harry S. Truman became President of the United States upon the, and ended on January 20, 1953. He had been Vice President of the United States for only days when he succeeded to the presidency. A Democrat, he ran for and won a full four–year term in the 1948 election. His victory in that election, over Republican Thomas E. Dewey, was one of the greatest upsets in presidential electoral history. Following the 1952 election, Truman was succeeded in office by Republican Dwight D. Eisenhower.

Truman, the 33rd United States president, presided over the final defeat of Germany and Japan in World War II, the launching of the Marshall Plan to rebuild the economy of Western Europe, the undertaking of the Korean War, and the inception of the Cold War against the Soviet Union. In domestic affairs, his liberal proposals were a continuation of Franklin Roosevelt's New Deal, but the Conservative Coalition-dominated Congress blocked most of them. He used presidential authority to mandate equal treatment for blacks in the military and put civil rights on the national political agenda.

Truman's presidency was a turning point in foreign affairs, as the United States engaged in an internationalist foreign policy and renounced isolationism. In mid-1945, Truman helped establish the United Nations as Roosevelt had planned it. When relations with the Soviet Union turned hostile in 1947, he issued the Truman Doctrine in 1947 to contain Communism; it is often used to mark the start of the Cold War. In 1948 he got the $13 billion Marshall Plan enacted to rebuild Western Europe. Fears of Soviet espionage led to a Red Scare and the rise of McCarthyism. Truman oversaw the Berlin Airlift of 1948 and the creation of NATO in 1949. When communist North Korea invaded South Korea in 1950, he sent U.S. troops and gained UN approval for seizing North Korea in the Korean War. After initial successes, however, American/UN forces were thrown back by Chinese intervention in late 1950. The bloody war was stalemated throughout the final years of Truman's presidency.

During his administration, the nation experienced an unexpected surge in prosperity as economic growth surged following many long years of depression and war. His political coalition was based on the white South, labor unions, farmers, ethnic groups, and traditional Democrats across the North. Truman rallied them to win election in his own right in 1948. His domestic agenda, known as the "Fair Deal," was largely defeated by a conservative Congress dominated by the Southern legislators. Despite wave after wave of strikes in 1946, he at times took a tough line against his labor union allies and successfully guided the American economy through the post-war economic challenges. He holds the record for vetoes at 180, and saw 12 overridden by Congress; Gerald Ford later tied that record. Truman put civil rights on the national agenda as a moral priority in 1948. He made it a campaign issue, appointed study groups, and issued Executive Orders to end racial discrimination in the military and federal agencies. Scholars typically rank Truman's presidency about #6 from the top. His reputation in textbooks was favorable from the 1950s onward. However revisionist historians in the 1960s attacked his foreign policy as too hostile to Communism, and his domestic policy as too favorable toward business. That revisionism largely faded after the 1991 dissolution of the Soviet Union.

Truman had been selected as Franklin D. Roosevelt's running mate in the 1944 presidential election. Roosevelt had favored incumbent Vice President Henry A. Wallace or James F. Byrnes as his running mate in 1944, but Wallace was unpopular among conservatives in the party, while Byrnes was opposed by liberals and many Catholics. At the behest of party leaders, Roosevelt accepted Missouri Senator Harry S. Truman, who was acceptable to all factions of the party. Truman had gained a national reputation during World War II for his leadership of the Truman Committee, which he created to fight wasteful practices in wartime production; it reflected Truman's demands for honest and efficient administration. Truman led the bipartisan committee "with extraordinary skill", choosing quiet but strong methods of correction rather than creating publicity and enmity. Roosevelt won the 1944 election, and Truman took office as vice president in January 1945. He played a minor role as Vice President and was not told about the atomic bomb until he became president.

On the night of April 12, 1945, Truman was urgently summoned to the White House. He was met by Eleanor Roosevelt, who informed him that President Roosevelt was dead. Shocked, Truman asked Mrs. Roosevelt, "Is there anything I can do for you?", to which she replied: "Is there anything "we" can do for "you"? For you are the one in trouble now." Truman took the presidential oath of office shortly after learning of Roosevelt's death. The day after assuming office Truman spoke to reporters: "Boys, if you ever pray, pray for me now. I don't know if you fellas ever had a load of hay fall on you, but when they told me what happened yesterday, I felt like the moon, the stars, and all the planets had fallen on me."

When he first took office, Truman asked all the members of Roosevelt's cabinet to remain in place for the time being, but by the end of 1946 only one Roosevelt appointee, Secretary of the Navy James Forrestal, remained. Fred M. Vinson succeeded Treasury Secretary Henry Morgenthau Jr. in July 1945; after Truman appointed Vinson to the Supreme Court the following year, John Wesley Snyder served as the Treasury Secretary. Truman also quickly replaced Secretary of State Edward Stettinius Jr. with James F. Byrnes, who had been referred to as the "Assistant President" under Roosevelt. Byrnes lost Truman's trust with his conciliatory policy towards the Soviet Union in late 1945, and he was replaced by former General George Marshall in January 1947. Along with Undersecretary of State Dean Acheson and a group of advisers known as the "Wise Men," Marshall emerged as one of the major architects of postwar foreign policy.

In 1947, Forrestal became the first Secretary of Defense, overseeing all branches of the United States Armed Forces. Illness sent Forrestal into retirement in 1949, and he was replaced successively by Louis A. Johnson, Marshall, and finally Robert A. Lovett. Acheson succeeded Marshall as Secretary of State in 1949 and served in that position until the end of Truman's term. Several of Truman's appointees were longtime personal friends, some of whom were appointed to positions that seemed well beyond their competence. These appointments included Vinson, Snyder, and military aide Harry H. Vaughan, who seemed to others like a huge joke. Outside of the cabinet, Clark Clifford and John R. Steelman emerged as particularly important advisers.

The office of vice president remained vacant during Truman's first ( partial) term, as the Constitution then had no provision for filling it intra-term prior to the 1967 ratification of the Twenty-fifth Amendment. Until the passage of the Presidential Succession Act of 1947, the Secretary of State was next in the presidential line of succession. After the passage of the act in July 1947, the Speaker of the House became the next-in-line. During different points of Truman's first term, Secretary of State Stettinius, Secretary of State Byrnes, Secretary of State Marshall, Speaker Joseph Martin, and Speaker Sam Rayburn would have succeeded to the presidency if Truman left office. Alben Barkley served as Truman's running mate in the 1948 election, and became Vice President during Truman's second term. Truman included him in Cabinet deliberations.

Truman made four appointments to the United States Supreme Court. After the resignation of Owen Roberts in 1945, Truman appointed Republican Senator Harold Hitz Burton of Ohio to the Supreme Court. Roberts was the lone remaining justice on the Supreme Court who had not been appointed or elevated to the position of chief justice by Roosevelt, and Truman believed it was important to nominate a Republican to succeed Roberts. Chief Justice Harlan F. Stone died in 1946, and Truman appointed Secretary of the Treasury Fred M. Vinson as Stone's successor. Two vacancies arose in 1949 due to deaths of Frank Murphy and Wiley Blount Rutledge. Truman appointed Attorney General Tom C. Clark to succeed Murphy and federal appellate judge Sherman Minton to succeed Rutledge. Vinson served for just seven years before his death in 1953, while Minton resigned from the Supreme Court in 1956. Burton served until 1958, often joining the conservative bloc led by Felix Frankfurter. Clark served until 1967, emerging as an important swing vote on the Vinson Court and the Warren Court. In addition to his Supreme Court appointments, Truman also appointed 27 judges to the courts of appeals and 101 judges to federal district courts.

By April 1945, the Allied Powers, led by the United States, Great Britain, and the Soviet Union, were close to defeating Germany, but Japan remained a formidable adversary in the Pacific War. As vice president, Truman had been uninformed about major initiatives relating to the war, including the top-secret Manhattan Project, which was about to test the world's first atomic bomb. Although Truman was told briefly on the afternoon of April 12 that the Allies had a new, highly destructive weapon, it was not until April 25 that Secretary of War Henry Stimson told him the details of the atomic bomb, which was almost ready. Germany surrendered on May 8, 1945, ending the war in Europe. Truman's attention turned to Japan, where he hoped to end the war as quickly, and with as little expense in lives or government funds, as possible.

With the end of the war drawing near, Truman flew to Berlin for the Potsdam Conference, to meet with Soviet leader Joseph Stalin and British leader Winston Churchill regarding the post-war order. Several major decisions were made at the Potsdam Conference: Germany would be divided into four occupation zones (among the three powers and France), Germany's border was to be shifted West to the Oder–Neisse line, a Soviet-backed group was recognized as the legitimate government of Poland, and Vietnam was to be partitioned at the 16th parallel. The Soviet Union also agreed to launch an invasion of Japanese-held Manchuria. While at the Potsdam Conference, Truman was informed that the Trinity test of the first atomic bomb on July 16 had been successful. He hinted to Stalin that the U.S. was about to use a new kind of weapon against the Japanese. Though this was the first time the Soviets had been officially given information about the atomic bomb, Stalin was already aware of the bomb project, having learned about it (through espionage) long before Truman did.

In August 1945, the Japanese government ignored surrender demands as specified in the Potsdam Declaration. With the support of most of his aides, Truman approved the schedule of the military's plans to drop atomic bombs on the Japanese cities of Hiroshima and Nagasaki. Hiroshima was bombed on August 6, and Nagasaki three days later, leaving approximately 135,000 dead; another 130,000 would die from radiation sickness and other bomb-related illnesses in the following five years. Japan agreed to surrender on August 10, on the condition that Emperor Hirohito would not forced to abdicate; after some internal debate, Truman accepted these terms of surrender.

The decision to drop atomic bombs on Hiroshima and Nagasaki provoked long-running debates. Supporters of the bombings argue that, given the tenacious Japanese defense of the outlying islands, the bombings saved hundreds of thousands of lives that would have been lost invading mainland Japan. Truman also wanted to end the war before the Soviet Union could invade Japanese-held territories and set up Communist governments. Critics have argued that the use of nuclear weapons was unnecessary, given that conventional tactics such as firebombing and blockade might induce Japan's surrender without the need for such weapons. Truman strongly defended himself in his memoirs in 1955–1956, stating that many lives could have been lost had the U.S. invaded mainland Japan without the atomic bombs. In 1963, he reaffirmed this defense, telling a journalist that "it was done to save 125,000 youngsters on the American side and 125,000 on the Japanese side from getting killed and that is what it did. It probably also saved a half million youngsters on both sides from being maimed for life."

When Truman took office, several international organizations that were designed to help prevent future wars and international economic crises were in the process of being established. Chief among those organizations was the United Nations, an intergovernmental organization similar to the League of Nations that would help ensure international cooperation. When Truman took office, delegates were about to meet at the United Nations Conference on International Organization in San Francisco. As a Wilsonian internationalist, Truman strongly supported the creation of the United Nations, and he signed United Nations Charter at that conference. Truman did not repeat Woodrow Wilson's partisan attempt to ratify the Treaty of Versailles in 1919. Instead Truman won Senate ratification by cooperating closely with Republican leaders, especially Arthur H. Vandenberg. The Republicans controlled both houses of Congress after their landslide election in 1946, so Vandenberg became chairman of the purple SenateForeign Relations Committee, so Truman's continuing commitment to bipartisanship was essential, Construction of the United Nations headquarters in New York City was funded by the Rockefeller Foundation and completed in 1952.

In 1934, Congress had passed the Reciprocal Tariff Act, giving the president an unprecedented amount of authority in setting tariff rates. The act allowed for the creation of reciprocal agreements in which the U.S. and other countries mutually agreed to lower tariff rates. Despite significant opposition from those who favored higher tariffs, Truman was able to win legislative extension of the reciprocity program, and his administration reached numerous agreements that lowered trade barriers. The Truman administration also sought to further lower global tariff rates by engaging in multilateral, rather than bilateral, trade negotiations, and the State Department proposed the establishment of the International Trade Organization (ITO). The ITO would be granted broad powers to regulate trade among the countries, and its charter was approved by the United Nations in 1948. However, the ITO's broad powers engendered opposition in Congress, and Truman declined to send the charter to the Senate for ratification. In the course of creating the ITO, the U.S. and 22 other countries signed the General Agreement on Tariffs and Trade (GATT), a set of principles governing trade policy. Under the terms of the agreement, each country agreed to reduce overall tariff rates and to treat each co-signatory as a "most favoured nation," meaning that no non-signatory country could benefit from more advantageous tariff rates. Due to a combination of the Reciprocal Tariff Act, the GATT, and inflation, U.S. tariff rates fell dramatically between the passage of the Smoot–Hawley Tariff Act in 1930 and the end of the Truman administration in 1953.

In March 1946, at an optimistic moment for postwar cooperation, the administration released the Acheson-Lilienthal Report. It proposed that all nations voluntarily abstain from constructing nuclear weapons; as part of the proposal, the U.S. would dismantle its nuclear program once all other countries agreed not to pursue nuclear weapons. Fearing that Congress would reject the proposal, Truman turned to the well-connected Bernard Baruch to represent the U.S. position to the United Nations. The Baruch Plan, largely based on the Acheson-Lilienthal Report, was not adopted due to opposition from Congress and the Soviet Union. The Soviet Union would develop its own nuclear arsenal, testing a nuclear weapon for the first time in August 1949.

The United States Atomic Energy Commission, directed by David E. Lilienthal until 1950, was in charge of designing and building nuclear weapons. The policy was 100% civilian control. The U.S. had only 9 atomic bombs in 1946, but the stockpile grew to 650 by 1951. Lilienthal wanted to give high priority to peaceful uses, especially nuclear power plants, but coal was cheap and the power industry was not interested. Construction of the first nuclear plant would not begin until 1954. In early 1950, Truman authorized the development of thermonuclear weapons, a more powerful version of atomic bombs. Truman's decision to develop thermonuclear weapons faced opposition from many liberals and some government officials, but Truman believed that the Soviet Union would likely develop the weapons, and he was unwilling to allow the Soviets to have such an advantage. The first test of thermonuclear weaponry was conducted by the United States in 1952, and the Soviet Union performed its own thermonuclear test in August 1953.

A major project for the United Nations in the late 1940s was dealing with millions of refugees in Europe. Truman helped convince the United Nations to found the International Refugee Organization (IRO), a temporary organization that helped resettle. The United States helped fund temporary camps, and admitted large numbers as permanent residents. Truman obtained ample funding from Congress for the Displaced Persons Act of 1948, which allowed many of the displaced people of World War II to immigrate into the United States. Of the approximately one million people resettled by the IRO, more than 400,000 settled in the United States. The most contentious issue facing the IRO was the resettlement of European Jews, many of whom, with the support of Truman, were allowed to immigrate to British-controlled Mandatory Palestine.

The administration helped create a new category of refugee, the "escapee," at the 1951 Geneva Convention Relating to the Status of Refugees. The American Escapee Program began in 1952 to help the flight and relocation of political refugees from communism in Eastern Europe. The motivation for the refugee and escapee programs was twofold: humanitarianism, and use as a political weapon against inhumane communism.

Facing new, global challenges, the Truman administration reorganized the military and intelligence establishment to provide for more centralized control and reduce rivalries. The National Security Act of 1947 combined and reorganized all military forces by merging the Department of War and the Department of the Navy into the National Military Establishment (later the Department of Defense). The law also created the U.S. Air Force, the Central Intelligence Agency (CIA), and the National Security Council (NSC). The CIA and the NSC were designed to be non-military, advisory bodies that would increase U.S. preparation against foreign threats without assuming the domestic functions of the Federal Bureau of Investigation. The National Security Act institutionalized the Joint Chiefs of Staff, which had been established on a temporary basis during World War II. The Joint Chiefs of Staff took charge of all military action, and the Secretary of Defense became the chief advisor of military matters to the president. In 1952, Truman secretly consolidated and empowered the cryptologic elements of the United States by creating the National Security Agency (NSA). Truman also sought to require one year of military service for all young men physically capable of such service, but this proposal never won more than modest support among members of Congress.

Truman had hoped that the National Security Act would minimize interservice rivalries, but each branch retained considerable autonomy and battles over the military budgets and other issues continued. In 1949, Secretary of Defense Louis Johnson announced that he would cancel a so-called "supercarrier," which many in the navy saw as an important part of the service's future. The cancellation sparked a crisis known as the "Revolt of the Admirals", when a number of retired and active-duty admirals publicly disagreed with the Truman administration's emphasis on less expensive strategic atomic bombs delivered by the air force. Defense Secretary Forrestal had supported the navy's position and obtained funding for an aircraft carrier. During congressional hearings public opinion shifted strongly against the navy. In the end the navy got its carrier and also kept marine aviation, but the revolting admirals were punished and the navy lost control over strategic bombing. Military budgets following the hearings prioritized the development of air force heavy bomber designs, and the United States accumulated a combat ready force of over 1,000 long-range strategic bombers capable of supporting nuclear mission scenarios.

Truman gave a low priority to defense budgets—it got whatever money was left over after domestic spending. From the beginning, he assumed that the American monopoly on the atomic bomb was adequate protection against any and all external threats. Military spending plunged from 39% of GNP in 1945 to only 5% in 1948. The number of military personnel fell from just over 3 million in 1946 to approximately 1.6 million in 1947, although the number of military personnel was still nearly five times larger than that of U.S. military in 1939. In 1949, Truman ordered a review of U.S. military policies in light of the Soviet Union's acquisition of nuclear weapons. The National Security Council drafted NSC 68, which called for a major expansion of the U.S. defense budget, increased aid to U.S. allies, and a more aggressive posture in the Cold War. Despite increasing Cold War tensions, Truman dismissed the document, as he was unwilling to commit to higher defense spending. The Korean War convinced Truman of the necessity for higher defense spending, and such spending soared between 1949 and 1953.

The Second World War dramatically upended the international system, as formerly-powerful nations like Germany, France, Japan, and even Britain had been devastated. At the end of the war, only the United States and the Soviet Union had the ability to exercise influence, and a bipolar international power structure replaced the multipolar structure of the Interwar period. On taking office, Truman privately viewed the Soviet Union as a "police government pure and simple," but he was initially reluctant to take a hard-line towards the Soviet Union, as he hoped to work with the Soviets in the aftermath of Second World War. Truman's suspicions deepened as the Soviets consolidated their control in Eastern Europe throughout 1945, and the February 1946 announcement of the Soviet five-year plan further strained relations as it called for the continuing build-up of the Soviet military. At the December 1945 Moscow Conference, Secretary of State Byrnes agreed to recognize the pro-Soviet governments in the Balkans, while the Soviet leadership accepted U.S. leadership in the occupation of Japan. U.S. concessions at the conference angered other members of the Truman administration, including Truman himself. By the beginning of 1946, it had become clear to Truman that Britain and the United States would have little influence in Soviet-dominated Eastern Europe.

Former Vice President Henry Wallace, former First Lady Eleanor Roosevelt, and many other prominent Americans continued to hope for cooperative relations with the Soviet Union. Some liberals, like Reinhold Niebuhr, distrusted the Soviet Union but believed that the United States should not try to counter Soviet influence in Eastern Europe, which the Soviets saw as their "strategic security belt." Truman was reluctant to fully break with the Soviet Union in early 1946, but he took an increasingly hard line towards the Soviet Union throughout the year. He personally approved of Winston Churchill's March 1946 "Iron Curtain" speech, which urged the United States to take the lead of an anti-Soviet alliance, but he did not publicly endorse it.

Post-war tensions arose in Iran, which the Soviets had occupied during World War II; pressure from the U.S. and the United Nations finally forced the withdrawal of Soviet soldiers. The Soviet Union and the United States clashed in Germany, which had been divided into four occupation zones. In the September 1946 Stuttgart speech, Secretary of State Byrnes announced that the United States would no longer seek reparations from Germany and would support the establishment of a democratic state. The United States, France, and Britain announced that they would combine their occupation zones, eventually forming West Germany. Turkey also emerged as a point of contention, as the Soviet Union demanded joint control over the Dardanelles and the Bosphorus, key straits that controlled movement between the Black Sea and the Mediterranean Sea. The U.S. forcefully opposed this proposed alteration to the 1936 Montreux Convention, which had granted Turkey sole control over the straits, and Truman dispatched a fleet to the Eastern Mediterranean to show his administration's commitment to the region. In East Asia, Truman denied the Soviet request to reunify Korea, and refused to allow the Soviets a role in the post-war occupation of Japan.

By September 1946, Truman was convinced that the Soviet Union sought world domination and that cooperation was futile. He adopted a policy of containment, based on a 1946 cable by diplomat George F. Kennan. Containment, a policy of preventing the further expansion of Soviet influence, represented a middle-ground position between friendly detente (as represented by Wallace), and aggressive rollback to regain territory already lost to Communism, as would be adopted in 1981 by Ronald Reagan. Kennan's doctrine was based on the notion that the Soviet Union was led by an uncompromising totalitarian regime, and that the Soviets were primarily responsible for escalating tensions. Wallace, who had been appointed Secretary of Commerce after the 1944 election, resigned from the cabinet in September 1946 due to Truman's hardening stance towards the Soviet Union.

In the first major step in implementing containment, Truman extended loans to Greece and Turkey to prevent the spread of Soviet-aligned governments. Prior to 1947, the U.S. had largely ignored Greece, which had an anti-communist government, because it was in the British sphere. Since 1944, the British had assisted the Greek government against a left-wing insurgency, but in early 1947 the British informed the United States that they could no longer afford to intervene in Greece. At the urging of Acheson, who warned that the fall of Greece could lead to the expansion of Soviet influence throughout Europe, Truman requested that Congress grant an unprecedented $400 million aid package to Greece and Turkey. In a March 1947 speech before a joint session of Congress, Truman articulated the Truman Doctrine, which called for the United States to support "free people who are resisting attempted subjugation by armed minorities or by outside pressures." Overcoming those who opposed U.S. involvement in Greek affairs, as well those who feared that the aid would weaken post-war cooperation, Truman won bipartisan approval of the aid package. The congressional vote represented a permanent break with the non-interventionism that had characterized U.S. foreign policy prior to World War II.

The United States became closely involved in the Greek Civil War, which ended with the defeat of the insurgency in 1949. Stalin and Yugoslavian leader Josip Broz Tito both provided aid to the insurgents, but a dispute over the aid led to the start of a split in the Communist bloc. American military and economic aid to Turkey also proved effective. Turkey did not have a civil war and was heavily funded well into the 1950s. The Truman administration also provided aid to the Italian government in advance of the 1948 general election. The aid package, combined with a covert CIA operation, anti-Communist mobilization by the Catholic Church, and pressure from prominent Italian-Americans, helped to ensure a Communist defeat in the election. The initiatives of the Truman doctrine solidified the post-war division between the United States and the Soviet Union, and the Soviet Union responded by tightening its control over Eastern Europe. Countries aligned with the Soviet Union became known as the Eastern Bloc, while the U.S. and its allies became known as the Western Bloc.

The liberal wing of the Democratic Party had an element that strongly opposed Truman doctrine. Eleanor Roosevelt wrote Truman in April 1947 calling him to rely on the United Nations instead of his Truman Doctrine. She denounced two Greece and Turkey because they were undemocratic and complained he was adopting “Mr. Churchill’s policies in the Near East.” Truman needed support from the Roosevelt wing, wrote her that while he held onto his long-term hopes for the United Nations, he insisted that and an "economically, ideologically and politically sound" peace would more likely come from American action, than from the United States. He emphasized the strategic geographical importance of the Greek-Turkish land bridge as a critical point in which democratic forces could stop the advance of communism that had so ravaged Eastern Europe.

The United States had terminated the war-time Lend-Lease program in August 1945, dashing the hopes of those who hoped it would continue as a post-war aid program, but the U.S. sent massive shipments of food to Europe in the years immediately following the end of the war. Despite this American aid, much of Europe continued to suffer from food and fuel shortages by 1947; as Churchill put it, Europe was "a rubble heap, a charnel house, a breeding ground for pestilence and hate." U.S. leaders feared that poor economic conditions could lead to the rise of Communism in states such as France and Italy. With the goal of stemming the spread of Communism and increasing trade between the U.S. and Europe, the Truman administration devised the Marshall Plan, which sought to rejuvenate the devastated economies of Western Europe. To fund the Marshall Plan, Truman asked Congress to approve an unprecedented, multi-year, $25 billion appropriation.

Congress, under the control of conservative Republicans, agreed to fund the program for multiple reasons. The 20-member conservative isolationist wing of the Republican Party, based in the rural Midwest, was led by Senator Kenneth S. Wherry. Wherry argued that it would be "a wasteful 'operation rat-hole'"; that it made no sense to oppose communism by supporting the socialist governments in Western Europe; and that American goods would reach Russia and increase its war potential. The opposition was outmaneuvered by the emerging internationalist wing, led by Senator Arthur H. Vandenberg. With support from Republican Senator Henry Cabot Lodge, Jr., Vandenberg admitted there was no certainty that the plan would succeed, but said it would halt economic chaos, sustain Western civilization, and stop further Soviet expansion. Senator Robert A. Taft, a leading conservative Republican who was generally skeptical of American commitments in Europe, chose to focus on domestic issues and deferred to Vandenberg on foreign policy. Major newspapers were highly supportive, including conservative outlets like "Time Magazine". Both houses of Congress approved of the initial appropriation, known as the Foreign Assistance Act, by large majorities, and Truman signed the act into law in April 1948. Congress would eventually allocate $12.4 billion in aid over the four years of the plan.

In addition to aid, the Marshall Plan also focused on efficiency along the lines of American industry and removing tariffs and trade barriers. Though the United States allowed each recipient to develop its own plan for the aid, it set several rules and guidelines on the use of the funding. Governments were required to exclude Communists, socialist policies were discouraged, and balanced budgets were favored. Additionally, the United States conditioned aid to the French and British on their acceptance of the reindustrialization of Germany and support for European integration. To avoid exacerbating tensions, the U.S. invited the Soviet Union to become a recipient in the program, but set terms that Stalin was likely to reject. The Soviet Union refused to consider joining the program and vetoed participation by its own satellites. The Soviets set up their own program for aid, the Molotov Plan, and the competing plans resulted in reduced trade between the Eastern bloc and the Western bloc.

The Marshall Plan helped European economies recover in the late 1940s and early 1950s. By 1952, industrial productivity had increased by 35 percent compared to 1938 levels. The Marshall Plan also provided critical psychological reassurance to many Europeans, restoring optimism to a war-torn continent. Though European countries did not adopt American economic structures and ideas to the degree hoped for by some Americans, they remained firmly rooted in mixed economic systems. The European integration process led to the creation of the European Economic Community, which eventually formed the basis of the European Union.

In reaction to Western moves aimed at reindustrializing their German occupation zones, Stalin ordered a blockade of the Western-held sectors of Berlin, which was deep in the Soviet occupation zone. Stalin hoped to prevent the creation of a western German state aligned with the U.S., or, failing that, to consolidate control over eastern Germany. The blockade began on June 24, 1948. The Allies had never negotiated a deal to guarantee supply of the sectors deep within the Soviet-occupied zone. The commander of the American occupation zone in Germany, General Lucius D. Clay, proposed sending a large armored column across the Soviet zone to West Berlin with instructions to defend itself if it were stopped or attacked. Truman believed this would entail an unacceptable risk of war. He instead approved Ernest Bevin's plan to supply the blockaded city by air. On June 25, the Allies initiated the Berlin Airlift, a campaign that delivered food and other supplies, such as coal, using military aircraft on a massive scale. Nothing like it had ever been attempted before, and no single nation had the capability, either logistically or materially, to accomplish it. The airlift worked; ground access was again granted on May 11, 1949. Nevertheless, the airlift continued for several months after that. The Berlin Airlift was one of Truman's great foreign policy successes, and it significantly aided his election campaign in 1948.

Rising tensions with the Soviets, along with the Soviet veto of numerous United Nations Resolutions, convinced Truman, Senator Vandenberg, and other American leaders of the necessity of creating a defensive alliance devoted to collective security. In 1949, the United States, Canada, and several European countries signed the North Atlantic Treaty, creating a trans-Atlantic military alliance and committing the United States to its first permanent alliance since the 1778 Treaty of Alliance with France. The treaty establishing NATO was widely popular and easily passed the Senate in 1949. NATO's goals were to contain Soviet expansion in Europe and to send a clear message to communist leaders that the world's democracies were willing and able to build new security structures in support of democratic ideals. The treaty also re-assured France that the United States would come to its defense, paving the way for continuing French cooperation in the re-establishment of an independent German state. The U.S., Britain, France, Italy, the Netherlands, Belgium, Luxembourg, Norway, Denmark, Portugal, Iceland, and Canada were the original treaty signatories. Shortly after the creation of NATO, Truman convinced Congress to pass the Mutual Defense Assistance Act, which created a military aid program for European allies.

Cold War tensions heightened following Soviet acquisition of nuclear weapons and the beginning of the Korean War. The U.S. increased its commitment to NATO and invited Greece and Turkey to join the alliance. It launched a second major foreign aid program following the passage of the Mutual Security Act. Truman permanently stationed 180,000 in Europe, while European defense spending grew from 5 percent to 12 percent of gross national product. NATO established a unified command structure, and Truman appointed General Dwight D. Eisenhower as the first Supreme Commander of NATO. West Germany fell under the aegis of NATO, and it would eventually be incorporated into NATO in 1955.

Truman usually worked well with his top advisors--the exceptions were Israel in 1948 and Spain 1945-50. Truman was a very strong opponent of Francisco Franco, the right-wing dictator of Spain. He withdrew the American ambassador (but diplomatic relations were not formally broken), kept Spain out of the UN, and rejected any Marshall Plan financial aid to Spain. However, as the Cold War escalated, support for Spain was strong in Congress, the Pentagon, the business community and other influential elements especially Catholics and cotton growers. Liberal opposition to Spain had faded after the Wallace element broke with the Democratic Party in 1948; the CIO became passive on the issue. As Secretary of State Acheson increased his pressure on Truman, the president, stood alone in his administration as his own top appointees wanted to normalize relations. When China entered the Korean War and pushed American forces back, the argument for allies became irresistible. Admitting that he was "overruled and worn down," Truman relented and sent an ambassador and made loans available. Military talks began and President Eisenhower later brought Spain into NATO.

Cold War tensions and competition reached across the globe, affecting Europe, Asia, North America, Latin America, and Africa. The United States had historically focused its foreign policy on upholding the Monroe Doctrine in the Western Hemisphere, but new commitments in Europe and Asia diminished U.S. focus on Latin America. Partially in reaction to fears of expanding Soviet influence, the U.S. led efforts to create collective security pact in the Western Hemisphere. In 1947, the United States and most Latin American nations joined the Rio Pact, a defensive military alliance. The following year, the independent states of the Americas formed the Organization of American States (OAS), an intergovernmental organization designed to foster regional unity. Many Latin American nations, seeking favor with the United States, cut off relations with the Soviet Union. Latin American countries also requested aid and investment similar to the Marshall Plan, but Truman believed that most U.S. foreign aid was best directed to Europe and other areas that could potentially fall under the influence of Communism.

Truman had long taken an interest in the history of the Middle East, and was sympathetic to Jews who sought a homeland in British-controlled Mandatory Palestine. In 1943, he had called for a homeland for those Jews who survived the Nazi regime. However, State Department officials were reluctant to offend the Arabs, who were opposed to the establishment of a Jewish state in the region. Secretary of Defense Forrestal warned Truman of the importance of Saudi Arabian oil in another war; Truman replied that he would decide his policy on the basis of justice, not oil. American diplomats with experience in the region were opposed, but Truman told them he had few Arabs among his constituents. Regarding policy in the Eastern Mediterranean and the Middle East, Palestine was secondary to the goal of protecting the "Northern Tier" of Greece, Turkey, and Iran from communism.

In 1947, the United Nations approved the partition of Mandatory Palestine into a Jewish state and an Arab state. The British announced that they would withdraw from Palestine in May 1948, and Jewish leaders began to organize a provisional government. In the months leading up to the British withdrawal, the Truman administration debated whether or not to recognize the fledgling state of Israel. Overcoming initial objections from Marshall, Clark Clifford convinced Truman that non-recognition would lead Israel to tilt towards the Soviet Union in the Cold War. Truman recognized the State of Israel on May 14, 1948, eleven minutes after it declared itself a nation. Israel would secure its independence with a victory in the 1948 Arab–Israeli War, but the Arab–Israeli conflict remains unresolved.

Following the defeat of the Japanese Empire, China descended into a civil war. The civil war baffled Washington, as both the Nationalists under Chiang Kai-shek and the Communists under Mao Zedong had American advocates who split along partisan and left-right lines. Truman sent George Marshall to China in early 1946 to broker a compromise featuring a coalition government. The mission failed, as both sides felt the issue would be decided on the battlefield, not at a conference table. Marshall returned to Washington in December 1946, blaming extremist elements on both sides. He said the Communists were "irreconcilable” and the Nationalists were "reactionary." In mid-1947, Truman sent General Albert Coady Wedemeyer to China to try again, but no progress was made.

Though the Nationalists held a numerical advantage in the aftermath of the war, the Communists gained the upper hand in the civil war after 1947. Corruption, poor economic conditions, and poor military leadership eroded popular support for the Nationalist government, and the Communists won many peasants to their side. As the Nationalists collapsed in 1948, the Truman administration faced the question of whether to intervene on the side of the Nationalists or seek good relations with Mao. Chiang's strong support among sections of the American public, along with desire to assure other allies that the U.S. was committed to containment, convinced Truman to increase economic and military aid to the Nationalists. However, Truman held out little hope for a Nationalist victory, and he refused to send U.S. soldiers.

In 1949 Mao Zedong and his Communists took control of the mainland of China, driving the Nationalists to Taiwan. The United States had a new enemy in Asia, and Truman came under fire from conservatives for "losing" China. Along with the Soviet detonation of a nuclear weapon, the Communist victory in the Chinese Civil War played a major role in escalating Cold War tensions and U.S. militarization during 1949. Truman would have been willing to maintain some relationship between the U.S. and the Communist government, but Mao was unwilling. Chiang established the Republic of China on Taiwan, which retained China's seat on the UN Security Council until 1971. In June 1950, after the outbreak of fighting in Korea, Truman ordered the Navy's Seventh Fleet into the Taiwan Strait to prevent further conflict between the communist government and the Republic of China.

Under the leadership of General Douglas MacArthur, the U.S. occupied Japan after the latter's surrender in August 1945. MacArthur presided over extensive reforms of the Japanese government and society, implementing a new constitution that established a parliamentary democracy and granted women the right to vote. He also reformed the Japanese educational system and oversaw major economic changes, although Japanese business leaders were able to resist the reforms to some degree. As the Cold War intensified in 1947, the Truman administration took greater control over the occupation, ending Japanese reparations to the Allied Powers and prioritizing economic growth over long-term reform. The Japanese suffered from poor economic conditions until the beginning of the Korean War, when U.S. purchases stimulated growth. In 1951, the United States and Japan signed the Treaty of San Francisco, which restored Japanese sovereignty but allowed the United States to maintain bases in Japan. Over the opposition of the Soviet Union and some other adversaries of Japan in World War II, the peace treaty did not contain punitive measures such as reparations, though Japan did lose control of the Kuril Islands and other pre-war possessions.

With the end of World War II, the United States fulfilled the commitment made by the 1934 Tydings–McDuffie Act and granted independence to the Philippines. The U.S. had encouraged decolonization throughout World War II, but the start of the Cold War changed priorities. The U.S. used the Marshall Plan to pressure the Dutch to grant independence to Indonesia under the leadership of the anti-Communist Sukarno, and the Dutch recognized Indonesia's independence in 1949. However, in French Indochina, the Truman administration recognized the French client state led by Emperor Bảo Đại. The U.S. feared alienating the French, who occupied a crucial position on the continent, and feared that the withdrawal of the French would allow the Communist faction of Ho Chi Minh to assume power. Despite initial reluctance to become involved in Indochina, by 1952, the United States was heavily subsidizing the French suppression of Ho's Việt Minh in the First Indochina War. The U.S. also established alliances in the region through the creation of the Mutual Defense Treaty with the Philippines and the ANZUS pact with Australia and New Zealand.

Following World War II, the United States and the Soviet Union occupied Korea, which had been a colony of the Japanese Empire. The Soviet Union, which occupied Korea north of the 38th parallel, established the Democratic People's Republic of Korea (North Korea) in 1948, while the United States established the Republic of Korea (South Korea) that same year. The 38th parallel was chosen since it was approximately halfway between Korea's northernmost and southernmost regions, and was always intended to mark a temporary separation before the eventual reunification of Korea. Hoping to avoid a long-term military commitment in the region, Truman withdrew U.S. soldiers from the Korean Peninsula in 1949. The Soviet Union also withdrew their soldiers from Korea in 1949, but continued to supply North Korea with military aid.

On June 25, 1950, Kim Il-sung's Korean People's Army invaded South Korea, starting the Korean War. In the early weeks of the war, the North Koreans easily pushed back their southern counterparts. The Soviet Union was not directly involved, though Kim did win Stalin's approval before launching the invasion. Truman, meanwhile, did not view Korea itself as a vital region in the Cold War, but he believed that allowing a Western-aligned country to fall would embolden Communists around the world and damage his own standing at home. The top officials of the Truman administration were heavily influenced by a desire to not repeat the "appeasement" of the 1930s; Truman stated to an aide, "there's no telling what they'll do, if we don't put up a fight right now." Truman turned to the United Nations to condemn the invasion. With the Soviet Union boycotting the United Nations Security Council due to the UN's refusal to recognize the People's Republic of China, Truman won approval of Resolution 84. The resolution denounced North Korea's actions and empowered other nations to defend South Korea.

North Korean forces experienced early successes, capturing the city of Seoul on June 28. Fearing the fall of the entire peninsula, General Douglas MacArthur, commander of U.S. forces in Asia, won Truman's approval to land U.S. troops on the peninsula. Rather than asking Congress for a declaration of war, Truman argued that the UN Resolution provided the presidency the constitutional power to deploy soldiers as a "police action" under the aegis of the UN. The intervention in Korea was widely popular in the United States at the time, and Truman's July 1950 request for $10 billion was approved almost unanimously. By August 1950, U.S. troops pouring into South Korea, along with American air strikes, stabilized the front around the Pusan Perimeter. Responding to criticism over unreadiness, Truman fired Secretary of Defense Louis Johnson and replaced him with the George Marshall. With UN approval, Truman decided on a "rollback" policy—conquest of North Korea. UN forces launched a counterattack, scoring a stunning surprise victory with an amphibious landing at the Battle of Inchon that trapped most of the invaders. UN forces marched north, toward the Yalu River boundary with China, with the goal of reuniting Korea under UN auspices.

As the UN forces approached the Yalu River, the CIA and General MacArthur both expected that the Chinese would remain out of the war. Defying those prediction, Chinese forces crossed the Yalu River in November 1950 and forced the overstretched UN soldiers to retreat. Fearing that the escalation of the war could spark a global conflict with the Soviet Union, Truman refused MacArthur's request to bomb Chinese supply bases north of the Yalu River. UN forces were pushed below the 38th parallel before the end of 1950. Under the leadership of General Matthew Ridgway, the UN launched another counterattack, pushing Chinese forces back up to the 38th parallel.

MacArthur made several public demands for an escalation of the war, leading to a break with Truman in late 1950 and early 1951. On April 5, House Minority Leader Joseph Martin made public a letter from MacArthur that strongly criticized Truman's handling of the Korean War and called for an expansion of the conflict against China. Truman believed that MacArthur's recommendations were wrong, but more importantly, he believed that MacArthur had overstepped his bounds in trying to make foreign and military policy, potentially endangering the civilian control of the military. After consulting with the Joint Chiefs of Staff and members of Congress, Truman decided to relieve MacArthur of his command. The dismissal of General Douglas MacArthur ignited a firestorm of outrage against Truman and support for MacArthur. Fierce criticism from virtually all quarters accused Truman of refusing to shoulder the blame for a war gone sour and blaming his generals instead. Others, including Eleanor Roosevelt, supported and applauded Truman's decision. MacArthur meanwhile returned to the U.S. to a hero's welcome, and addressed a joint session of Congress. In part due to the dismissal of MacArthur, Truman's approval mark in February 1952 stood at 22% according to Gallup polls, which was, until George W. Bush in 2008, the all-time lowest approval mark for an active American president. Though the public generally favored MacArthur over Truman immediately after MacArthur's dismissal, congressional hearings and newspaper editorials helped turn public opinion against MacArthur's advocacy for escalation.

The war remained a frustrating stalemate for two years. UN and Chinese forces fought inconclusive conflicts like the Battle of Heartbreak Ridge and the Battle of Pork Chop Hill, but neither side was able to advance far past the 38th parallel. Throughout late 1951, Truman sought a cease fire, but disputes over prisoner exchanges led to the collapse of negotiations. Of the 116,000 Chinese and Korean prisoners-of-war held by the United States, only 83,000 were willing to return to their home countries, and Truman was unwilling to forcibly return the prisoners. The Korean War ended with an armistice in 1953 after Truman left office; the armistice divided North and South Korea along a border close to the 38th parallel. Over 30,000 Americans and approximately 3 million Koreans died in the conflict. The United States maintained a permanent military presence in South Korea after the war.

Truman made five international trips during his presidency: His only trans-Atlantic trip was to participate in the 1945 Potsdam Conference with British Prime Ministers Churchill and Attlee and Soviet Premier Stalin. He also visited neighboring Bermuda, Canada and Mexico, plus Brazil in South America. Truman only left the continental United States on two other occasions (to Puerto Rico, the Virgin Islands, Guantanamo Bay Naval Base, Cuba, February 20-March 5, 1948; and to Wake Island, October 11–18, 1950) during his nearly eight years in office.

Although foreign affairs dominated much of Truman's time in office, economic reconversion became his administration's central focus in late 1945. Truman faced several major challenges in presiding over the transition to a post-war economy, including a large national debt and persistent inflation. The United States had emerged from the Great Depression in part due to the war production that accompanied World War II, and many Americans feared that the nation would sink into another depression with the end of the war. While the country had been unified in winning the war, there was no consensus on the best methods of economic reconversion after the war, or the level of involvement that the federal government should have in economic affairs. Truman faced a Congress that was dominated by the conservative coalition, an informal alliance of Republicans and conservative Southern Democrats. This group, which had generally dominated Congress since Roosevelt's second term, opposed many of Truman's policies and did not welcome strong presidential leadership. Truman asked Congress for a host of measures, including a bill that would make the Fair Employment Practice Committee a permanent institution, but his focus on foreign affairs during this period prevented him from effectively advocating for his programs with members of Congress.

Truman was particularly concerned about keeping unemployment levels low; nearly 2 million people lost jobs within days of the Japanese surrender, and he feared that even more would lose their jobs in the following months. Liberal New Dealers pushed for an explicit federal commitment to ensuring "full employment," but Congress instead passed the Employment Act of 1946. The act created the Council of Economic Advisers and mandated the federal government to:

The United States had instituted price controls and wage controls during the war in order to avoid large-scale inflation or deflation. Within the Truman administration, some advocated lifting these controls immediately in order to allow private industries to hire new workers, while others feared that immediately lifting the controls would lead to runaway inflation. Truman sought to find a middle course between the two camps; price controls on many nonessential items were lifted by the end of September 1945, but others remained in place by the end of 1945. Increasingly concerned about inflation, Truman reimposed some price controls in December 1945, but the unpopularity of those controls led the administration to seek other ways to curb inflation, including cuts to federal spending. In July 1946, after average prices rose at the unprecedented rate of 5.5 percent, Truman won passage of a bill that extended his authority to institute price controls on some items. Though unemployment remained low, labor unrest, inflation, and other issues badly damaged Truman's popularity, which in turn contributed to a poor Democratic showing in the November 1946 mid-term elections. After the Republican victory in those elections, Truman announced the end of all federal wage and price controls, with the exception of rent controls.

Conflict between management and labor presented one of the biggest challenges to the conversion of the economy to peacetime production. Organized labor had adhered to its pledge to refrain from striking during the war, but labor leaders were eager to share in the gains from a postwar economic resurgence. After several labor disputes broke out in September and October 1945, Truman convened a national conference between leaders of business and organized labor in November, at which he advocated collective bargaining in order to avoid labor-related economic disruptions. The conference failed to have a major impact; an unprecedented wave of major strikes affected the United States, and by February 1946 nearly 2 million workers were engaged in strikes or other labor disputes. Many of the strikes were led by John L. Lewis of the Congress of Industrial Organizations (CIO), who Truman despised.

When a national rail strike threatened in May 1946, Truman seized the railroads. Two key railway unions struck anyway and the entire national railroad system was shut down—24,000 freight trains and 175,000 passenger trains a day stopped moving. For two days public anger mounted; no one was angrier than Truman himself. He drafted a message to Congress that called on veterans to form a lynch mob and destroy the union leaders. His staff was stunned, and top aide Clark Clifford rewrote and toned down the speech. Truman did go to Congress and he called for a new law to draft all the railroad strikers into the Army. As he was concluding his speech he read a message just handed to him that the strike was settled on presidential terms. Truman nevertheless finished the speech, and a few hours later the House voted to draft the strikers. Taft killed the bill in the Senate. Truman's speech marked the end of the strike wave, as business and labor leaders both generally avoided actions that would provoke a strong response from the administration. The strikes damaged the political standing of unions, and the real wages of blue collar workers fell by over 12 percent in the year after the surrender of Japan. Additionally, "Operation Dixie", the CIO's efforts to expand massively into the South, failed.

Historians generally agree that the G.I. Bill was one of the most important and successful government programs of the late 1940s. The G.I. Bill had been passed in 1944 by a conservative coalition that wanted to restrict benefits to "deserving" wartime veterans, as opposed to the Roosevelt administration, which favored a new welfare program that would reach both veterans and non-veterans. In addition to education and housing benefits, the bill included aid to veterans who wanted to start a small business or farm, as well one year of unemployment compensation. The GI benefits were not legally restricted by gender or race, but in practice white men were by far the most active users of the program.

The most famous component of the G.I. Bill provided free collegiate, vocational, and high school education for veterans – not only free tuition, but also full housing and subsistence allowances for the veterans and their families. There was a remarkable transformation of higher education, as 2.2 million veterans crowded into hastily built classrooms, showing a fierce determination to make up for lost time by maximizing their career opportunities. Due in large part to the G.I. Bill, the number of college degrees awarded rose from just over 200,000 in 1940 to nearly 500,000 in 1950.

The G.I. Bill also guaranteed low cost loans for veterans, with very low down payments and low interest rates. In 1947 alone, 540,000 veterans bought a house at the average price of $7,300. Developers purchased empty land just outside the city, installed tract houses based on a handful of designs, and provided streets and utilities. Local public officials raced to build schools to meet the public's need. The most famous development was Levittown in Long Island. It offered a new house for $1000 down, and $70 a month; houses featured three bedrooms, a fireplace, a gas range and gas furnace, and a landscaped lot of 75 by 100 feet, all for a total price of $10,000. Veterans could get one with a much lower down payment. 15 million housing units were built between 1945 and 1955, and the home-ownership rate grew from 50 percent in 1945 to 60 percent in 1960. Together with the growth of the automobile industry, the G.I. Bill's housing benefits helped provide for a major expansion of suburbs in the United States.

The 1946 mid-term election left Republicans in control of Congress for the first time since the early 1930s. Truman initially hoped to work Republican leaders in Congress, focusing on the passage of housing programs and other potential areas of common ground. Truman and the 80th Congress were able to agree on a balanced budget, albeit one that spent less on defense and some other programs that Truman favored. Congress also assented to the creation of the Hoover Commission, which proposed a series of reorganizations to the executive branch. However, the 80th Congress proved strongly resistant to Truman's policies. One of its first major acts was to approve what would become the Twenty-second Amendment, which created presidential term limits. The amendment was designed by Republican leaders as an implicit rebuke to Franklin Roosevelt, the only president who had ever served more than two terms. The Twenty-second Amendment was ratified by the requisite 36 states on February 27, 1951. A major area of contention was Truman's attempts to use price and wage controls to restrain inflation. Congress also passed bills designed to cut taxes, weaken the Interstate Commerce Commission, reduce the number of employees covered by Social Security, and amend the National Labor Relations Act of 1935 (also known as the Wagner Act); all of these bills, except for the revisions to the Wagner Act, were vetoed by Truman in 1947. Upon returning to session in 1948, Congress passed the Revenue Act of 1948, another major tax cut; Truman again vetoed the bill, but this time his veto was overridden by Congress.

The Labor Management Relations Act of 1947, also known as the Taft–Hartley Act, was passed in response to the labor unrest seen in 1945 and 1946. Truman denounced the act as a "slave-labor bill" in his veto, but he used its emergency provisions a number of times to halt strikes and lockouts. Repeated union efforts to repeal or modify it always failed, and it remains in effect today. The Taft-Hartley Act added a list of prohibited union actions to the National Labor Relations Act of 1935 (also known as the Wagner Act), which had defined several types of employer actions as unfair labor practices. Taft-Hartley prohibited jurisdictional strikes, in which a union strikes in order to pressure an employer to assign particular work to the employees that union represents, and secondary boycotts and "common situs" picketing, in which unions picket, strike, or refuse to handle the goods of a business with which they have no primary dispute but which is associated with a targeted business. The act outlawed closed shops, which were contractual agreements that required an employer to hire only union members. Union shops, in which new recruits must join the union within a certain amount of time, were permitted under Taft-Hartley, but only as part of a collective bargaining agreement and only if the contract allows the worker at least thirty days after the date of hire or the effective date of the contract to join the union. The Taft–Hartley Act also granted states power to pass "right-to-work laws," which ban union shops. All union officials were required to sign an affidavit that they were not Communists or else the union would lose its federal bargaining powers guaranteed by the National Labor Relations Board.

Historian James T. Patterson concludes that:

In his first major address to Congress, Truman articulated a liberal domestic program, but his first-term domestic policy was dominated by post-war reconversion. As he readied for the 1948 election, Truman made clear his identity as a Democrat in the New Deal tradition, advocating national health care, repeal of the Taft–Hartley Act, federal aid to education, expanded public housing programs, a higher minimum wage, more public power projects like the Tennessee Valley Authority, and a more progressive tax structure. The administration also put forth the Brannan Plan, which would have removed the government's production controls and price supports in agriculture in favor of direct payments to farmers. Taken together, Truman's proposals constituted a broad legislative agenda that came to be called the "Fair Deal." A major difference between the New Deal and the Fair Deal was that the latter included an aggressive civil rights program, which Truman termed a moral priority. Truman's proposals were not well received by Congress, even with renewed Democratic majorities in Congress after 1948. The conservative coalition of Republicans and conservative Southern Democrats played a key role in blocking passage of the Fair Deal, but the inability of liberal to agree on the details of many programs also contributed to legislative gridlock.

Only one of the major Fair Deal bills, the Housing Act of 1949, was ever enacted. Truman did win other victories in the 81st Congress, as the minimum wage was raised from forty cents an hour to seventy-five cents an hour, and Social Security benefits for the retired were doubled. Congress also passed the Celler–Kefauver Act, which closed loopholes in the Sherman Antitrust Act. The 1950 mid-term elections bolstered Republicans and conservative Democrats, ending any chance of passing further Fair Deal programs. Despite Truman's inability to pass the Fair Deal, the major New Deal programs still in operation were not repealed, and there were minor improvements and extensions in many of them. The Fair Deal would also serve as an inspiration for many of the Great Society programs passed during the presidency of Lyndon B. Johnson.

Truman sought to reduce discrimination against African-Americans, but due to the power of Southern congressmen, he was unable to advance any significant civil rights legislation through Congress and eventually turned to executive actions. A 1947 report by the President's Committee on Civil Rights titled "To Secure These Rights" presented a detailed ten-point agenda of civil rights reforms. In February 1948, the president submitted a civil rights agenda to Congress that proposed creating several federal offices devoted to issues such as voting rights and fair employment practices. This provoked a storm of criticism from Southern Democrats in the runup to the national nominating convention, but Truman refused to compromise, saying: "My forebears were Confederates ... but my very stomach turned over when I had learned that Negro soldiers, just back from overseas, were being dumped out of Army trucks in Mississippi and beaten." At the start of the 81st Congress, an effort to reform the Senate's filibuster rules so that a filibuster could be defeated by a simple majority vote was defeated. The failure to reform the filibuster rules ensured that civil rights would not emerge as an important legislative issue until the late 1950s.

Tales of the abuse, violence, and persecution suffered by many African American veterans upon their return from World War II infuriated Truman, and were a major factor in his decision to issue Executive Order 9981, in July 1948, requiring equal opportunity in the Armed Forces. After several years of planning, recommendations and revisions between Truman, the Committee on Equality of Treatment and Opportunity, and the various branches of the military, military units started to be racially integrated in the early 1950s. The 1948 Women's Armed Services Integration Act allowed women to serve in the peacetime military. Another executive order, also in 1948, made it illegal to discriminate against persons applying for civil service positions based on race. A third, in 1951, established the Committee on Government Contract Compliance (CGCC), which sought to prevent defense contractors from discriminating because of race.

Truman also appointed non-whites to unprecedented positions of power in the executive and judicial branches. Among his appointments was William Henry Hastie, the first African American to serve as a federal appellate judge. In civil rights cases like "Sweatt v. Painter", the Justice Department issued amicus curiae briefs that supported ending segregation. In December 1952, the Truman administration filed an amicus curiae brief for the case of "Brown v. Board of Education"; two years later, the Supreme Court's holding in that case would effectively overturn the "separate but equal" doctrine that allowed for racial segregation in public education.

Historians Donald R. McCoy and Richard T. Ruetten argue that even if Truman:

National health insurance had been on the table for decades but never gained much traction. Starting in the late 1930s hospitals promoted private insurance plans such as Blue Cross. Between 1940 and 1950, the percentage of Americans with health insurance rose from 9 percent to above 50 percent, as hospitals and doctors increasingly offered more healthcare services. Truman proposed a national health insurance plan in November 1945 that had the support of the American Federation of Labor (AFL) support. Truman's plan was defeated by conservatives led by Senator Taft, as well as the American Medical Association, which rallied the medical community against the bill. The business community also overwhelmingly opposed national plans. Even labor unions discovered they could negotiate with business to obtain even better health benefits for their own members, so they focused increasingly on that goal. The failure of Truman's healthcare plan solidified the status of private employers as the primary sponsors of health insurance in the United States.

Organized crime in major cities, most of which were Democratic Party strongholds, was a favorite attack theme of Republican politicians and the media. Petty crime rates went up after 1945, now that far more young men were back on the streets, and much more money was in circulation. Far more serious was organized crime run by professional criminal gangs. The Justice Department in 1947 organized a 'racket squad' to build evidence for grand jury investigations in several major cities. The income tax returns of gambling entrepreneurs and racketeers were audited. However, federal officials were reluctant to share their new information with local law enforcement. Truman and his Attorney General J. Howard McGrath in February 1950, told local officials that they had to bear the chief burden in defeating organized crime. Senator Estes Kefauver, a liberal Democrat from Tennessee, launched a major Senate investigation in 1950 as chairman of the Special Committee to Investigate Organized Crime in Interstate Commerce. Kefauver, although only a freshman in the Senate, received large-scale national coverage and became a presidential contender.

Truman's long record of association with the Pendergast machine in Kansas City proved increasingly embarrassing, even though Pendergast himself had died a few months before Truman became president. The various scandals of organized crime did not directly touch Truman, but they highlighted and exacerbated his problems with scandals inside his administration, such as influence peddling. The Kefauver committee exposed numerous charges of corruption among senior administration officials, some of whom received expensive fur coats and deep freezers in exchange for favors. Senator J. William Fulbright, a Democrat from Arkansas, reported that officials of the Reconstruction Finance Corporation were illegally coordinating their work with the Democratic National Committee. Kefauver found that over 160 Internal Revenue Service officials took bribes, use their offices to run private businesses, tolerated this behavior by their subordinates, or embezzled federal funds. When Attorney McGrath fired the special prosecutor in early 1952 for being too zealous, Truman fired McGrath. In the 1952 campaign, Eisenhower made "corruption" one of his three main attack points in the 1952 election, along with Korea and communism.

The left wing of the Democratic Party was in turmoil over foreign policy issues, especially the role of the Soviet Union and domestic communism. The powerful labor unions played a central role, including the strongly anti-communist American Federation of Labor. The more liberal Congress of Industrial Organizations (CIO), under the leadership of Walter Reuther, purged the communist element that held powerful positions in numerous unions. If they could not be purged, unions were expelled from the CIO. The conservative big-city machines, likewise, expelled or minimized left-wing forces. Expelled leftists coalesced around Henry Wallace, who ran an independent campaign for president in 1948. To Wallace's dismay, however, his own campaign was increasingly controlled by communist elements and he was left bitterly frustrated. An anti-Communist liberal group, Americans for Democratic Action (ADA), was established to provide a counterweight to Wallace's supporters. Though often critical of the far-right's unrestrained attacks on alleged Communists, members of the ADA attacked left-wing activists who, they feared, took orders from Communist leaders in the Soviet Union.

Truman established the Temporary Commission on Employee Loyalty in November 1946 to create employee loyalty standards designed to weed out communist sympathizers from the federal workforce. In March 1947, Truman issued Executive Order 9835, which ordered purges of left-wingers who refused to disavow communism. It removed about 300 federal employees who currently were members of or associated with any organization identified by the Attorney General as communist, fascist, or totalitarian. Anti-communist liberals by 1947–48 thus played a central role in the Democratic Party, and enthusiastically supported Truman's anti-communist foreign policy.

In August 1948, Whittaker Chambers, a former spy for the Soviets and a senior editor at "Time" magazine, testified before the House Un-American Activities Committee (HUAC). He said that an underground communist network had been working within the U.S. government since the 1930s, of which Chambers had been a member, along with Alger Hiss, until recently a senior State Department official. Although Hiss denied the allegations, he was convicted in January 1950 for perjury for his denials under oath. The Soviet Union's success in exploding an atomic weapon in 1949 and the fall of the nationalist Chinese the same year led many Americans to conclude that subversion by Soviet spies was responsible, and to demand that communists be rooted out from the government and other places of influence. However, Truman did not fully share such opinions, and throughout his tenure he would balance a desire to maintain internal security against the fear that a red scare could hurt innocents and impede government operations. He famously called the Hiss trial a "red herring," and the Justice Department was moving to indict Chambers instead of Hiss for perjury. Yet Truman also presided over the prosecution of numerous Communist leaders under the terms of the Smith Act.

Following Hiss' conviction, Secretary of State Acheson announced that he stood by him. This and other events, such as the revelation that British atomic bomb scientist Klaus Fuchs was a spy, led current and former members of HUAC to decry the Truman administration, especially the State Department, as soft on communism. Republican Congressmen Karl E. Mundt of South Dakota and Richard Nixon of California emerged as particularly vocal and prominent critics on HUAC. Wisconsin Senator Joseph McCarthy used a speech in West Virginia to accuse the State Department of harboring communists, and rode the controversy to political fame. In the following years, Republicans used Hiss' conviction to castigate the Democrats for harboring communists in government. Truman responded by arguing that McCarthy's efforts would undermine the bipartisan foreign policy that had prevailed since the end of World War II and thereby give a political gift to the Soviet Union, but few Republicans spoke out against McCarthy during Truman's tenure in office. Democratic Senator William Benton sponsored a motion to expel McCarthy from Congress, but the motion was defeated and Benton lost his 1952 re-election campaign; McCarthy, meanwhile, was re-elected. McCarthy's anti-Communist campaigns, part of a larger Red Scare, played a major role in shaping a more confrontational Cold War foreign policy. It also affected members of Congress and other political leaders, who now worried that the embrace of left-wing policies would leave themselves vulnerable to accusations of being "soft" on Communism.

The outbreak of the Korean War led to renewed interest in such an internal security bill, which had previously been debated during the 80th Congress. Senator Pat McCarran of Nevada put forward a bill that would require Communist organizations to register with the government, and allowed the president to indefinitely detain those who were suspected of having engaged in espionage. The bill received little opposition from members of Congress, who feared being labeled as pro-Communist, and it passed both the House and the Senate as the McCarran Internal Security Act. Truman vetoed the bill in September 1950, arguing that it infringed on personal liberties and would be ineffective at protecting against subversion. Congress overrode Truman's veto, and the McCarran Internal Security Act passed into law.

Immigration had been at a low level in the Great Depression and war years. It surged as the war ended, with the arrival of refugees and family members of citizens. The issue was not a high priority for the Truman administration, but there was great interest in Congress and among ethnic groups. In 1945, the War Brides Act allowed foreign-born wives of U.S. citizens who had served in the U.S. Armed Forces to immigrate to the United States. In 1946, the law was extended to include the fiancés of American soldiers. In 1946, the Luce-Celler Act extended the right to become naturalized citizens to form the newly independent nation of the Philippines and to Asian Indians. The immigration quota was set at 100 people per year.

In 1952, the McCarran Walter Immigration Act passed over Truman's veto. It kept the old 1924 quota system but added many new opportunities for immigration from Europe and elsewhere. In practice two-thirds of the new arrivals entered outside the old quota system. Immigration law was effectively controlled by Congressman Francis E. Walter of Pennsylvania, a Democrat who wanted to minimize immigration.

Labor disruptions continued to affect the country after 1946, though they did not reach the severity of strike wave of 1945–1946. When a steel strike loomed in April 1952, Truman instructed Secretary of Commerce Charles W. Sawyer to seize and continue operations of the nation's steel mills. Truman cited his authority as Commander in Chief and the need to maintain an uninterrupted supply of steel for munitions to be used in the war in Korea. The Supreme Court found the seizure unconstitutional, and reversed the order in a major separation-of-powers decision, "Youngstown Sheet & Tube Co. v. Sawyer" (1952). The 6–3 decision, which held that Truman's assertion of authority was too vague and was not rooted in any legislative action by Congress, was delivered by a Court composed entirely of Justices appointed by either Truman or Roosevelt. The high court's reversal of Truman's order was his most notable legal defeat. The Supreme Court decision left the country with the possibility of a critical steel shortage, but Truman was able to convince the steel managers and organized labor to reach a settlement in July 1952.

Truman sought to grant greater rights to the territories and dependencies of the United States. He unsuccessfully pushed for the admission of Hawaii and Alaska as states in order to solidify their status as integral parts of the United States, but Congress did not act on this proposal. Truman was more successful in pushing Organic legislation for Guam, Samoa, and the Trust Territory of the Pacific Islands, the latter of which had been acquired from Japan after World War II. This legislation, passed in 1950 and 1951, transferred the territories from military to civilian administration, though the Navy continued to exercise considerable influence. In 1952, Congress passed a bill to recognize Puerto Rico's newly-written constitution.

In the 1946 mid-term elections, Truman's Democrats suffered losses in both houses of Congress. Republicans, who had not controlled a chamber of Congress since the 1932 elections, took control of both the House and the Senate. Truman's party was hurt by a disappointing postwar economy. The election was a major blow to Truman's hopes of passing his domestic policies. However, Dallek points to the 1946 elections as the moment when Truman became more sure of himself as president, and stopped trying to appease all factions of the public. Truman would strongly criticize the subsequent 80th United States Congress as a "Do Nothing Congress."

Labor unions suffered heavy losses in the election, and responded afterwards by taking strong actions. The Congress of Industrial Organizations (CIO) systematically purged communists and far-left sympathizers from leadership roles in its unions. The CIO expelled some unions that resisted the purge, notably its third-largest affiliate the United Electrical, Radio and Machine Workers of America (UE), and set up a new rival union to take away the UE membership.

Meanwhile, the AFL set up its first explicitly political unit, Labor's League for Political Education. The AFL increasingly abandoned its historic tradition of nonpartisanship, since neutrality between the major parties was impossible. By 1952, the AFL had given up on decentralization, local autonomy, and non-partisanship, and had developed instead a new political approach marked by the same style of centralization, national coordination, and partisan alliances that characterized the CIO. After these moves, the CIO and AFL were in a good position to fight off Henry Wallace in 1948 and work enthusiastically for Truman's reelection. The CIO and AFL no longer had major points of conflict, so they merged in amicably 1955 as the AFL–CIO.

The 1948 presidential election is famous for Truman's stunning come-from-behind victory. In the spring of 1948, Truman's public approval rating stood at 36%, and the president was nearly universally regarded as incapable of winning reelection. The "New Deal" loyalists within the party—including FDR's son James—tried to swing the Democratic nomination to General Dwight D. Eisenhower, a highly popular figure whose political views and party affiliation were totally unknown. Other liberals favored Associate Justice William O. Douglas, but both Eisenhower and Douglas refused to enter the race, and the "Stop Truman" movement failed to unite around any other candidate.

At the 1948 Democratic National Convention, Truman attempted to unify the Northern delegations with a vague civil rights plank in the party platform. He was upstaged by liberals. A sharp address given by Mayor Hubert Humphrey of Minneapolis—as well as the local political interests of a number of urban bosses—convinced the Convention to adopt a stronger civil rights plank, which Truman accepted. All of Alabama's delegates, and a portion of Mississippi's, walked out of the convention in protest. Unfazed, Truman delivered an aggressive acceptance speech attacking the 80th Congress, which Truman called the "Do Nothing Congress," and called it back into session to demonstrate its continuing failures. For his running mate, Truman accepted Kentucky Senator Alben W. Barkley, though he really wanted Justice William O. Douglas, who turned down the nomination.

Within two weeks of the convention, Truman issued an executive order banning racial discrimination in the armed services. as well as Executive Order 9980 to integrate federal agencies. Truman took a political risk in backing civil rights, and many seasoned Democrats were concerned that the loss of Dixiecrat support might destroy the Democratic Party in the South. South Carolina Governor Strom Thurmond, a segregationist, declared his candidacy for the presidency on a Dixiecrat ticket and led a full-scale revolt of Southern "states' rights" proponents. This rebellion on the right was matched by one on the left, led by Wallace on the Progressive Party ticket. Immediately after its first post-FDR convention, the Democratic Party seemed to be disintegrating. Victory in November seemed unlikely as the party was not simply split but divided three ways. The Republicans, meanwhile, nominated New York Governor Thomas E. Dewey, who had been the party's 1944 presidential nominee.

The campaign was a presidential odyssey. In a personal appeal to the nation, Truman crisscrossed the U.S. by train; his "whistle stop" speeches from the rear platform of the observation car, "Ferdinand Magellan," came to represent his campaign. His combative appearances, such as those at the town square of Harrisburg, Illinois, captured the popular imagination and drew huge crowds. Six stops in Michigan drew a combined half-million people; a full million turned out for a New York City ticker-tape parade. The large, mostly spontaneous gatherings at Truman's whistle stop events were an important sign of a change in momentum in the campaign, but this shift went virtually unnoticed by the national press corps. It continued reporting Republican leads in polls that were increasingly out of date. Pollsters assumed few people changed their minds late in a campaign and did spot what was happening. The three major polling organizations stopped polling well before the November 2 election date—Roper in September, and Crossley and Gallup in October—thus failing to measure the period when Truman appears to have surged past Dewey. Truman's surge was aided by the mistakes of his opponents. Dewey waged a low-risk campaign and issued vague generalities on his plans once in office, while Thurmond found less support in the South than many had expected, as most white Southerners believed him to be too extreme. Wallace was unable to galvanize support behind his domestic policies, and his conciliatory attitude towards the Soviet Union alienated many potential supporters.

In the end, Truman held his progressive Midwestern base, won most of the Southern states despite the civil rights plank, and squeaked through with narrow victories in a few critical states, notably Ohio, California, and Illinois. He won over 50 percent of the popular vote and secured 303 electoral votes. Dewey received only 189 electoral votes; Thurmond garnered 39, and Henry Wallace none. Dewey carried several Northeastern states that had generally voted for Roosevelt, and the 1948 election was the closest presidential election since the 1916 election. In the concurrent congressional elections, the Democrats re-took control of the House and the Senate. The defining image of the campaign was a photograph snapped in the early morning hours of the day after the election, when an ecstatic Truman held aloft the erroneous front page of the "Chicago Tribune" with a huge headline proclaiming "Dewey Defeats Truman."

In Truman's second mid-term election, Republicans again picked up seats in both houses of Congress. However, unlike the 1946 elections, Democrats retained control of Congress. Republicans ran against Truman's proposed domestic policies and his handling of the Korean War. Truman was particularly upset by the apparent success of those who campaigned on McCarthyism.

At the time of the 1952 New Hampshire primary, Truman had not stated whether he would seek re-election, and no other candidate had won Truman's backing. Although a Constitutional amendment limiting presidents to two terms had been ratified in 1951, Truman could run for another term due to a grandfather clause in the amendment. Truman's first choice to succeed him, Chief Justice Vinson, had declined to run, Illinois Governor Adlai Stevenson had also turned Truman down, Vice President Barkley was considered too old, and Truman disliked Senator Kefauver. Accordingly, Truman let his name be entered in the New Hampshire primary by supporters. The highly unpopular Truman was handily defeated by Kefauver; 18 days later the president announced he would not seek a second full term. Truman was eventually able to persuade Stevenson to run, and the governor gained the nomination at the 1952 Democratic National Convention.

Eisenhower's public stature, along with his unknown views on domestic issues, had made him appealing as a potential candidate for both parties in the 1948 election. Though he had generally supported Truman's foreign policy, Eisenhower privately held conservative views on most domestic issues and never seriously considered running for office as a Democrat. Beginning in 1951, eastern, internationalist Republicans, led by Thomas Dewey and Henry Cabot Lodge Jr., coordinated a draft movement designed to help Eisenhower win the 1952 Republican presidential nomination. Eisenhower initially resisted these efforts, but in March 1952 he agreed to allow his name to be entered into the New Hampshire primary. He was motivated in part by his desire to defeat Robert Taft, the other major contender for the Republican nomination. The 1952 Republican primaries became a battle between Dewey's internationalist wing of the party and Taft's conservative, isolationist wing. Eisenhower narrowly prevailed over Taft at the 1952 Republican National Convention; with the approval of Eisenhower, the convention nominated Richard Nixon for vice president.

The once good Truman-Eisenhower relationship soured during the campaign. Truman was appalled when Eisenhower appeared on the same platform with Joseph McCarthy in Wisconsin, and failed to defend General George C. Marshall, who McCarthy had recently denounced as part of a Communist conspiracy. Similarly, Eisenhower was outraged when Truman, who made a whistle-stop tour in support of Stevenson, accused the former general of disregarding "sinister forces ... Anti-Semitism, anti-Catholicism, and anti-foreignism" within the Republican Party.

Though Stevenson's public service and issue-oriented campaign appealed to many liberals, he was unable to rally support among blacks, ethnic whites, and the working class. Eisenhower campaigned against what he denounced as Truman's failures: "Korea, Communism and Corruption." Polls consistently indicated that Eisenhower would win the race, and Nixon deftly handled a potentially dangerous controversy over his finances with his Checkers speech, delivered live on national television. In part due to the Checkers speech, television emerged as an important medium in the race; the number of households with televisions had grown from under 200,000 in 1948 to over 15 million in 1952. On election day, as widely expected, Eisenhower defeated Stevenson by a wide margin. Eisenhower took 55.4 percent of the popular vote and won 442 electoral votes, taking almost every state outside of the South. Though Eisenhower ran ahead of most congressional Republicans, his party nonetheless took control of both the House and Senate, giving the Republican Party unified control of Congress and the presidency for the first time since the 1930 elections.

Scholars have on average ranked Truman in the top ten American presidents, most often at #7. In 1962, a poll of 75 historians conducted by Arthur M. Schlesinger, Sr. ranked Truman among the "near great" presidents. Truman's ranking in polls of political scientists and historians, never fallen lower than ninth, and ranking as high as fifth in a "C-SPAN" poll in 2009. A 2018 poll of the American Political Science Association’s Presidents and Executive Politics section ranked Truman as the seventh best president. A 2017 C-Span poll of historians ranked Truman as the sixth best president.

When he left office in 1953, the American public saw Truman as one of the most unpopular chief executives in history. His job approval rating of 22% in the Gallup Poll of February 1952 was lower than Richard Nixon's 24% in August 1974, the month that Nixon resigned. Truman famously boasted "the buck stops here"—he took final responsibility for his administration.

Journalist Samuel Lubell challenged the claim. Citing continuing divisions within the Democratic Party, the ongoing Cold War, and the boom and bust cycle, Lubell in 1952 stated that "after seven years of Truman's hectic, even furious, activity the nation seemed to be about on the same general spot as when he first came to office ... Nowhere in the whole Truman record can one point to a single, decisive break-through ... All his skills and energies—and he was among our hardest-working Presidents—were directed to standing still".

Truman's image in university textbooks was quite favorable in the 1950s. During the years of campus unrest in the 1960s and 1970s revisionist historians on the left attacked his foreign policy as too hostile to Communism, and his domestic policy as too favorable toward business. That revisionism was not accepted by more established scholars. The harsh perspective faded with the decline in Communism's appeal after 1980, leading to a more balanced view.

American public feeling towards Truman grew steadily warmer with the passing years. Truman died when the nation was consumed with crises in Vietnam and Watergate, and his death brought a new wave of attention to his political career. During this period, Truman captured the popular imagination, emerging as a kind of political folk hero, a president who was thought to exemplify an integrity and accountability many observers felt was lacking in the Nixon White House. This public reassessment of Truman was aided by the popularity of a book of reminiscences which Truman had told to journalist Merle Miller beginning in 1961, with the agreement that they would not be published until after Truman's death. Scholars who have compared the audio tapes with the published transcripts have concluded the Miller often distorted what Truman said or fabricated statements Truman never said. Nevertheless, Truman continued to receive criticism. After a review of information available to Truman about the presence of espionage activities in the U.S. government, Democratic Senator Daniel Patrick Moynihan concluded that Truman was "almost willfully obtuse" concerning the danger of American communism. In 2002, historian Alonzo Hamby concluded that "Harry Truman remains a controversial president."

The fall of the Soviet Union in 1991 caused Truman advocates to claim vindication for Truman's decisions in the postwar period. According to Truman biographer Robert Dallek, "His contribution to victory in the cold war without a devastating nuclear conflict elevated him to the stature of a great or near-great president." The 1992 publication of David McCullough's favorable biography of Truman further cemented the view of Truman as a highly regarded Chief Executive. According to historian Daniel R. McCoy in his book on the Truman presidency,

With the basic fusion reactions confirmed, and with a feasible design in hand, there was nothing to prevent Los Alamos from testing a thermonuclear device. On 1 November 1952, the first thermonuclear explosion occurred when Ivy Mike was detonated on Enewetak Atoll, within the US Pacific Proving Grounds. This device, which used liquid deuterium as its fusion fuel, was immense and utterly unusable as a weapon. Nevertheless, its success validated the Teller–Ulam design, and stimulated intensive development of practical weapons.

Stanislaw Ulam

Stanisław Marcin Ulam (; 13 April 1909 – 13 May 1984) was a Polish scientist in the fields of mathematics and nuclear physics. He participated in the Manhattan Project, originated the Teller–Ulam design of thermonuclear weapons, discovered the concept of cellular automaton, invented the Monte Carlo method of computation, and suggested nuclear pulse propulsion. In pure and applied mathematics, he proved some theorems and proposed several conjectures.

Born into a wealthy Polish Jewish family, Ulam studied mathematics at the Lwów Polytechnic Institute, where he earned his PhD in 1933 under the supervision of Kazimierz Kuratowski. In 1935, John von Neumann, whom Ulam had met in Warsaw, invited him to come to the Institute for Advanced Study in Princeton, New Jersey, for a few months. From 1936 to 1939, he spent summers in Poland and academic years at Harvard University in Cambridge, Massachusetts, where he worked to establish important results regarding ergodic theory. On 20 August 1939, he sailed for the United States for the last time with his 17-year-old brother Adam Ulam. He became an assistant professor at the University of Wisconsin–Madison in 1940, and a United States citizen in 1941.

In October 1943, he received an invitation from Hans Bethe to join the Manhattan Project at the secret Los Alamos Laboratory in New Mexico. There, he worked on the hydrodynamic calculations to predict the behavior of the explosive lenses that were needed by an implosion-type weapon. He was assigned to Edward Teller's group, where he worked on Teller's "Super" bomb for Teller and Enrico Fermi. After the war he left to become an associate professor at the University of Southern California, but returned to Los Alamos in 1946 to work on thermonuclear weapons. With the aid of a cadre of female "computers", including his wife Françoise Aron Ulam, he found that Teller's "Super" design was unworkable. In January 1951, Ulam and Teller came up with the Teller–Ulam design, which is the basis for all thermonuclear weapons.

Ulam considered the problem of nuclear propulsion of rockets, which was pursued by Project Rover, and proposed, as an alternative to Rover's nuclear thermal rocket, to harness small nuclear explosions for propulsion, which became Project Orion. With Fermi, John Pasta, and Mary Tsingou, Ulam studied the Fermi–Pasta–Ulam–Tsingou problem, which became the inspiration for the field of non-linear science. He is probably best known for realising that electronic computers made it practical to apply statistical methods to functions without known solutions, and as computers have developed, the Monte Carlo method has become a common and standard approach to many problems.

Ulam was born in Lemberg, Galicia, on 13 April 1909. At this time, Galicia was in the Kingdom of Galicia and Lodomeria of the Austro-Hungarian Empire, known to Poles as the Austrian partition. In 1918, it became part of the newly restored Poland, the Second Polish Republic, and the city took its Polish name again, Lwów.

The Ulams were a wealthy Polish Jewish family of bankers, industrialists, and other professionals. Ulam's immediate family was "well-to-do but hardly rich". His father, Józef Ulam, was born in Lwów and was a lawyer, and his mother, Anna (née Auerbach), was born in Stryj. His uncle, Michał Ulam, was an architect, building contractor, and lumber industrialist. From 1916 until 1918, Józef's family lived temporarily in Vienna. After they returned, Lwów became the epicenter of the Polish–Ukrainian War, during which the city experienced a Ukrainian siege.
In 1919, Ulam entered Lwów Gymnasium Nr. VII, from which he graduated in 1927. He then studied mathematics at the Lwów Polytechnic Institute. Under the supervision of Kazimierz Kuratowski, he received his Master of Arts degree in 1932, and became a Doctor of Science in 1933. At the age of 20, in 1929, he published his first paper "Concerning Function of Sets" in the journal "Fundamenta Mathematicae". From 1931 until 1935, he traveled to and studied in Wilno (Vilnius), Vienna, Zurich, Paris, and Cambridge, England, where he met G. H. Hardy and Subrahmanyan Chandrasekhar.

Along with Stanisław Mazur, Mark Kac, Włodzimierz Stożek, Kuratowski, and others, Ulam was a member of the Lwów School of Mathematics. Its founders were Hugo Steinhaus and Stefan Banach, who were professors at the Jan Kazimierz University. Mathematicians of this "school" met for long hours at the Scottish Café, where the problems they discussed were collected in the Scottish Book, a thick notebook provided by Banach's wife. Ulam was a major contributor to the book. Of the 193 problems recorded between 1935 and 1941, he contributed 40 problems as a single author, another 11 with Banach and Mazur, and an additional 15 with others. In 1957, he received from Steinhaus a copy of the book, which had survived the war, and translated it into English. In 1981, Ulam's friend R. Daniel Maudlin published an expanded and annotated version.

In 1935, John von Neumann, whom Ulam had met in Warsaw, invited him to come to the Institute for Advanced Study in Princeton, New Jersey, for a few months. In December of that year, Ulam sailed to the US. At Princeton, he went to lectures and seminars, where he heard Oswald Veblen, James Alexander, and Albert Einstein. During a tea party at von Neumann's house, he encountered G. D. Birkhoff, who suggested that he apply for a position with the Harvard Society of Fellows. Following up on Birkhoff's suggestion, Ulam spent summers in Poland and academic years at Harvard University in Cambridge, Massachusetts from 1936 to 1939, where he worked with John C. Oxtoby to establish results regarding ergodic theory. These appeared in Annals of Mathematics in 1941.

On 20 August 1939, in Gdynia, Józef Ulam, along with his brother Szymon, put his two sons, Stanislaw and 17 year old Adam, on a ship headed for the US. Eleven days later, the. Within two months, the Germans completed their occupation of western Poland, and the Soviets and occupied eastern Poland. Within two years, Józef Ulam and the rest of his family were victims of the Holocaust, Hugo Steinhaus was in hiding, Kazimierz Kuratowski was lecturing at the underground university in Warsaw, Włodzimierz Stożek and his two sons had been killed in the massacre of Lwów professors, and the last problem had been recorded in the Scottish Book. Stefan Banach survived the Nazi occupation by feeding lice at Rudolf Weigl's typhus research institute. In 1963, Adam Ulam, who had become an eminent kremlinologist at Harvard, received a letter from George Volsky, who hid in Józef Ulam's house after deserting from the Polish army. This reminiscence gave a chilling account of Lwów's chaotic scenes in late 1939. In later life Ulam described himself as "an agnostic. Sometimes I muse deeply on the forces that are for me invisible. When I am almost close to the idea of God, I feel immediately estranged by the horrors of this world, which he seems to tolerate".

In 1940, after being recommended by Birkhoff, Ulam became an assistant professor at the University of Wisconsin–Madison. Here, he became a United States citizen in 1941. That year, he married Françoise Aron. She had been a French exchange student at Mount Holyoke College, whom he met in Cambridge. They had one daughter, Claire. In Madison, Ulam met his friend and colleague C. J. Everett, with whom he would collaborate on a number of papers.

In early 1943, Ulam asked von Neumann to find him a war job. In October, he received an invitation to join an unidentified project near Santa Fe, New Mexico. The letter was signed by Hans Bethe, who had been appointed as leader of the theoretical division of Los Alamos National Laboratory by Robert Oppenheimer, its scientific director. Knowing nothing of the area, he borrowed a New Mexico guide book. On the checkout card, he found the names of his Wisconsin colleagues, Joan Hinton, David Frisch, and Joseph McKibben, all of whom had mysteriously disappeared. This was Ulam's introduction to the Manhattan Project, which was US's wartime effort to create the atomic bomb.

A few weeks after Ulam reached Los Alamos in February 1944, the project experienced a crisis. In April, Emilio Segrè discovered that plutonium made in reactors would not work in a gun-type plutonium weapon like the "Thin Man", which was being developed in parallel with a uranium weapon, the "Little Boy" that was dropped on Hiroshima. This problem threatened to waste an enormous investment in new reactors at the Hanford site and to make slow separation of uranium isotopes the only way to prepare fissile material suitable for use in bombs. To respond, Oppenheimer implemented, in August, a sweeping reorganization of the laboratory to focus on development of an implosion-type weapon and appointed George Kistiakowsky head of the implosion department. He was a professor at Harvard and an expert on precise use of explosives.

The basic concept of implosion is to use chemical explosives to crush a chunk of fissile material into a critical mass, where neutron multiplication leads to a nuclear chain reaction, releasing a large amount of energy. Cylindrical implosive configurations had been studied by Seth Neddermeyer, but von Neumann, who had experience with shaped charges used in armor-piercing ammunition, was a vocal advocate of spherical implosion driven by explosive lenses. He realized that the symmetry and speed with which implosion compressed the plutonium were critical issues, and enlisted Ulam to help design lens configurations that would provide nearly spherical implosion. Within an implosion, because of enormous pressures and high temperatures, solid materials behave much like fluids. This meant that hydrodynamical calculations were needed to predict and minimize asymmetries that would spoil a nuclear detonation. Of these calculations, Ulam said:

Nevertheless, with the primitive facilities available at the time, Ulam and von Neumann did carry out numerical computations that led to a satisfactory design. This motivated their advocacy of a powerful computational capability at Los Alamos, which began during the war years, continued through the cold war, and still exists. Otto Frisch remembered Ulam as "a brilliant Polish topologist with a charming French wife. At once he told me that he was a pure mathematician who had sunk so low that his latest paper actually contained numbers with decimal points!"

Even the inherent statistical fluctuations of neutron multiplication within a chain reaction have implications with regard to implosion speed and symmetry. In November 1944, David Hawkins and Ulam addressed this problem in a report entitled "Theory of Multiplicative Processes". This report, which invokes probability-generating functions, is also an early entry in the extensive literature on statistics of branching and multiplicative processes. In 1948, its scope was extended by Ulam and Everett.

Early in the Manhattan project, Enrico Fermi's attention was focused on the use of reactors to produce plutonium. In September 1944, he arrived at Los Alamos, shortly after breathing life into the first Hanford reactor, which had been poisoned by a xenon isotope. Soon after Fermi's arrival, Teller's "Super" bomb group, of which Ulam was a part, was transferred to a new division headed by Fermi. Fermi and Ulam formed a relationship that became very fruitful after the war.

In September 1945, Ulam left Los Alamos to become an associate professor at the University of Southern California in Los Angeles. In January 1946, he suffered an acute attack of encephalitis, which put his life in danger, but which was alleviated by emergency brain surgery. During his recuperation, many friends visited, including Nicholas Metropolis from Los Alamos and the famous mathematician Paul Erdős, who remarked: "Stan, you are just like before." This was encouraging, because Ulam was concerned about the state of his mental faculties, for he had lost the ability to speak during the crisis. Another friend, Gian-Carlo Rota, asserted in a 1987 article that the attack changed Ulam's personality: afterwards, he turned from rigorous pure mathematics to more speculative conjectures concerning the application of mathematics to physics and biology; Rota also cites Ulam's former collaborator Paul Stein as noting that Ulam was sloppier in his clothing afterwards, and John Oxtoby as noting that Ulam before the encephalitis could work for hours on end doing calculations, while when Rota worked with him, was reluctant to solve even a quadratic equation. This assertion was not accepted by Françoise Aron Ulam.

By late April 1946, Ulam had recovered enough to attend a secret conference at Los Alamos to discuss thermonuclear weapons. Those in attendance included Ulam, von Neumann, Metropolis, Teller, Stan Frankel, and others. Throughout his participation in the Manhattan Project, Teller's efforts had been directed toward the development of a "super" weapon based on nuclear fusion, rather than toward development of a practical fission bomb. After extensive discussion, the participants reached a consensus that his ideas were worthy of further exploration. A few weeks later, Ulam received an offer of a position at Los Alamos from Metropolis and Robert D. Richtmyer, the new head of its theoretical division, at a higher salary, and the Ulams returned to Los Alamos.

Late in the war, under the sponsorship of von Neumann, Frankel and Metropolis began to carry out calculations on the first general-purpose electronic computer, the ENIAC at the Aberdeen Proving Ground in Maryland. Shortly after returning to Los Alamos, Ulam participated in a review of results from these calculations. Earlier, while playing solitaire during his recovery from surgery, Ulam had thought about playing hundreds of games to estimate statistically the probability of a successful outcome. With ENIAC in mind, he realized that the availability of computers made such statistical methods very practical. John von Neumann immediately saw the significance of this insight. In March 1947 he proposed a statistical approach to the problem of neutron diffusion in fissionable material. Because Ulam had often mentioned his uncle, Michał Ulam, "who just had to go to Monte Carlo" to gamble, Metropolis dubbed the statistical approach "The Monte Carlo method". Metropolis and Ulam published the first unclassified paper on the Monte Carlo method in 1949.

Fermi, learning of Ulam's breakthrough, devised an analog computer known as the Monte Carlo trolley, later dubbed the FERMIAC. The device performed a mechanical simulation of random diffusion of neutrons. As computers improved in speed and programmability, these methods became more useful. In particular, many Monte Carlo calculations carried out on modern massively parallel supercomputers are embarrassingly parallel applications, whose results can be very accurate.

On 29 August 1949, the Soviet Union tested its first fission bomb, the RDS-1. Created under the supervision of Lavrentiy Beria, who sought to duplicate the US effort, this weapon was nearly identical to Fat Man, for its design was based on information provided by spies Klaus Fuchs, Theodore Hall, and David Greenglass. In response, on 31 January 1950, President Harry S. Truman announced a crash program to develop a fusion bomb.

To advocate an aggressive development program, Ernest Lawrence and Luis Alvarez came to Los Alamos, where they conferred with Norris Bradbury, the laboratory director, and with George Gamow, Edward Teller, and Ulam. Soon, these three became members of a short-lived committee appointed by Bradbury to study the problem, with Teller as chairman. At this time, research on the use of a fission weapon to create a fusion reaction had been ongoing since 1942, but the design was still essentially the one originally proposed by Teller. His concept was to put tritium and/or deuterium in close proximity to a fission bomb, with the hope that the heat and intense flux of neutrons released when the bomb exploded, would ignite a self-sustaining fusion reaction. Reactions of these isotopes of hydrogen are of interest because the energy per unit mass of fuel released by their fusion is much larger than that from fission of heavy nuclei.
Because the results of calculations based on Teller's concept were discouraging, many scientists believed it could not lead to a successful weapon, while others had moral and economic grounds for not proceeding. Consequently, several senior people of the Manhattan Project opposed development, including Bethe and Oppenheimer. To clarify the situation, Ulam and von Neumann resolved to do new calculations to determine whether Teller's approach was feasible. To carry out these studies, von Neumann decided to use electronic computers: ENIAC at Aberdeen, a new computer, MANIAC, at Princeton, and its twin, which was under construction at Los Alamos. Ulam enlisted Everett to follow a completely different approach, one guided by physical intuition. Françoise Ulam was one of a cadre of women "computers" who carried out laborious and extensive computations of thermonuclear scenarios on mechanical calculators, supplemented and confirmed by Everett's slide rule. Ulam and Fermi collaborated on further analysis of these scenarios. The results showed that, in workable configurations, a thermonuclear reaction would not ignite, and if ignited, it would not be self-sustaining. Ulam had used his expertise in combinatorics to analyze the chain reaction in deuterium, which was much more complicated than the ones in uranium and plutonium, and he concluded that no self-sustaining chain reaction would take place at the (low) densities that Teller was considering. In late 1950, these conclusions were confirmed by von Neumann's results.

In January 1951, Ulam had another idea: to channel the mechanical shock of a nuclear explosion so as to compress the fusion fuel. On the recommendation of his wife, Ulam discussed this idea with Bradbury and Mark before he told Teller about it. Almost immediately, Teller saw its merit, but noted that soft X-rays from the fission bomb would compress the thermonuclear fuel more strongly than mechanical shock and suggested ways to enhance this effect. On 9 March 1951, Teller and Ulam submitted a joint report describing these innovations. A few weeks later, Teller suggested placing a fissile rod or cylinder at the center of the fusion fuel. The detonation of this "spark plug" would help to initiate and enhance the fusion reaction. The design based on these ideas, called staged radiation implosion, has become the standard way to build thermonuclear weapons. It is often described as the "Teller–Ulam design".
In September 1951, after a series of differences with Bradbury and other scientists, Teller resigned from Los Alamos, and returned to the University of Chicago. At about the same time, Ulam went on leave as a visiting professor at Harvard for a semester. Although Teller and Ulam submitted a joint report on their design and jointly applied for a patent on it, they soon became involved in a dispute over who deserved credit. After the war, Bethe returned to Cornell University, but he was deeply involved in the development of thermonuclear weapons as a consultant. In 1954, he wrote an article on the history of the H-bomb, which presents his opinion that both men contributed very significantly to the breakthrough. This balanced view is shared by others who were involved, including Mark and Fermi, but Teller persistently attempted to downplay Ulam's role. "After the H-bomb was made," Bethe recalled, "reporters started to call Teller the father of the H-bomb. For the sake of history, I think it is more precise to say that Ulam is the father, because he provided the seed, and Teller is the mother, because he remained with the child. As for me, I guess I am the midwife."

With the basic fusion reactions confirmed, and with a feasible design in hand, there was nothing to prevent Los Alamos from testing a thermonuclear device. On 1 November 1952, the first thermonuclear explosion occurred when Ivy Mike was detonated on Enewetak Atoll, within the US Pacific Proving Grounds. This device, which used liquid deuterium as its fusion fuel, was immense and utterly unusable as a weapon. Nevertheless, its success validated the Teller–Ulam design, and stimulated intensive development of practical weapons.

When Ulam returned to Los Alamos, his attention turned away from weapon design and toward the use of computers to investigate problems in physics and mathematics. With John Pasta, who helped Metropolis to bring MANIAC on line in March 1952, he explored these ideas in a report "Heuristic Studies in Problems of Mathematical Physics on High Speed Computing Machines", which was submitted on 9 June 1953. It treated several problems that cannot be addressed within the framework of traditional analytic methods: billowing of fluids, rotational motion in gravitating systems, magnetic lines of force, and hydrodynamic instabilities.

Soon, Pasta and Ulam became experienced with electronic computation on MANIAC, and by this time, Enrico Fermi had settled into a routine of spending academic years at the University of Chicago and summers at Los Alamos. During these summer visits, Pasta, Ulam, and Mary Tsingou, a programmer in the MANIAC group, joined him to study a variation of the classic problem of a string of masses held together by springs that exert forces linearly proportional to their displacement from equilibrium. Fermi proposed to add to this force a nonlinear component, which could be chosen to be proportional to either the square or cube of the displacement, or to a more complicated "broken linear" function. This addition is the key element of the Fermi–Pasta–Ulam–Tsingou problem, which is often designated by the abbreviation FPUT.

A classical spring system can be described in terms of vibrational modes, which are analogous to the harmonics that occur on a stretched violin string. If the system starts in a particular mode, vibrations in other modes do not develop. With the nonlinear component, Fermi expected energy in one mode to transfer gradually to other modes, and eventually, to be distributed equally among all modes. This is roughly what began to happen shortly after the system was initialized with all its energy in the lowest mode, but much later, essentially all the energy periodically reappeared in the lowest mode. This behavior is very different from the expected equipartition of energy. It remained mysterious until 1965, when Kruskal and Zabusky showed that, after appropriate mathematical transformations, the system can be described by the Korteweg–de Vries equation, which is the prototype of nonlinear partial differential equations that have soliton solutions. This means that FPUT behavior can be understood in terms of solitons.

Starting in 1955, Ulam and Frederick Reines considered nuclear propulsion of aircraft and rockets. This is an attractive possibility, because the nuclear energy per unit mass of fuel is a million times greater than that available from chemicals. From 1955 to 1972, their ideas were pursued during Project Rover, which explored the use of nuclear reactors to power rockets. In response to a question by Senator John O. Pastore at a congressional committee hearing on "Outer Space Propulsion by Nuclear Energy", on January 22, 1958, Ulam replied that "the future as a whole of mankind is to some extent involved inexorably now with going outside the globe."

Ulam and C. J. Everett also proposed, in contrast to Rover's continuous heating of rocket exhaust, to harness small nuclear explosions for propulsion. Project Orion was a study of this idea. It began in 1958 and ended in 1965, after the Partial Nuclear Test Ban Treaty of 1963 banned nuclear weapons tests in the atmosphere and in space. Work on this project was spearheaded by physicist Freeman Dyson, who commented on the decision to end Orion in his article, "Death of a Project".

Bradbury appointed Ulam and John H. Manley as research advisors to the laboratory director in 1957. These newly created positions were on the same administrative level as division leaders, and Ulam held his until he retired from Los Alamos. In this capacity, he was able to influence and guide programs in many divisions: theoretical, physics, chemistry, metallurgy, weapons, health, Rover, and others.

In addition to these activities, Ulam continued to publish technical reports and research papers. One of these introduced the Fermi–Ulam model, an extension of Fermi's theory of the acceleration of cosmic rays. Another, with Paul Stein and Mary Tsingou, titled "Quadratic Transformations", was an early investigation of chaos theory and is considered the first published use of the phrase "chaotic behavior".

During his years at Los Alamos, Ulam was a visiting professor at Harvard from 1951 to 1952, MIT from 1956 to 1957, the University of California, San Diego, in 1963, and the University of Colorado at Boulder from 1961 to 1962 and 1965 to 1967. In 1967, the last of these positions became permanent, when Ulam was appointed as professor and Chairman of the Department of Mathematics at Boulder, Colorado. He kept a residence in Santa Fe, New Mexico, which made it convenient to spend summers at Los Alamos as a consultant.

In Colorado, where he rejoined his friends Gamow, Richtmyer, and Hawkins, Ulam's research interests turned toward biology. In 1968, recognizing this emphasis, the University of Colorado School of Medicine appointed Ulam as Professor of Biomathematics, and he held this position until his death. With his Los Alamos colleague Robert Schrandt he published a report, "Some Elementary Attempts at Numerical Modeling of Problems Concerning Rates of Evolutionary Processes", which applied his earlier ideas on branching processes to biological inheritance. Another, report, with William Beyer, Temple F. Smith, and M. L. Stein, titled "Metrics in Biology", introduced new ideas about biometric distances.

When he retired from Colorado in 1975, Ulam had begun to spend winter semesters at the University of Florida, where he was a graduate research professor. Except for sabbaticals at the University of California, Davis from 1982 to 1983, and at Rockefeller University from 1980 to 1984, this pattern of spending summers in Colorado and Los Alamos and winters in Florida continued until Ulam died of an apparent heart attack in Santa Fe on 13 May 1984.
Paul Erdős noted that "he died suddenly of heart failure, without fear or pain, while he could still prove and conjecture." In 1987, Françoise Ulam deposited his papers with the American Philosophical Society Library in Philadelphia. She continued to live in Santa Fe until she died on 30 April 2011, at the age of 93. Both Françoise and her husband are buried with her French family in Montparnasse Cemetery in Paris.

From the publication of his first paper as a student in 1929 until his death, Ulam was constantly writing on mathematics. The list of Ulam's publications includes more than 150 papers. Topics represented by a significant number of papers are: set theory (including measurable cardinals and abstract measures), topology, transformation theory, ergodic theory, group theory, projective algebra, number theory, combinatorics, and graph theory. In March 2009, the Mathematical Reviews database contained 697 papers with the name "Ulam".

Notable results of this work are:

With his pivotal role in the development of thermonuclear weapons, Stanislaw Ulam changed the world. According to Françoise Ulam: "Stan would reassure me that, barring accidents, the H-bomb rendered nuclear war impossible." In 1980, Ulam and his wife appeared in the television documentary "The Day After Trinity".
The Monte Carlo method has become a ubiquitous and standard approach to computation, and the method has been applied to a vast number of scientific problems. In addition to problems in physics and mathematics, the method has been applied to finance, social science, environmental risk assessment, linguistics, radiation therapy, and sports.

The Fermi–Pasta–Ulam–Tsingou problem is credited not only as "the birth of experimental mathematics", but also as inspiration for the vast field of Nonlinear Science. In his Lilienfeld Prize lecture, David K. Campbell noted this relationship and described how FPUT gave rise to ideas in chaos, solitons, and dynamical systems. In 1980, Donald Kerr, laboratory director at Los Alamos, with the strong support of Ulam and Mark Kac, founded the Center for Nonlinear Studies (CNLS). In 1985, CNLS initiated the "Stanislaw M. Ulam Distinguished Scholar" program, which provides an annual award that enables a noted scientist to spend a year carrying out research at Los Alamos.

The fiftieth anniversary of the original FPUT paper was the subject of the March 2005 issue of the journal Chaos, and the topic of the 25th Annual International Conference of CNLS. The University of Southern Mississippi and the University of Florida supported the "Ulam Quarterly", which was active from 1992 to 1996, and which was one of the first online mathematical journals. Florida's Department of Mathematics has sponsored, since 1998, the annual "Ulam Colloquium Lecture", and in March 2009, the "Ulam Centennial Conference".

Ulam's work on non-Euclidean distance metrics in the context of molecular biology made a significant contribution to sequence analysis and his contributions in theoretical biology are considered watersheds in the development of cellular automata theory, population biology, pattern recognition, and biometrics generally. Colleagues noted that some of his greatest contributions were in clearly identifying problems to be solved and general techniques for solving them.

In 1987, Los Alamos issued a special issue of its "Science" publication, which summarized his accomplishments, and which appeared, in 1989, as the book "From Cardinals to Chaos". Similarly, in 1990, the University of California Press issued a compilation of mathematical reports by Ulam and his Los Alamos collaborators: "Analogies Between Analogies". During his career, Ulam was awarded honorary degrees by the Universities of New Mexico, Wisconsin, and Pittsburgh.

Nuclear weapons brought an entirely new and terrifying possibility to warfare: a nuclear holocaust. While at first the United States held a monopoly on the production of nuclear weapons, the Soviet Union, with some assistance from espionage, managed to detonate its first weapon (dubbed "Joe-1" by the West) in August 1949. The post-war relations between the two, which had already been deteriorating, began to rapidly disintegrate. Soon the two were locked in a massive stockpiling of nuclear weapons. The United States began a crash program to develop the first hydrogen bomb in 1950, and detonated its first thermonuclear weapon in 1952. This new weapon was alone over 400 times as powerful as the weapons used against Japan. The Soviet Union detonated a primitive thermonuclear weapon in 1953 and a full-fledged one in 1955.

20th-century events

From 1914 to 1918, the First World War, and its aftermath, caused major changes in the power balance of the world, destroying or transforming some of the most powerful empires.

The First World War (or simply WWI), termed "The Great War" by contemporaries, started in 1914 and ended in 1918. It was ignited by the Assassination in Sarajevo of the Austro-Hungarian Empire's heir to the throne, Erzherzog Franz Ferdinand, by Gavrilo Princip of the organization "Young Bosnia," Bosnian Serbs' liberation movement. Bound by Slavic nationalism to help the small Serbian state, the Russians came to the aid of the Serbs when they were attacked. Interwoven alliances, an increasing arms race, and old hatreds dragged Europe into the war. The Allies, known initially as "The Triple Entente", comprised the British Empire, France and Russia. Germany, Austria-Hungary, Italy, Bulgaria, and later the Ottoman Empire, were known as "The Central Powers".

In 1917, Russia ended hostile actions against the Central Powers after the fall of the Tsar. The Bolsheviks negotiated the Treaty of Brest-Litovsk with Germany, although it was at huge cost to Russia. In the treaty, Bolshevik Russia ceded the Baltic States to Germany, and its province of Kars Oblast in the south Caucasus to the Ottoman Empire. It also recognized the independence of Ukraine. Although Germany shifted huge forces from the eastern to the western front after signing the treaty, it was unable to stop the Allied advance, especially with the entrance of American troops in 1918.

The war itself was also a chance for the combatant nations to show off their military strength and technological ingenuity. The Germans introduced the machine gun, U-Boats and deadly gases. The British first used the tank. Both sides had a chance to test out their new aircraft to see if they could be used in warfare. It was widely believed that the war would be short. Unfortunately, since trench warfare was the best form of defense, advances on both sides were very slow, and came at a terrible cost in lives.

When the war was finally over in 1918, the results would set the stage for the next twenty years. First and foremost, the Germans were forced to sign the Treaty of Versailles, forcing them to make exorbitant payments to repair damages caused during the War. Many Germans felt these reparations were unfair because they did not actually "lose" the war nor did they feel they caused the war (see Stab-in-the-back legend). Germany was never occupied by Allied troops, yet it had to accept a liberal democratic government imposed on it by the victors after the abdication of Kaiser Wilhelm.

Much of the map of Europe was redrawn by the victors based upon the theory that future wars could be prevented if all ethnic groups had their own "homeland". New states like Yugoslavia and Czechoslovakia were created out of the former Austro-Hungarian Empire to accommodate the nationalist aspirations of these groups. An international body called the League of Nations was formed to mediate disputes and prevent future wars, although its effectiveness was severely limited by, among other things, its reluctance and inability to act.

The Russian Revolution of 1918 (ending in the overthrow of the Tsarist regime and the brutal execution of His Imperial Majesty Nicholas II and his family) sparked a wave of communist revolutions across Europe, prompting many to believe that a socialist world revolution could be realized in the near future. However, the European revolutions were defeated, Vladimir Lenin died in 1924, and within a few years, Joseph Stalin displaced Leon Trotsky as the "de facto" leader of the Soviet Union. The idea of worldwide revolution was no longer in the forefront, as Stalin concentrated on "socialism in one country" and embarked on a bold plan of collectivization and industrialization. The majority of socialists and even many communists became disillusioned with Stalin's autocratic rule, his purges and the assassination of his "enemies", as well as the news of famines he imposed on his own people.

Communism was strengthened as a force in Western democracies when the global economy crashed in 1929 in what became known as the Great Depression. Many people saw this as the first stage of the end of the capitalist system and were attracted to Communism as a solution to the economic crisis, especially as the Soviet Union's economic development in the 1930s was strong, unaffected by the capitalist world's crisis.

After World War I, the global economy remained strong through the 1920s. The war had provided a stimulus for industry and for economic activity in general. There were many warning signs foretelling the collapse of the global economic system in 1929 that were generally not understood by the political leadership of the time. The responses to the crisis often made the situation worse, as millions of people watched their savings become next to worthless and the idea of a steady job with a reasonable income fading away.

Many sought answers in alternative ideologies such as communism and fascism. They believed that the capitalist economic system was collapsing, and that new ideas were required to meet the crisis. The early responses to the crisis were based upon the assumption that the free market would correct itself. This, however, did very little to correct the crisis or to alleviate the suffering of many ordinary people. Thus, the idea that the existing system could be reformed by government intervention in the economy, rather than by continuing the "laissez-faire" approach, became prominent as a solution to the crisis. Democratic governments assumed the responsibility to provide needed services in society, and to alleviate poverty. Thus was born the welfare state. These two politico-economic principles, the belief in government intervention and the welfare state, as opposed to the belief in the free market and private institutions, would define many political battles for the rest of the century.

Fascism first appeared in Italy with the rise to power of Benito Mussolini in 1922. The ideology was supported by a large proportion of the upper classes as a strong challenge to the threat of communism.

When Adolf Hitler came to power in Germany in 1933, a new variant of fascism called Nazism took over Germany and ended the German experiment with democracy. The Nazi Party in Germany was dedicated to the restoration of German honor and prestige, the unification of German-speaking peoples, and the annexation of Central and Eastern Europe as vassal states, with the Slavic population to act as slave labor to serve German economic interests. There was also a strong appeal to a mythical racial purity (the idea that Germans were the "Herrenvolk" or the "master race"), and a vicious anti-semitism which promoted the idea of Jews as subhuman ("Untermensch") and worthy only of extermination.

Many people in Western Europe and the United States greeted the rise of Hitler with relief or indifference. They could see nothing wrong with a strong Germany ready to take on the communist menace to the east. Anti-semitism during the Great Depression was widespread as many were content to blame the Jews for causing the economic downturn.

Hitler began to put his plan in motion, annexing Austria in the "Anschluss", or reunification of Austria to Germany, in 1938. He then negotiated the annexation of the Sudetenland, a German-speaking mountainous area of Czechoslovakia, in the Munich Conference. The British were eager to avoid war and believed Hitler's assurance to protect the security of the Czech state. Hitler annexed the rest of the Czech state shortly afterwards, indicating that he had ulterior motives.

Fascism was not the only form of dictatorship to rise in the post-war period. Almost all of the new democracies in the nations of Eastern Europe collapsed and were replaced by authoritarian regimes. Spain also became a dictatorship under the leadership of General Francisco Franco after the Spanish Civil War. Totalitarian states attempted to achieve total control over their subjects as well as their total loyalty. They held the state above the individual, and were often responsible for some of the worst acts in history, such as the Holocaust Adolf Hitler perpetrated on European Jews, or the Great Purge Stalin perpetrated in the Soviet Union in the 1930s.

"This section provides a conversational overview of World War II in Europe. See main article for a fuller discussion."

Soon after the events in Czechoslovakia, Britain and France issued assurances of protection to Poland, which seemed to be next on Hitler's list. World War II officially began on September 1, 1939, when Hitler unleashed his Blitzkrieg, or lightning war, against Poland. Britain and France, much to Hitler's surprise, immediately declared war upon Germany, but the help they delivered to Poland was negligible. Sixteen days later, Poland was attacked from the East by Soviet Union, acting in a secret alliance with Nazi Germany. After only a few weeks, the Polish forces were overwhelmed, and its government fled to exile in London (see Polish government in Exile).

In starting World War II, the Germans had unleashed a new type of warfare, characterized by highly mobile forces and the use of massed aircraft. The German strategy concentrated upon the devotion of the "Wehrmacht", or German army, to the use of tank groups, called panzer divisions, and groups of mobile infantry, in concert with relentless attacks from the air. Encirclement was also a major part of the strategy. This change smashed any expectations that the Second World War would be fought in the trenches like the first.

As Hitler's forces conquered Poland, the Soviet Union, under General Secretary Joseph Stalin, was acting out guarantees of territory under a secret part of a nonaggression pact between the USSR and Germany known as the Molotov–Ribbentrop Pact. This treaty gave Stalin free rein to take the Baltic republics of Estonia, Latvia, and Lithuania, as well as Eastern Poland, all of which would remain in Soviet possession after the war. Stalin also launched an attack on Finland, which he hoped to reduce to little more than a Soviet puppet state, but the Red Army met staunch Finnish resistance in what became known as the Winter War and succeeded in gaining only limited territory from the Finns. This action would later cause the Finns to ally with Germany when its attack on the Soviet Union came in 1941.

After the defeat of Poland, a period known as the Phony War ensued during the winter of 1939–1940, which saw only limited military land operation on the Western Front. All of this changed on May 10, 1940, when the Germans launched a massive attack on the Low Countries (Belgium, the Netherlands, and Luxembourg), most probably to surmount the Maginot Line of defenses on the Franco-German border. This witnessed the incredible fall of Eben Emael, a Belgian fort considered impregnable and guarded by 600 Belgians, to a force of only 88 German paratroopers. The worst of this was that King Léopold III of Belgium surrendered to the Germans on May 28 without warning his allies, exposing the entire flank of the Allied forces to German panzer groups. Following the conquest of the Low Countries, Hitler occupied Denmark and Norway, beginning on April 9, 1940. Norway was strategically important because of its sea routes which supplied crucial Swedish ore to the Nazi war machine. Norway held on for a few crucial weeks, but Denmark surrendered after only four days. Sweden was the only Scandinavian country to successfully maintain its neutrality throughout the war, with occasional breaches of neutrality in favor of both Germany and the Western Allies.

With the disaster in the Low Countries, France, considered at the time to have had the finest army in the world, lasted only four weeks, with Paris being occupied on June 14. Three days later, Marshal Philippe Pétain surrendered to the Germans. The debacle in France also led to one of the war's greatest mysteries, and Hitler's first great blunder, Dunkirk, where a third of a million trapped British and French soldiers were evacuated by not only British war boats, but every boat the army could find, including fishing rafts. Hitler refused to "risk" his panzers on an action at Dunkirk, listening to the advice of Air Minister Hermann Göring and allowing the "Luftwaffe", or German Air Force, to handle the job. The irony of this was that the escaped men would form the core of the army that was to invade the beaches of Normandy in 1944. Hitler did not occupy all of France, but about three-quarters, including all of the Atlantic coast, allowing Marshal Pétain to remain as dictator of an area known as Vichy France. However, members of the escaped French Army formed around General Charles de Gaulle to create the Free French forces, which would continue to battle Hitler in the stead of an independent France. At this moment, Italy, under Benito Mussolini, declared war on the Allies on June 10, thinking that the war was almost over, but he managed only to occupy a few hundred yards of French territory. Throughout the war, the Italians would be more of a burden to the Nazis than a boon, and would later cost them precious time in Greece.

Hitler now turned his eyes on Great Britain, which stood alone against him. He ordered his generals to draw up plans for an invasion, code named Operation Sea Lion, and ordered the Luftwaffe to launch a massive air war against the British isles, which would come to be known as the Battle of Britain. The British at first suffered steady losses, but eventually managed to turn the air war against Germany, taking down 2,698 German planes throughout the summer of 1940 to only 915 Royal Air Force (RAF) losses. The key turning point came when the Germans discontinued successful attacks against British airplane factories and radar command and coordination stations and turned to civilian bombing known as terror bombing using the distinctive "bomb" sound created by the German dive-bomber, the Stuka. The switch came after a small British bombing force had attacked Berlin. Hitler was infuriated. However, his decision to switch the attacks' focus allowed the British to rebuild the RAF and eventually force the Germans to indefinitely postpone Sea Lion.

The importance of the Battle of Britain is that it marked the first of Hitler's defeats, however its overall impact was overshadowed by his later blunders in the east. Secondly, it marked the advent of radar as a major weapon in modern air war. With radar, squadrons of fighters could be quickly assembled to respond to incoming bombers attempting to bomb civilian targets. It also allowed the identification of the type and a guess at the number of incoming enemy aircraft, as well as tracking of friendly airplanes.

Hitler, taken aback by his defeat over the skies of Britain, now turned his gaze eastward to the Soviet Union. Despite having signed the non-aggression pact with Stalin, Hitler despised communism and wished to destroy it in the land of its birth. He originally planned to launch the attack in early spring of 1941 to avoid the disastrous Russian winter. However, a pro-allied coup in Yugoslavia and Mussolini's almost utter defeat in his invasion of Greece from occupied Albania prompted Hitler to launch a personal campaign of revenge in Yugoslavia and to occupy Greece at the same time. The Greeks would have a bitter revenge of sorts; the attack caused a delay of several crucial weeks of the invasion of the USSR, potentially hampering it.

On June 22, 1941, Hitler attacked Stalin with the largest army the world had ever seen. Over three million men and their weapons were put into service against the Soviet Union. Stalin had been warned about the attack, both by other countries and by his own intelligence network, but he had refused to believe it. Therefore, the Soviet army was largely unprepared and suffered massive setbacks in the early part of the war, despite Stalin's orders to counterattack the Germans. Throughout 1941, German forces, divided into 3 army groups (Army Group A, Army Group B, and Army Group C), occupied the territories of the present day Ukraine and Belarus, laid siege to Leningrad (present day Saint Petersburg), and advanced to within 15 miles of Moscow. At this critical moment, the Soviet people stalled the German "Wehrmacht" to a halt at the gates of Moscow. Stalin had planned to evacuate the city, and had already moved important government functions, but decided to stay and rally the city. Recently arrived troops from the east under the command of Marshal Georgi Zhukov counterattacked the Germans and drove them from Moscow.

Mussolini had launched an offensive in North Africa from Italian-controlled Libya into British-controlled Egypt. However, the British smashed the Italians and were on the verge of taking Libya. Hitler decided to help by sending in a few thousand troops, a "Luftwaffe" division, and the first-rate general Erwin Rommel. Rommel managed to use his small force to repeatedly smash massively superior British forces and to recapture the port city of Tobruk and advance into Egypt. However, Hitler, embroiled in his invasion of the Soviet Union, refused to send Rommel any more troops, causing Rommel to retreat and preventing him from seizing the Middle East, where Axis-friendly regimes had taken root in Iraq and Persia (present-day Iran).

After the winter, Hitler launched a fresh offensive in the spring of 1942, with the aim of capturing the oil-rich Caucacus and the city of Stalingrad. However, he repeatedly switched his troops to where they were not needed. The offensive bogged down, and the entire 6th Army, considered the best of German troops, was trapped in Stalingrad. Hitler now refused to let 6th Army break out. He insisted that the German army would force its way in. Hermann Göring also assured Hitler that the "Luftwaffe" could supply the 6th Army adequately, when it could in reality only supply a minute fraction of the needed ammunition and rations. Eventually, the starved 6th Army surrendered, dealing a severe blow to the Germans. In the end, the defeat at Stalingrad was the turning point for the war in the east.

Meanwhile, the Japanese had attacked the United States at Pearl Harbor in Hawaii on December 7, 1941. This disastrous attack forced the Americans into the war. Parts of the German brass advised against declaring war on the US, arguing that since Japan was the aggressor, the Tripartite Pact didn't bound Germany to do so. However, Hitler hoped that Japan would be able to quickly defeat the US and then turn its attention on helping the effort against Russia. Both Germany and Italy declared war on the United States a few days after the attack.

Throughout the rest of 1942 and 1943, the Soviets began to gain ground against the Germans after some victories like the tank battle of Kursk. By this time, Rommel had been forced to abandon North Africa after a defeat by Montgomery at El Alamein in what was the first decisive victory of the Allies over the Germany army, leading Churchill to declare it "the end of the beginning". On several fronts, the "Wehrmacht" had encountered serious casualties that it could not replace. Hitler also insisted on a "hold at all costs" policy which forbade relinquishing any ground. He followed a "fight to the last man" policy that was completely ineffective. By the beginning of 1944, Hitler had lost all initiative in the Soviet Union, and was struggling even to hold back the tide turning against him.

From 1942 to 1944, the United States and Britain acted in only a limited manner in the European theater, much to the chagrin of Stalin. They drove out the Germans in Africa, invading Morocco and Algeria on November 8, 1942. Then, on July 10, 1943, the Allies invaded Sicily, in preparation for an advance through Italy, the "soft underbelly" of the Axis, as Winston Churchill called it. On September 9, the invasion of Italy began. By the winter of 1943, the southern half of Italy was in Allied hands. The Italians, most of whom did not really support the war, had already turned against Mussolini. In July, he had been stripped of power and taken prisoner, though the Italians feigned continued support of the Axis. On September 8, the Italians formally surrendered, but most of Italy not in Allied hands was controlled by German troops and those loyal to Mussolini's (Mussolini had been freed by German paratroopers) new Italian Social Republic, which in reality consisted of the shrinking zone of German control. The Germans offered staunch resistance, but by June 4, 1944, Rome had fallen.

The Battle of the Atlantic took place from 1942 to 1944 and was described as "longest, largest, and most complex naval battle in history". The Germans hoped to sever the vital supply lines between Britain and America, sinking many tons of shipping with U-boats, German submarines. However, the development of the destroyer and aircraft with a longer patrol range were effective at countering the U-boat threat and by December 1943, the Germans had lost the battle.

On June 6, 1944, the Western Allies finally launched the long-awaited assault on "Fortress Europe" so wanted by Stalin. The offensive, codenamed Operation Overlord, began the early morning hours of June 6. The day, known as D-day, was marked by foul weather. Rommel, who was now in charge of defending France against possible Allied attack, thought the Allies would not attack during the stormy weather, and was on holiday in Germany. Besides this, the Germans were expecting an attack, but at the natural harbor of Calais and not the beaches of Normandy; They did not know about the Allies' artificial harbours, and false leads planted by the Allies suggested Calais as the landing site.

By this time, the war was looking ever darker for Germany. On July 20, 1944, a group of conspiring German officers attempted to assassinate Hitler. The bomb they used did injure him, but the second was not used, and a table shielded Hitler in a stroke of luck. The plotters still could have launched a coup, but only the head of occupied Paris acted, arresting SS and Gestapo forces in the city. The German propaganda minister, Joseph Goebbels, rallied the Nazis and hunted down the conspirators, arresting up to 7000 people in the wake of the plot, according to some estimates.

In France, the Allies took Normandy and finally Paris on August 25. In the east, the Soviets had advanced almost to the former Polish-Soviet border. At this time, Hitler introduced the V-weapons, the V-1 flying bomb and, later, the V-2, the first rockets used in modern warfare. The V-1 was often intercepted by air pilots, but the V-2 was extremely fast and carried a large payload. However, this advance came too late in the war to have any real effect. The Germans were also on the verge of introducing a number of terrifying new weapons, including advanced jet aircraft, which were too fast for ordinary propeller aircraft, and submarine improvements which would allow the Germans to again fight effectively in the Atlantic. All this came too late to save Hitler. Although a September invasion of The Netherlands failed, the Allies made steady advances. In the winter of 1944, Hitler put everything into one last desperate gamble in the West, known as the Battle of the Bulge, which, despite an initial advance, was a failure, because the introduction of new Allied tanks and low troop numbers among the Germans prevented any real action being taken. Nevertheless, it was one of the bloodiest battles of the war and the second costliest battle in the history of the American Army.

In early February 1945, the three Allied leaders, Franklin Roosevelt, Winston Churchill, and Joseph Stalin, met at newly liberated Yalta in the Crimea in the Soviet Union in the Yalta Conference. Here, they agreed upon a plan to divide post-war Europe. Most of the east went to Stalin, who agreed to allow free elections in Eastern Europe, which he never did. The west went to Britain, France, and the U.S. Post-war Germany would be split between the four, as would Berlin. This division of spheres of influence would set up international diplomacy for Cold War that would dominated the second half of the century.

At the beginning of 1945, Hitler was on his last strings. The Soviets launched a devastating attack from Poland into Germany and Eastern Europe, intending to take Berlin. The Germans collapsed in the West, allowing the Allies to fan out across Germany. However, the Supreme Allied Commander, American general Dwight D. Eisenhower, refused to strike for Berlin, and instead became obsessed with reports of possible guerrilla activity in southern Germany, which in reality existed mainly in the propaganda of Joseph Goebbels. By April 25, the Soviets had besieged Berlin. Hitler remained in the city in a bunker under the Chancellery garden. On April 30, he committed suicide by shooting himself, after a ritual wedding with his longtime mistress Eva Braun. The Germans held out another 7 days under Admiral Doenitz, their new leader, but the Germans surrendered unconditionally on May 7, 1945, ending the war in Europe (see V-E Day).

Rivalries that had begun during the war, combined with the sense of strength in the victorious powers, laid the foundations of the Iron Curtain and of the Cold War.

The first event that is usually linked to the later Pacific conflict was the Mukden Incident of 18 September 1931, during which the Japanese military staged the bombing of the South Manchuria Railway and pinned the blame on Chinese dissidents. Japan then used this as pretext to invade northeastern China the next day and turn the region of Manchuria into the Japanese puppet state of Manchukuo. After the League of Nations commissioned the 1932 Lytton Report, which exposed the Mukden Incident as a Japanese ruse, Japan was left internationally isolated, withdrawing from the League of Nations in March 1933. In 1934 Manchukuo became a constitutional monarchy and the former Chinese Emperor Pu Yi was placed on the throne, despite the real power being held by Japan.

Despite relations between Japan and China suffering because the occupation of Manchuria, the situation did not turn into all-out war until 7 July 1937, when a dispute between Chinese and Japanese soldiers turned into a battle during the Marco Polo Bridge Incident. This, in turn, lead to a further escalation of the hostilities, leading to the Second Sino-Japanese War, one of the deadliest conflicts in human history. Japan enjoyed some early major victories, capturing both Shanghai and the Chinese capital of Nanjing in 1937 and forcing the Chinese central government's relocation to Chongqing. Following Chinese victories in Changsha and Guangxi in 1939, the war reached somewhat of a stalemate, with Japan controlling the large cities, but being unable to rule the vast countryside. On 27 September 1940, Japan became cosignatories of the Tripartite Pact, joining a military alliance with Germany and Italy.

On December 7, 1941, Japan launched a surprise attack on the American base at Pearl Harbor as a mean of preventing the U.S. Pacific Fleet from interfering with its planned expansion in the area. The next day, following Roosevelt's Infamy Speech, the United States declared war on Japan, marking the official entry of both nations in World War 2. At the same time, Japan also launched attacks on Thailand, the British colonies of Malaya, Singapore, Hong Kong and American military bases in Guam, Wake Island and the Philippines.

The next six months were dominated by Japanese victories against the Allied forces, already war-weary and stretched thin after two years of war in Europe and North Africa. Japan managed to capture British Burma, New Guinea, the Dutch East Indies, the Solomon Islands, Bali and Timor. In March 1942, days before the surrender of the Philippines, general Douglas MacArthur, who came out of retirement to become commander of United States Army Forces in the Far East at the onset of the war, was forced to flee and narrowly escape to Australia. On 19 February 1942, Japan also launched a devastating aerial attack on the Australian city of Darwin, in what was the first attack by a foreign power on Australian soil.

The turning of the tide came in early May, during the Battle of the Coral Sea, which proved the first strategic victory of the Allies against Japanese expansion. This was followed one month later by the Battle of Midway, in which the US Navy defeated an attacking Japanese fleet, inflicting devastating damage on the attacking force. The Battle of Midway is widely considered the most important naval battle of the Pacific Campaign and has even been described as "the most stunning and decisive blow in the history of naval warfare" The reasons usually given for the resounding victory are Japanese overconfidence in their own forces, poor planning and the intercepting of Japanese battle plans by the United States. The losses suffered during this Battle of Midway, paired with the campaign of attrition that was taking place in Solomon Islands, left Japan unable to replenish its forces and turned the tides of war against it. By August 1942, the Allied forces were on the offensive, earning victories in the New Guinea campaign at Guadalcanal, Milne Bay and Buna-Gona.

Over the next two years the Allies slowly captured one island base after another, trying to get closer to Japan itself, where it planned to launch massive strategic air attacks and, only if absolutely necessary, execute a ground invasion. By July 1944, the US recaptured Guam and captured Tinian, which finally put them in attacking range of mainland Japan using the new Boeing B-29 bombers. Despite the advances of the Allies, massive fighting was still taking place, both in South-East Asia with the Japanese offensive in India and in the Pacific, with the Battle of the Philippine Sea, where Japan suffered major losses and definitively lost the ability to rely on aircraft carriers. In later October, a combined attack of American and Australian forces took on the Imperial Japanese Navy during the Battle of Leyte Gulf in what was to be the largest naval battle of World War II and possibly the largest naval battle of all time. The Allied victory achieved its stated goal of further severing oil supplied to the Japanese army, which was already struggling to fuel the remainder of its naval power. The battle of Leyte Gulf is also notable as the first battle in which Japanese aircraft carried out organized "kamikaze" attacks.

By early 1945, the Americans set their eyes on the Japanese island of Iwo Jima, which was of great strategic importance, being situated halfway between Tokyo and the Mariana Islands. Capturing the island meant not only a better launch station for aerial attacks against mainland Japan, but also the prevention of its use as an early-warning station against air raids. The Japanese were expecting an attack and have been heavily fortifying the entire island for over an year. Despite the American troops outnumbering the Japanese to a rate of more than 3 to 1, the USA suffered heavy casualties in the 36 days of fighting, with over 6,800 Marines killed and another 20,000 wounded, leading some historians to question the strategic worth of the action. With Japan struggling to defend the homeland islands, counteroffensives in other parts of the Japanese Empire became more feasible, with important battles taking place in Burma, Borneo and China.

Continuing to come nearer the main Japanese islands, on 1 April 1945 the US Marines mounted the largest amphibious assault of the Pacific Theater on the island of Okinawa. After a long campaign of island hopping, the Allies were planning to use Kadena Air Base on the large island of Okinawa as a base for Operation Downfall, the planned invasion of the Japanese home islands which were only 340 miles away. The battle was one of the bloodiest in the Pacific, with approximately 160,000 casualties on both sides: at least 75,000 Allied and 84,166–117,000 Japanese. Around half of the island's population of 300,000 were killed, committed suicide or went missing.
The assaults on both Iwo Jima and Okinawa proves incredibly costly in terms of American lives and president Truman was horrified at the prospect of Operation Downfall, a planned boots-on-the-ground invasion of mainland Japan that was estimated to lead to over a million casualties among American soldiers. Despite a devastating campaign of fire-bombing in Tokyo and dozens of other cities, the Japanese showed no sign of planning to surrender. Following the Potsdam Declaration of July 26, where the Allies threatened "prompt and utter destruction" should Japan not surrender, the decision was made to resort to the first use of atomic bombs. On 6 August, the 393d Bombardment Squadron B-29 "Enola Gay", took off from Tinian and dropped the bomb on the city of Hiroshima. Three days later, a second bomb was dropped on Nagasaki. The two bombing killed 129,000–226,000 people, most of them civilians. On 9 August, the Soviet Union also invaded Manchukuo in what was to be the last campaign of the war.

On 10 August the "sacred decision" was made by Japanese Cabinet to accept the Potsdam terms and on 15 August Emperor Hirohito broadcast to the nation and to the world at large the rescript of surrender, ending the Second World War. The formal Japanese Instrument of Surrender was signed on 2 September 1945, on the battleship USS "Missouri", in Tokyo Bay. The surrender was accepted by General Douglas MacArthur as Supreme Commander of the Allied Powers.

The Holocaust (which roughly means "great fire") was the deliberate, systematic murder of millions of Jews and other minorities during World War II by the Nazi regime in Germany. Several differing views exist regarding whether it was intended to occur from the war's beginning, or if the plans for it came about later. Regardless, persecution of Jews extended well before the war even started, such as in the "Kristallnacht" (literally "Crystal Night", Night of Broken Glass). The Nazis used propaganda to great effect to stir up anti-Semitic feelings within ordinary Germans.

After the conquest of Poland, the Third Reich, which had previously deported Jews and other "undesirables", suddenly had within its borders the largest concentration of Jews in the world. The solution was to round up Jews and place them in Nazi concentration camps or in ghettos, cordoned off sections of cities where Jews were forced to live in deplorable conditions, often with tens of thousands starving to death, and the bodies decaying in the streets. As appalling as this sounds, they were the lucky ones. After the invasion of the Soviet Union, armed killing squads of SS men known as "Einsatzgruppen" systematically rounded up Jews and murdered an estimated one million Jews within the country. As barbaric and inhuman as this seems, it was too slow and inefficient by Nazi standards.

In 1942, the top leadership met in Wannsee, a suburb of Berlin, and began to plan a more efficient way to slaughter the Jews. The Nazis created a system of extermination camps throughout Poland, and began rounding up Jews from the Soviet Union, and from the Ghettos. Not only were Jews shot or gassed to death en masse, but they were forced to provide slave labor and they were used in horrific medical experiments (see Human experimentation in Nazi Germany). Out of the widespread condemnation of the Nazis' medical experiments, the Nuremberg Code of medical ethics was devised.

The Nazis took a sadistic pleasure in the death camps; the entrance to the most notorious camp, Auschwitz, stated ""Arbeit Macht Frei""—"Work Sets You Free". In the end, six million Jews and up to 5 million homosexuals, Jehovah's Witnesses, Roma and political prisoners were killed by various means, mainly in the death camps. An additional several million Soviet and other Allied prisoners of war died in camps and holding areas.

There is some controversy over whether ordinary Germans knew about the Holocaust. It appears that many Germans knew about the concentration camps; such things were prominently displayed in magazines and newspapers. In many places, Jews had to walk past towns and villages on their way to work as slaves in German industry. In any case, Allied soldiers reported that the smell of the camps carried for miles. A very small number of people deny the Holocaust occurred entirely, though these claims have been routinely discredited by mainstream historians.

During the 1930s, innovations in physics made it apparent that it could be possible to develop nuclear weapons of incredible power using nuclear reactions. When World War II broke out, scientists and advisors among the Allies feared that Nazi Germany may have been trying to develop its own atomic weapons, and the United States and the United Kingdom pooled their efforts in what became known as the Manhattan Project to beat them to it. At the secret Los Alamos laboratory in New Mexico, scientist Robert Oppenheimer led a team of the world's top scientists to develop the first nuclear weapons, the first of which was tested at the Trinity site in July 1945. However, Germany had surrendered in May 1945, and it had been discovered that the German atomic bomb program had not been very close to success.The Allied team produced two nuclear weapons for use in the war, one powered by uranium-235 and the other by plutonium as fissionable material, named "Little Boy" and "Fat Man". These were dropped on the Japanese cities of Hiroshima and Nagasaki on August 6 and 9, 1945 each. This, in combination with the Soviet invasion of Japanese-controlled territory, convinced the Japanese government to surrender unconditionally. These two weapons remain the only two nuclear weapons ever used against other countries in war.

Nuclear weapons brought an entirely new and terrifying possibility to warfare: a nuclear holocaust. While at first the United States held a monopoly on the production of nuclear weapons, the Soviet Union, with some assistance from espionage, managed to detonate its first weapon (dubbed "Joe-1" by the West) in August 1949. The post-war relations between the two, which had already been deteriorating, began to rapidly disintegrate. Soon the two were locked in a massive stockpiling of nuclear weapons. The United States began a crash program to develop the first hydrogen bomb in 1950, and detonated its first thermonuclear weapon in 1952. This new weapon was alone over 400 times as powerful as the weapons used against Japan. The Soviet Union detonated a primitive thermonuclear weapon in 1953 and a full-fledged one in 1955.

The conflict continued to escalate, with the major superpowers developing long-range missiles (such as the ICBM) and a nuclear strategy which guaranteed that any use of the nuclear weapons would be suicide for the attacking nation (Mutually Assured Destruction). The creation of early warning systems put the control of these weapons into the hands of newly created computers, and they served as a tense backdrop throughout the Cold War.

Since the 1940s there were concerns about the rising proliferation of nuclear weapons to new countries, which was seen as being destabilizing to international relations, spurring regional arms races, and generally increasing the likelihood of some form of nuclear war. Eventually, nine nations would overtly develop nuclear weapons, and still maintain stockpiles today: the United States, the Soviet Union (and later Russia would inherit these), the United Kingdom, France, China, India, Pakistan, Israel and North Korea. South Africa developed six crude weapons in the 1980s (which it later dismantled), and Israel almost certainly developed nuclear weapons though it never confirmed nor denied it. The creation of the Nuclear Non-proliferation Treaty in 1968 was an attempt to curtail such proliferation, but a number of countries developed nuclear weapons since it was signed (and many did not sign it), and a number of other countries, including Libya and Iran were suspected of having clandestine nuclear weapons programs.

Following World War II, the majority of the industrialized world lay in ruins as a result of aerial bombings, naval bombardment, and protracted land campaigns. The United States was a notable exception to this; barring Pearl Harbor and some minor incidents, the U.S. had suffered no attacks upon its territory. The United States and the Soviet Union, which, despite the devastation of its most populated areas, rebuilt quickly, found themselves the world's two dominant superpowers.

Much of Western Europe was rebuilt after the war with assistance from the Marshall Plan. Germany, chief instigator of the war, was placed under joint military occupation by the United States, Great Britain, France, and the Soviet Union. Berlin, although in Soviet-controlled territory, was also divided among the four powers. Occupation of Berlin would continue until 1990. Japan was also placed under U.S. occupation, that would last seven years, until 1952. Oddly, these two Axis powers, despite military occupation, soon rose to become the second (Japan) and third (West Germany) most powerful economies in the world.

Following the end of the war, the Allies famously prosecuted numerous German officials for war crimes and other offenses in the Nuremberg Trials. Although Adolf Hitler had committed suicide, many of his cronies, including Hermann Göring, were convicted. Less well-known trials of other Axis officials also occurred, including the Tokyo War Crime Trial.

The failure of the League of Nations to prevent World War II essentially discredited the organization, and it was dissolved. A new attempt at world peace was begun with the founding of the United Nations on October 24, 1945, in San Francisco. Today, nearly all countries are members, but despite its many successes, the organization's success at achieving its goal of world peace is disputed. The organization was never given enough power to overcome the conflicting interests and priorities of its member nations.

Almost all of the major nations that were involved in World War II began shedding their overseas colonies soon after the conflict. The tactics employed by the revolutionaries ranged from non-violent forms of protest to armed rebellions, depending on the nation involved. Immediately after the war, European powers began a decades-long process of withdrawing from their possessions in Africa and Asia.

In India, Mahatma Gandhi became a global icon for his non-violent struggle to achieve Indian independence. This was achieved in 1947 with the end of British rule in India and the partition of the territory into modern day India and Pakistan, which would later also further divide, leading to the creation of the People's Republic of Bangladesh in 1971. Elsewhere in Asia, The United States granted independence to the Philippines, its major Pacific possession in 1946. In French Indochina, armed insurrections forced the French out in the early 1950s, leading to the formation of Laos, Cambodia and Vietnam.

In Africa, nationalists such as Jomo Kenyatta of Kenya and Kwame Nkrumah of Ghana led their respective nations to independence from foreign rule. Mere decades before, the British Empire controlled almost half of the continent, but by 1968, the only British possession in Africa was Seychelles (which would also become independent in 1976). Between 1956 and 1962, almost 20 African countries achieved their independence from France. Through the efforts of Amílcar Cabral and others, the Portuguese colonies of Guinea-Bissau, Cape Verde, Angola, Mozambique and São Tomé and Príncipe followed suit and achieved independence during the mid-1970s, in what was to be the last important wave of African decolonization.

In 1946, there were 35 member states in the United Nations, but as the newly independent nations joined the organization, by 1970 membership increased to 127.

The emergence of newly independent countries in Africa and Asia that used the borders of the imperial partitions later led to further conflict. In many cases this meant that historically antagonistic ethnic or religious groups now needed to share the same country or that several nations held territorial claims over regions which were more or less arbitrarily divided by European powers. Some conflicts which aroused from these tensions, especially the Nigerian Civil War, the Second Congo War, the Second Sudanese Civil War and the Bangladesh Liberation War have been among the bloodiest wars of the 20th Century.

During the Yalta Conference, where the Western, capitalist powers, and the communist Soviet Union agreed on separate spheres of influence in Europe, they set up the stage for a geopolitical rivalry that would dominate international relations for the next five decades. In March 1946, Winston Churchill gave a now famous speech while visiting Westminster College in the US which is usually credited as the first use of the term iron curtain to refer to the separation of Soviet and Western areas of influence in Europe:From Stettin in the Baltic to Trieste in the Adriatic an iron curtain has descended across the Continent. Behind that line lie all the capitals of the ancient states of Central and Eastern Europe. Warsaw, Berlin, Prague, Vienna, Budapest, Belgrade, Bucharest and Sofia; all these famous cities and the populations around them lie in what I must call the Soviet sphere, and all are subject, in one form or another, not only to Soviet influence but to a very high and in some cases increasing measure of control from Moscow.Indeed, the Soviet Union have already annexed several countries as Soviet Socialist Republics during the war. Eastern Poland (incorporated into the Byelorussian SSR and the Ukrainian SSR), Latvia (which became the Latvian SSR), Estonia (which became the Estonian SSR), Lithuania ( which became the Lithuanian SSR), part of eastern Finland (which became the Karelo-Finnish SSR) and eastern Romania (which became the Moldavian SSR) were now wholly part of Soviet Union. Furthermore, between 1945 and 1949, Yugoslavia, Albania, Bulgaria, Poland, Romania, Czechoslovakia, Hungary and East Germany saw communist regimes coming to power and their transformation into People's Republics. While they remained independent countries and their relation with the Soviet authorities would fluctuate over the next half century, they were widely seen as Soviet satellite states. Outside of Europe, countries that would see the rise of communism and would ally themselves to the communist block include Mongolia, China, North Korea, Cuba and Vietnam.

The spread of the communist ideology in general and the Soviet influence in particular made Western leaders nervous and it even lead to Churchill considering a preemptive attack on the USSR even before World War II was formally ended. Fearful of a possible invasion of Western Europe by Soviet forces or, more realistically, Soviet subversion of Western governments, the US set up clandestine "stay-behind" operations of armed resistance in collaboration with Western European leaders, which later evolved into Operation Gladio.

The two rival blocks also coalesced into formal mutual defense organizations, with the formation of the North Atlantic Treaty Organization (NATO) in 1949 and the Warsaw Pact in 1955, each determine to expand its own influence among unaligned countries and limit the influence of their rivals.

Two wars and a near-war in the 1950s became the focus for capitalist versus communist struggle. The first war was the Korean War, fought between People's Republic of China-backed North Korea and mainly United States-backed South Korea. The Korean Peninsula was a Japanese colony between 1910 and 1945, when Soviet and American troops invaded and divided it along the 38th parallel. A communist government controlled the territory north of the border and a capitalist one controlled the South, with both authorities considering the other one illegitimate and claiming sovereignty over the entire peninsula. North Korea's invasion of South Korea on June 25, 1950 led to United Nations intervention. General Douglas MacArthur led troops from the United States, Canada, Australia, Great Britain, and other countries in repulsing the Northern invasion. However, the war reached a stalemate after Chinese intervention pushed U.N. forces back, and an ended hostilities in July 1953, leaving the two Koreas divided and tense for the rest of the century.

The second war, the Vietnam War, was perhaps the second most visible war of the 20th century, after World War II. After the French withdrawal from its former colony, on 21 July 1954, Vietnam became partitioned into two halves, much like Korea, along the 17th parallel. Fighting between North and South eventually escalated into a regional war.The United States provided aid to South Vietnam and contributed to propaganda efforts against the North, but was not directly involved until the Gulf of Tonkin Resolution, passed in reaction to a supposed North Vietnamese attack upon American destroyers, brought the U.S. into the war as a belligerent. The war was initially viewed as a fight to contain communism (see containment, Truman Doctrine, and Domino Theory), but, as more Americans were drafted and news of events such as the Tet Offensive and My Lai massacre leaked out, American sentiment turned against the war. U.S. President Richard Nixon was elected partially on claims of a "secret plan" to stop the war. This Nixon Doctrine involved a gradual pullout of American forces; South Vietnamese units were supposed to replace them, backed up by American air power. The plan went awry, with Nixon deliberately sabotaging peace talks for political gain, and the war spilled into neighboring Cambodia while South Vietnamese forces were pushed further back. Eventually, the U.S. and North Vietnam signed the Paris Peace Accords, ending U.S. involvement in the war. With the threat of U.S. retaliation gone, the North proceeded to violate the ceasefire and invaded the South with full military force. Saigon was captured on April 30, 1975, and Vietnam was unified under Communist rule a year later, effectively bringing an end to one of the most unpopular wars of all time.

The Cuban Missile Crisis illustrates just how close to the brink of nuclear war the world came during the Cold War. Cuba, under Fidel Castro's socialist government, had formed close ties with the Soviet Union. This was obviously disquieting to the United States, given Cuba's proximity. When Lockheed U-2 spy plane flights over the island revealed that Soviet missile launchers were being installed, U.S. President John F. Kennedy instituted a naval blockade and publicly confronted the Soviet Union. After a tense week, the Soviet Union backed down and ordered the launchers removed, not wanting to risk igniting a new world war.

With Cold War tensions running high, the Soviet Union and United States took their rivalry to the stars in 1957 with the Soviet launch of Sputnik. A "space race" between the two powers followed. Although the USSR reached several important milestones, such as the first craft on the Moon (Luna 2) and the first human in space (Yuri Gagarin), the U.S. pulled ahead eventually with its Mercury, Gemini, and Apollo programs, which culminated in Apollo 11's manned landing on the moon on 20 July 1969. Five more manned landings followed (Apollo 13 was forced to abort its mission). Nevertheless, despite its successes, the U.S. space program could not match many major achievements of the Soviet space program, such as unmanned rover-based space exploration and image and video transfer from the surface of another planet, until the early 21st century.

In addition, both countries launched numerous probes into space, such as the Venera 7 and Voyager 2.

In later decades, space became a somewhat friendlier place. Regular manned space flights were made possible with the American space shuttle, which was the first reusable spacecraft to be successfully used. Mir and Skylab enabled prolonged human habitation in space. In the 1990s, work on the International Space Station began, and by the end of the century, while still incomplete, it was in continual use by astronauts from the United States, Europe, Russia, Japan, and Canada.

By the 1980s, the Soviet Union was weakening. The Sino-Soviet split had removed the USSR's most powerful ally, the People's Republic of China. Its arms race with the U.S. was draining the country of funds, and further weakened by internal pressures, ethnic and political. Mikhail Gorbachev, its last leader, attempted to reform the country with "glasnost" and "perestroika", but the formation of Solidarity, the fall of the Berlin Wall, and the breaking-off of several Soviet republics, such as Lithuania, started a slippery slope of events that culminated in a coup to overthrow Gorbachev, organized by Communist Party hard-liners. Boris Yeltsin, president of Russia, organized mass opposition, and the coup failed. On December 26, 1991, the Soviet Union was officially disbanded into its constituent republics, thus putting a final line under the already exhausted Cold War.

The creation of the transistor revolutionized the development of the computer. The first computers, room-sized electro-mechanical devices built to break cryptographical codes during World War II, quickly became at least 20 times smaller using transistors. Computers became reprogrammable rather than fixed-purpose devices. The invention of programming languages meant computer operators could concentrate on problem solving at a high-level, without having to think in terms of the individual instructions to the computer itself. The creation of operating systems also vastly improved programming productivity. Building on this, computer pioneers could now realize what they had envisioned. The graphical user interface, piloted by a computer mouse made it simple to harness the power of the computer and made it more accessible to new users. Storage for computer programs progressed from punched cards and paper tape to magnetic tape, floppy disks and hard disks. Core memory and bubble memory fell to random access memory.

The invention of the word processor, spreadsheet and database greatly improved office productivity over the old paper, typewriter and filing cabinet methods. The economic advantage given to businesses led to economic efficiencies in computers themselves. Cost-effective CPUs led to thousands of industrial and home-brew computer designs, many of which became successful; a home-computer boom was led by the Apple II, the ZX80 and the Commodore PET.
IBM, seeking to embrace the microcomputer revolution, devised its IBM Personal Computer (PC). Crucially, IBM developed the PC from third-party components that were available on the open market. The only impediment to another company duplicating the system's architecture was the proprietary BIOS software. Other companies, starting with Compaq, reverse engineered the BIOS and released PC compatible computers that soon became the dominant architecture. Microsoft, which produced an operating system for the PC, rode this wave of popularity to become the world's leading software company.

The 1980s heralded the Information Age. The rise of computer applications and data processing made ethereal "information" as valuable as physical commodities. This brought about new concerns surrounding intellectual property issues. The U.S. Government made algorithms patentable, forming the basis of software patents. The controversy over these and proprietary software led Richard Stallman to create the Free Software Foundation and begin the GNU Project, paving the way for a free software movement.

Computers also became a usable platform for entertainment. Computer games were first developed by software programmers exercising their creativity on large systems at universities, but these efforts became commercially successful in arcade games such as "Pong" and "Space Invaders". Once the home computer market was established, young programmers in their bedrooms became the core of a youthful gaming industry. In order to take advantage of advancing technology, games consoles were created. Like arcade systems, these machines had custom hardware designed to do game-oriented operations (such as sprites and parallax scrolling) in preference to general purpose computing tasks.Computer networks appeared in two main styles; the local area network, linking computers in an office or school to each other, and the wide area network, linking the local area networks together. Initially, computers depended on the telephone networks to link to each other, spawning the Bulletin Board sub-culture. However, a DARPA project to create bomb-proof computer networks led to the creation of the Internet, a network of networks. The core of this network was the robust TCP/IP network protocol. Thanks to efforts from Al Gore, the Internet grew beyond its military role when universities and commercial businesses were permitted to connect their networks to it. The main impetus for this was electronic mail, a far faster and convenient form of communication than a conventional letter and memo distribution, and the File Transfer Protocol (FTP). However, the Internet remained largely unknown to the general public, who were used to Bulletin Boards and services like Compuserve and America Online. This changed when Tim Berners-Lee devised a simpler form of Vannevar Bush's hypertext, which he dubbed the World Wide Web. "The Web" suddenly changed the Internet into a printing press beyond the geographic boundaries of physical countries; it was termed "cyberspace". Anyone with a computer and an Internet connection could write pages in the simple HTML format and publish their thoughts to the world.

The Web's immense success also fueled the commercial use of the Internet. Convenient home shopping had been an element of "visions of the future" since the development of the telephone, but now the race was on to provide convenient, interactive consumerism. Companies trading through websites became known as "dot coms", due to the ".com" suffix of commercial Internet addresses.

It was a long hurtful century and by the end of the century, more technological advances had been made than in all of preceding history. Communications and information technology, transportation technology, and medical advances had radically altered daily lives. Europe appeared to be at a sustainable peace for the first time in recorded history. The people of the Indian subcontinent, a sixth of the world population at the end of the century, had attained an indigenous independence for the first time in centuries. China, an ancient nation comprising a fifth of the world population, was finally open to the world in a new and powerful synthesis of west and east, creating a new state after the near-complete destruction of the old cultural order. With the end of colonialism and the Cold War, nearly a billion people in Africa were left with truly independent new nation states, some cut from whole cloth, standing up after centuries of foreign domination.

The world was undergoing its second major period of globalization; the first, which started in the 19th century, having been terminated by World War I. Since the US was in a position of almost unchallenged domination, a major part of the process was Americanization. This led to anti-Western and anti-American feelings in parts of the world, especially the Middle East. The influence of China and India was also rising, as the world's largest populations, long marginalized by the West and by their own rulers, were rapidly integrating with the world economy.

However, several problems faced the world. The gap between rich and poor nations continued to widen. Some said that this problem could not be fixed, that there was a set amount of wealth and it could only be shared by so many. Others said that the powerful nations with large economies were not doing enough to help improve the rapidly evolving economies of the Third World. However, developing countries faced many challenges, including the scale of the task to be surmounted, rapidly growing populations, and the need to protect the environment and the cost that goes along with it.

Terrorism, dictatorship, and the spread of nuclear weapons were other issues requiring attention. The world was still blighted by small-scale wars and other violent conflicts, fueled by competition over resources and by ethnic conflicts. Despots such as Kim Jong-il of North Korea continued to lead their nations toward the development of nuclear weapons.

Disease threatened to destabilize many regions of the world. New viruses such as SARS and West Nile continued to spread. In poor nations, malaria and other diseases affected the majority of the population. Millions were infected with HIV, the virus which causes AIDS. The virus was becoming an epidemic in southern Africa.

Perhaps most importantly, it was speculated that in the long term, environmental problems threatened the planet's livability. The most serious problem was global warming, which was predicted to frequently flood coastal areas, due to human-caused emission of greenhouse gases, particularly carbon dioxide produced by the burning of fossil fuels. This prompted many nations to negotiate and sign the Kyoto treaty, which set mandatory limits on carbon dioxide emissions.

The celebration of the ending of the 20th century and the beginning of the 21st century was at New Year's Day, 2000.

In 1953, shortly after the Americans tested a thermonuclear weapon in 1952, followed by the Soviets with Joe 4, and before the UK government took a decision in July 1954 to develop a thermonuclear weapon, the Atomic Weapons Research Establishment at Aldermaston was asked about the possibilities for a very large pure fission bomb with a yield of one megaton. This study referred to the Zodiak Mk.3 bomb, but progressed no further than a rudimentary study. At this time studies were also started that ultimately led to a decision in 1954 to develop a thermonuclear weapon, and the design studies were split into two tracks because the British at that time had not yet discovered the Teller-Ulam technique necessary to initiate fusion. One track led to an intermediate design, the so-called Type A thermonuclear design, similar to the Alarm Clock and layer cake hybrid designs of other nuclear powers; although these designs are now regarded as large boosted fission weapons, and no longer regarded as thermonuclear weapons that derive a very large part of their energy from a fusion reaction, designated by the British as Type B, but as hybrids. The British hybrid weapon was known as Green Bamboo, weighed approx 4,500 lb (2,045 kg) and its spherical shape measured approx 45 inches diameter with a 72-point implosion system. This Green Bamboo weapon was intended as the warhead for all projected British strategic delivery systems of the period; the Yellow Sun Stage 1 air-dropped bomb, and the Blue Steel air-launched stand-off missile. The large girth of these weapons was designed to accommodate the diameter of Green Bamboo's implosion sphere.

Violet Club

Violet Club was a nuclear weapon deployed by the United Kingdom during the Cold War. It was Britain's first operational "high yield" weapon, and was intended to provide an emergency capability until a thermonuclear weapon could be developed from the 1956–1958 Operation Grapple tests.

A variant with a smaller implosion sphere, fewer explosive lenses, and some other changes was intended for the Blue Streak missile that was unable to carry Green Bamboo. However, the reduced size was achieved by reducing the size of the surrounding HE layers, and this resulted in less compression at the fissile core when detonated, and a reduced nuclear efficiency, and yield; meaning that less of the fissile material was consumed before the bomb blew itself apart. To compensate for the reduced compression more fissile material was required to maintain yield at the desired level, and this in turn made excessive demands on scarce and expensive fissile material. Estimates computed from reliable sources of actual core cost and cost per kilogram of HEU put the core sizes of Green Bamboo and Orange Herald as 98 kg and 125 kg respectively, although some other published (and unverified) sources claim lower figures of 87 kg and 117 kg respectively. This design later became known as Orange Herald, and was tested at Christmas Island, yielding 720 kt. These two weapons, Green Bamboo and Orange Herald were intended as the two hybrid weapon predecessors of the first British thermonuclear weapons, based on the Granite design series that began with Short Granite, Purple Granite, Grapple X, Grapple Y, and Flagpole and Halliard at Grapple Z.

Delays and failures in the Granite programme and the abandonment of Green Bamboo without a test, left a gap in the programme and an emergency capability weapon to fill that gap was devised from elements of both Green Bamboo and Orange Herald; being known as Knobkerry, or Green Grass, and the Interim Megaton Weapon. There were also other factors involved in the decision to build an Interim Megaton Weapon. One being that the UK HEU production programme was by 1957 producing quantities of HEU for which there was no immediate need; and the Chiefs of Staff were reluctant to see it continue to languish in stores, unused for weapons, when it was being produced at great cost. The Green Grass warhead containing perhaps 70–86 kg of HEU (although there are no reliable declassified sources for this figure) was therefore hurriedly produced and installed in a modified Blue Danube casing, to be known as Violet Club, until a better solution based on the Yellow Sun casing could be produced, known then with Green Grass installed, as Yellow Sun Mk.1. Only five Violet Club weapons were produced, known by the RAF description of Bomb, Aircraft, HE 9,000 lb HC, and they were stripped of their Green Grass warheads for transfer to the better casings when these became available.

Violet Club (and to a lesser degree Yellow Sun Mk.1) was not considered a satisfactory design and suffered from numerous design defects. An implosion design, the fissile core of the weapon was a hollow sphere of highly enriched uranium (HEU) which was surrounded by a high-explosive supercharge and 72-lens implosion system. The HEU core was greater than one uncompressed critical mass and to maintain it in a sub-critical condition AWRE fashioned it into a hollow thin-walled sphere. The HEU sphere was collapsed inwards by the supercharge and 72 explosive lenses. However, a fire in the bomb store or a traffic collision on the airfield could easily lead to a partial crushing or collapse of the unremovable uranium shell, and in turn a spontaneous nuclear chain reaction. AWRE responded by inserting (though a hole in the shell) a rubber bag, rather akin to an outsize female condom, and filled this with 20,000 steel ball bearings of, weighing around. Subsequently, the number of these steel balls was increased to 133,000, with a reduction in size to approx diameter. The balls were retained in the device by sealing the hole with a plastic bung.

The steel balls were intended to prevent a nuclear detonation even if the explosives fired accidentally, or in any conceivable accident. The ball bearings had to be removed through the hole in the bomb casing during flight preparation, and after the bomb was winched into the aircraft. The ball bearings then had to be re-inserted into the lowered and upturned bomb before transport back to the bomb store. Quite obviously, without the ball bearings installed, these weapons were armed and live, and too dangerous to allow to be flown on exercises. Bomber Command exercises demonstrated that flight preparation followed by a scramble take-off could not be reduced below thirty minutes, and on exercises in bad weather and at night a ninety-minute scramble was the norm. At least one accident, dated 1960, was reported in the press when the plastic bung was removed and 133,000 steel ball bearings exited onto the aircraft hangar floor, leaving the bomb armed and vulnerable. The Royal Air Force were so nervous of the outcome of a fire in storage that permission was sought to store the bombs inverted, so that a loss of the plastic bung could not end with the steel balls on the floor, leaving the HEU unprotected against a subsequent explosion. Even without the partial nuclear detonation feared by the RAF, there was "a risk of catastrophe".

AWRE's trickery with the steel balls had other unintended outcomes. Even an event as inconsequential as early morning frost was an issue. Violet Club could be loaded into a bomber for up to thirty days on standby while parked overnight on a remote base where the bomb could get very cold. If the steel balls froze together inside the bomb cavity and could not be removed, the bomb was useless. AWRE's solution was to fit the bomb with an electric blanket. Because the bombs were armed before flight, the take-off was hazardous; the bombs could not be jettisoned, and landing with an armed bomb on return to base was too hazardous to contemplate. As a consequence, Violet Club could not be used on an airborne alert, or even flown to a remote dispersal base.

Other design flaws centred on a requirement for a strip-down and inspection at six-monthly intervals, this took three weeks per weapon using AWRE civilian staff. The unstable nature of the bomb required that the work be done in-situ at RAF facilities, causing considerable disruption to operational duties. There were three main reasons for the strip-down and these were deterioration of the rubber bag, corrosion of the steel balls, and deterioration of the HE, which was prone to cracking. Replacement of a weapon's HE would cost the RAF in excess of £90,000 adjusted to 2006 prices.

Green Grass/Violet Club was the first UK-deployed weapon to dispense with the crude polonium and beryllium impact or crush type initiators used in Fat Man and other early US weapons together with the British Blue Danube and Red Beard. Instead it used an electronic neutron initiator (ENI) known as Blue Stone. This had the great advantage of being adjustable, allowing the neutron burst to be triggered at precisely the right moment. The burst heights of Green Grass were optimized for either maximum overpressure without allowing fireball contact with the ground, or to maximise the ground area subjected to a 6 psi overpressure; respectively and above ground level A barometric fuze was used, backed-up by an adjustable clockwork timer fuze with impact fuses as a final fail-safe. Most of these mechanisms originated from Blue Danube although the radar altimeter fuze was omitted. The electrical power for these fuzing mechanisms and the firing mechanisms of the 72-lens, implosion device came from lead-acid accumulators located in the tail of the Blue Danube casing. These were commercially sourced six-volt motorcycle batteries.

Green Grass struggled to meet the Chiefs of Staff requirement for an Interim Megaton [yield] Weapon. It was never tested, and initially, AWRE estimated its yield at 500 kt based on the Orange Herald test of 720kt, and non-nuclear tests of the HE implosion sphere fitted with non-fissile cores. A Mr Challens of AWRE who later became the Director of AWRE then claimed to the Air Staff that
A play-on-words that later returned to haunt AWRE when later estimates revised the yield to 400kt. Challens also stated on behalf of AWRE that
The Royal Air Force was not amused, with Bomber Command Staff officers minuting their seniors with remarks like this one.
Summing up, senior levels of the RAF believed they had been sold a 'lemon', and this further extract from the archives does much to illustrate the uncertain safety of the weapon in the hands of the inexperienced RAF.

The fifth and last Violet Club weapon was due for delivery to the RAF by the end of May 1959 and all were withdrawn by 1960, but in their Yellow Sun Mk.1 form they survived until 1963, when all had been replaced with Yellow Sun Mk.2 with Red Snow warheads installed. They were only used by the Avro Vulcan bomber.

These very large and dirty fission bombs were the largest pure-fission bombs deployed by any state, and unlike their predecessors, Blue Danube and Red Beard, they used HEU as a fissile material rather than plutonium, the reason being primarily economic. The cost of HEU to the Royal Air Force was (at 1958-9 prices) £19,200 per kg, with plutonium priced at £143,000 per kg. Although a HEU weapon needed more fissile material for a given yield than a plutonium weapon, a saving per weapon was of the order of £22.7M at 2006 prices. Thirty-seven Green Grass warheads were built (twelve as Violet Club) saving the Treasury £840M. The influence of the Treasury on weapons procurement should not be underestimated: it reaches even into weapons design. Clearly, there were economic benefits in building large and dirty U-235 fission bombs, rather than cleaner, but more expensive plutonium weapons, especially given the shortage of plutonium. By 1958 Britain's accumulated production of plutonium was only 472.2 kg and a proportion of that was bartered to the United States in exchange for HEU and other items. Up to 1958, British output of HEU was only 860 kg, while the United States supplied the UK with approximately seven tons of HEU from their less costly production process. So HEU used for Green Grass was purchased cheaply from the U.S. while selling to the U.S. the unwanted and high-priced weapons-grade plutonium.

Knobkerry, alias Green Grass, alias Interim Megaton Weapon, alias Violet Club and Yellow Sun Mk.1, had one other distinction. It was the last entirely British nuclear weapon deployed with the UK Armed Services. The British Operation Grapple thermonuclear weapon tests at Christmas Island in 1957-58 were the end of evolution. There were no more wholly home-grown designs. Britain never deployed a true thermonuclear weapon of wholly home-grown design. All the weapons tested at Operation Grapple were abandoned, because AWRE no longer needed them, although some of their features were undoubtedly incorporated into later weapons. These Granite-type devices were all experimental devices, needing to be developed further into reliable Service-engineered warheads at considerable cost in time and money. The U.S. designs offered after 1958 were fully tested and engineered, and cheap to produce. They were manufactured in Britain from British materials and U.S. blueprints. They were British property and there were no American political constraints on their use, they were a favourable deal for the Treasury, and Violet Club and Yellow Sun Mk.1 bridged the gap until the American designs could be manufactured.