When were nuclear weapons first used in war?

Research into the development of nuclear weapons was initially undertaken during World War II by the United States (in cooperation with the United Kingdom and Canada), Germany, Japan, and the USSR. The United States was the first and is the only country to have used a nuclear weapon in war, when it used two bombs against Japan in August 1945. After surrendering to end the war, Germany and Japan ceased to be involved in any nuclear weapon research. In August 1949, the USSR tested a nuclear weapon, becoming the second country to detonate a nuclear bomb. The United Kingdom first tested a nuclear weapon in October 1952. France first tested a nuclear weapon in 1960. The People's Republic of China detonated a nuclear weapon in 1964. India conducted its first nuclear test in 1974, which prompted Pakistan to develop its own nuclear program and, when India conducted a second series of nuclear tests in 1998, Pakistan followed with a series of tests of its own. In 2006, North Korea conducted its first nuclear test.

Nuclear proliferation

Nuclear proliferation is the spread of nuclear weapons, fissionable material, and weapons-applicable nuclear technology and information to nations not recognized as "Nuclear Weapon States" by the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), commonly known as the Non-Proliferation Treaty or NPT. Proliferation has been opposed by many nations with and without nuclear weapons, as governments fear that more countries with nuclear weapons will increase the possibility of nuclear warfare (up to and including the so-called "countervalue" targeting of civilians with nuclear weapons), de-stabilize international or regional relations, or infringe upon the national sovereignty of states.

Four countries besides the five recognized Nuclear Weapons States have acquired, or are presumed to have acquired, nuclear weapons: India, Pakistan, North Korea, and Israel. None of these four is a party to the NPT, although North Korea acceded to the NPT in 1985, then withdrew in 2003 and conducted announced nuclear tests in 2006, 2009, 2013, 2016, and 2017. One critique of the NPT is that the treaty is discriminatory in the sense that only those countries that tested nuclear weapons before 1968 are recognized as nuclear weapon states while all other states are treated as non-nuclear-weapon states who can only join the treaty if they forswear nuclear weapons.

Early efforts to prevent nuclear proliferation involved intense government secrecy, the wartime acquisition of known uranium stores (the Combined Development Trust), and at times even outright sabotage—such as the bombing of a heavy-water facility thought to be used for a German nuclear program. These efforts began immediately after the discovery of nuclear fission and its military potential. None of these efforts were explicitly public, because the weapon developments themselves were kept secret until the bombing of Hiroshima.

Earnest international efforts to promote nuclear non-proliferation began soon after World War II, when the Truman Administration proposed the Baruch Plan of 1946, named after Bernard Baruch, America's first representative to the United Nations Atomic Energy Commission. The Baruch Plan, which drew heavily from the Acheson–Lilienthal Report of 1946, proposed the verifiable dismantlement and destruction of the U.S. nuclear arsenal (which, at that time, was the only nuclear arsenal in the world) after all governments had cooperated successfully to accomplish two things: (1) the establishment of an "international atomic development authority," which would actually own and control all military-applicable nuclear materials and activities, and (2) the creation of a system of automatic sanctions, which not even the U.N. Security Council could veto, and which would proportionately punish states attempting to acquire the capability to make nuclear weapons or fissile material.

Baruch's plea for the destruction of nuclear weapons invoked basic moral and religious intuitions. In one part of his address to the UN, Baruch said, "Behind the black portent of the new atomic age lies a hope which, seized upon with faith, can work out our salvation. If we fail, then we have damned every man to be the slave of Fear. Let us not deceive ourselves. We must elect World Peace or World Destruction... We must answer the world's longing for peace and security." With this remark, Baruch helped launch the field of nuclear ethics, to which many policy experts and scholars have contributed.

Although the Baruch Plan enjoyed wide international support, it failed to emerge from the UNAEC because the Soviet Union planned to veto it in the Security Council. Still, it remained official American policy until 1953, when President Eisenhower made his "Atoms for Peace" proposal before the U.N. General Assembly. Eisenhower's proposal led eventually to the creation of the International Atomic Energy Agency (IAEA) in 1957. Under the "Atoms for Peace" program thousands of scientists from around the world were educated in nuclear science and then dispatched home, where many later pursued secret weapons programs in their home country.

Efforts to conclude an international agreement to limit the spread of nuclear weapons did not begin until the early 1960s, after four nations (the United States, the Soviet Union, the United Kingdom and France) had acquired nuclear weapons (see List of states with nuclear weapons for more information). Although these efforts stalled in the early 1960s, they renewed once again in 1964, after China detonated a nuclear weapon. In 1968, governments represented at the Eighteen Nation Disarmament Committee (ENDC) finished negotiations on the text of the NPT. In June 1968, the U.N. General Assembly endorsed the NPT with General Assembly Resolution 2373 (XXII), and in July 1968, the NPT opened for signature in Washington, DC, London and Moscow. The NPT entered into force in March 1970.

Since the mid-1970s, the primary focus of non-proliferation efforts has been to maintain, and even increase, international control over the fissile material and specialized technologies necessary to build such devices because these are the most difficult and expensive parts of a nuclear weapons program. The main materials whose generation and distribution is controlled are highly enriched uranium and plutonium. Other than the acquisition of these special materials, the scientific and technical means for weapons construction to develop rudimentary, but working, nuclear explosive devices are considered to be within the reach of industrialized nations.

Since its founding by the United Nations in 1957, the International Atomic Energy Agency (IAEA) has promoted two, sometimes contradictory, missions: on the one hand, the Agency seeks to promote and spread internationally the use of civilian nuclear energy; on the other hand, it seeks to prevent, or at least detect, the diversion of civilian nuclear energy to nuclear weapons, nuclear explosive devices or purposes unknown. The IAEA now operates a safeguards system as specified under Article III of the Nuclear Non-Proliferation Treaty (NPT) of 1968, which aims to ensure that civil stocks of uranium and plutonium, as well as facilities and technologies associated with these nuclear materials, are used only for peaceful purposes and do not contribute in any way to proliferation or nuclear weapons programs. It is often argued that proliferation of nuclear weapons to many other states has been prevented by the extension of assurances and mutual defence treaties to these states by nuclear powers, but other factors, such as national prestige, or specific historical experiences, also play a part in hastening or stopping nuclear proliferation.

Dual-use technology refers to the possibility of military use of civilian nuclear power technology. Many technologies and materials associated with the creation of a nuclear power program have a dual-use capability, in that several stages of the nuclear fuel cycle allow diversion of nuclear materials for nuclear weapons. When this happens a nuclear power program can become a route leading to the atomic bomb or a public annex to a secret bomb program. The crisis over Iran’s nuclear activities is a case in point.

Many UN and US agencies warn that building more nuclear reactors unavoidably increases nuclear proliferation risks. A fundamental goal for American and global security is to minimize the proliferation risks associated with the
expansion of nuclear power. If this development is "poorly managed or efforts to contain risks are unsuccessful, the nuclear future will be dangerous". For nuclear power programs to be developed and managed safely and securely, it is important that countries have domestic “good governance” characteristics that will encourage proper nuclear operations and management:

These characteristics include low degrees of corruption (to avoid officials selling materials and technology for their own personal gain as occurred with the A.Q. Khan smuggling network in Pakistan), high degrees of political stability (defined by the World Bank as “likelihood that the government will be destabilized or overthrown by unconstitutional or violent means, including motivated violence and terrorism”), high governmental effectiveness scores (a World Bank aggregate measure of “the quality of the civil service and the degree of its independence from political pressures [and] the quality of policy formulation and implementation”), and a strong degree of regulatory competence.

At present, 189 countries are States Parties to the "Treaty on the Nonproliferation of Nuclear Weapons", more commonly known as the Nuclear Non-Proliferation Treaty or NPT. These include the five Nuclear Weapons States (NWS) recognized by the NPT: the People's Republic of China, France, Russian Federation, the UK, and the United States.

Notable non-signatories to the NPT are Israel, Pakistan, and India (the latter two have since tested nuclear weapons, while Israel is considered by most to be an unacknowledged nuclear weapons state). North Korea was once a signatory but withdrew in January 2003. The legality of North Korea's withdrawal is debatable but as of 9 October 2006, North Korea clearly possesses the capability to make a nuclear explosive device.

The IAEA was established on 29 July 1957 to help nations develop nuclear energy for peaceful purposes. Allied to this role is the administration of safeguards arrangements to provide assurance to the international community that individual countries are honoring their commitments under the treaty. Though established under its own international treaty, the IAEA reports to both the United Nations General Assembly and the Security Council.

The IAEA regularly inspects civil nuclear facilities to verify the accuracy of documentation supplied to it. The agency checks inventories, and samples and analyzes materials. Safeguards are designed to deter diversion of nuclear material by increasing the risk of early detection. They are complemented by controls on the export of sensitive technology from countries such as UK and United States through voluntary bodies such as the Nuclear Suppliers Group. The main concern of the IAEA is that uranium not be enriched beyond what is necessary for commercial civil plants, and that plutonium which is produced by nuclear reactors not be refined into a form that would be suitable for bomb production.

Traditional safeguards are arrangements to account for and control the use of nuclear materials. This verification is a key element in the international system which ensures that uranium in particular is used only for peaceful purposes.

Parties to the NPT agree to accept technical safeguard measures applied by the IAEA. These require that operators of nuclear facilities maintain and declare detailed accounting records of all movements and transactions involving nuclear material. Over 550 facilities and several hundred other locations are subject to regular inspection, and their records and the nuclear material being audited. Inspections by the IAEA are complemented by other measures such as surveillance cameras and instrumentation.

The inspections act as an alert system providing a warning of the possible diversion of nuclear material from peaceful activities. The system relies on;

All NPT non-weapons states must accept these full-scope safeguards. In the five weapons states plus the non-NPT states (India, Pakistan and Israel), facility-specific safeguards apply. IAEA inspectors regularly visit these facilities to verify completeness and accuracy of records.

The terms of the NPT cannot be enforced by the IAEA itself, nor can nations be forced to sign the treaty. In reality, as shown in Iraq and North Korea, safeguards can be backed up by diplomatic, political and economic measures.

While traditional safeguards easily verified the correctness of formal declarations by suspect states, in the 1990s attention turned to what might not have been declared. While accepting safeguards at declared facilities, Iraq had set up elaborate equipment elsewhere in an attempt to enrich uranium to weapons grade. North Korea attempted to use research reactors (not commercial electricity-generating reactors) and a reprocessing plant to produce some weapons-grade plutonium.

The weakness of the NPT regime lay in the fact that no obvious diversion of material was involved. The uranium used as fuel probably came from indigenous sources, and the nuclear facilities were built by the countries themselves without being declared or placed under safeguards. Iraq, as an NPT party, was obliged to declare all facilities but did not do so. Nevertheless, the activities were detected and brought under control using international diplomacy. In Iraq, a military defeat assisted this process.

In North Korea, the activities concerned took place before the conclusion of its NPT safeguards agreement. With North Korea, the promised provision of commercial power reactors appeared to resolve the situation for a time, but it later withdrew from the NPT and declared it had nuclear weapons.

In 1993 a program was initiated to strengthen and extend the classical safeguards system, and a model protocol was agreed by the IAEA Board of Governors 1997. The measures boosted the IAEA's ability to detect undeclared nuclear activities, including those with no connection to the civil fuel cycle.

Innovations were of two kinds. Some could be implemented on the basis of IAEA's existing legal authority through safeguards agreements and inspections. Others required further legal authority to be conferred through an Additional Protocol. This must be agreed by each non-weapons state with IAEA, as a supplement to any existing comprehensive safeguards agreement. Weapons states have agreed to accept the principles of the model additional protocol.

Key elements of the model Additional Protocol:

As of 3 July 2015, 146 countries have signed Additional Protocols and 126 have brought them into force. The IAEA is also applying the measures of the Additional Protocol in Taiwan. Under the Joint Comprehensive Plan of Action, Iran has agreed to implement its protocol provisionally. Among the leading countries that have not signed the Additional Protocol are Egypt, which says it will not sign until Israel accepts comprehensive IAEA safeguards, and Brazil, which opposes making the protocol a requirement for international cooperation on enrichment and reprocessing, but has not ruled out signing.

The greatest risk from nuclear weapons proliferation comes from countries which have not joined the NPT and which have significant unsafeguarded nuclear activities; India, Pakistan, and Israel fall within this category. While safeguards apply to some of their activities, others remain beyond scrutiny.

A further concern is that countries may develop various sensitive nuclear fuel cycle facilities and research reactors under full safeguards and then subsequently opt out of the NPT. Bilateral agreements, such as insisted upon by Australia and Canada for sale of uranium, address this by including fallback provisions, but many countries are outside the scope of these agreements. If a nuclear-capable country does leave the NPT, it is likely to be reported by the IAEA to the UN Security Council, just as if it were in breach of its safeguards agreement. Trade sanctions would then be likely.

IAEA safeguards can help ensure that uranium supplied as nuclear fuel and other nuclear supplies do not contribute to nuclear weapons proliferation. In fact, the worldwide application of those safeguards and the substantial world trade in uranium for nuclear electricity make the proliferation of nuclear weapons much less likely.

The Additional Protocol, once it is widely in force, will provide credible assurance that there are no undeclared nuclear materials or activities in the states concerned. This will be a major step forward in preventing nuclear proliferation.

The Nuclear Suppliers Group communicated its guidelines, essentially a set of export rules, to the IAEA in 1978. These were to ensure that transfers of nuclear material or equipment would not be diverted to unsafeguarded nuclear fuel cycle or nuclear explosive activities, and formal government assurances to this effect were required from recipients. The Guidelines also recognised the need for physical protection measures in the transfer of sensitive facilities, technology and weapons-usable materials, and strengthened retransfer provisions. The group began with seven membersthe United States, the former USSR, the UK, France, Germany, Canada and Japanbut now includes 46 countries including all five nuclear weapons states.

The International Framework for Nuclear Energy Cooperation is an international project involving 25 partner countries, 28 observer and candidate partner countries, and the International Atomic Energy Agency, the Generation IV International Forum, and the European Commission. Its goal is to "[..] provide competitive, commercially-based services as an alternative to a state’s development of costly, proliferation-sensitive facilities, and address other issues associated with the safe and secure management of used fuel and radioactive waste."

According to Kenneth D. Bergeron's "Tritium on Ice: The Dangerous New Alliance of Nuclear Weapons and Nuclear Power", tritium is not classified as a "special nuclear material" but rather as a by-product. It is seen as an important litmus test on the seriousness of the United States' intention to nuclear disarm. This radioactive super-heavy hydrogen isotope is used to boost the efficiency of fissile materials in nuclear weapons. The United States resumed tritium production in 2003 for the first time in 15 years. This could indicate that there is a potential nuclear arm stockpile replacement since the isotope naturally decays.

In May 1995, NPT parties reaffirmed their commitment to a Fissile Materials Cut-off Treaty to prohibit the production of any further fissile material for weapons. This aims to complement the Comprehensive Nuclear-Test-Ban Treaty of 1996 (not entered into force as of 2011) and to codify commitments made by the United States, the UK, France and Russia to cease production of weapons material, as well as putting a similar ban on China. This treaty will also put more pressure on Israel, India and Pakistan to agree to international verification.

On 9 August 2005, Ayatollah Ali Khamenei issued a fatwa forbidding the production, stockpiling and use of nuclear weapons. Khamenei's official statement was made at the meeting of the International Atomic Energy Agency (IAEA) in Vienna. As of February 2006 Iran formally announced that uranium enrichment within their borders has continued. Iran claims it is for peaceful purposes but the United Kingdom, France, Germany, and the United States claim the purpose is for nuclear weapons research and construction.

India, Pakistan and Israel have been "threshold" countries in terms of the international non-proliferation regime. They possess or are quickly capable of assembling one or more nuclear weapons. They have remained outside the 1970 NPT. They are thus largely excluded from trade in nuclear plant or materials, except for safety-related devices for a few safeguarded facilities.

In May 1998 India and Pakistan each exploded several nuclear devices underground. This heightened concerns regarding an arms race between them, with Pakistan involving the People's Republic of China, an acknowledged nuclear weapons state. Both countries are opposed to the NPT as it stands, and India has consistently attacked the Treaty since its inception in 1970 labeling it as a lopsided treaty in favor of the nuclear powers.

Relations between the two countries are tense and hostile, and the risks of nuclear conflict between them have long been considered quite high. Kashmir is a prime cause of bilateral tension, its sovereignty being in dispute since 1948. There is persistent low level bilateral military conflict due to alleged backing of insurgency by Pakistan in India and infiltration of Pakistani state backed militants in the Indian state of Jammu and Kashmir, along with the disputed status of Kashmir.

Both engaged in a conventional arms race in the 1980s, including sophisticated technology and equipment capable of delivering nuclear weapons. In the 1990s the arms race quickened. In 1994 India reversed a four-year trend of reduced allocations for defence, and despite its much smaller economy, Pakistan was expected to push its own expenditures yet higher. Both have lost their patrons: India, the former USSR, and Pakistan, the United States.

But it is the growth and modernization of China's nuclear arsenal and its assistance with Pakistan's nuclear power programme and, reportedly, with missile technology, which exacerbate Indian concerns. In particular, as viewed by Indian strategists, Pakistan is aided by China's People's Liberation Army.

Nuclear power for civil use is well established in India. Its civil nuclear strategy has been directed towards complete independence in the nuclear fuel cycle, necessary because of its outspoken rejection of the NPT. This self-sufficiency extends from uranium exploration and mining through fuel fabrication, heavy water production, reactor design and construction, to reprocessing and waste management. It has a small fast breeder reactor and is planning a much larger one. It is also developing technology to utilise its abundant resources of thorium as a nuclear fuel.

India has 14 small nuclear power reactors in commercial operation, two larger ones under construction, and ten more planned. The 14 operating ones (2548 MWe total) comprise:

The two under construction and two of the planned ones are 450 MWe versions of these 200 MWe domestic products. Construction has been seriously delayed by financial and technical problems. In 2001 a final agreement was signed with Russia for the country's first large nuclear power plant, comprising two VVER-1000 reactors, under a Russian-financed US$3 billion contract. The first unit is due to be commissioned in 2007. A further two Russian units are under consideration for the site. Nuclear power supplied 3.1% of India's electricity in 2000.

Its weapons material appears to come from a Canadian-designed 40 MW "research" reactor which started up in 1960, well before the NPT, and a 100 MW indigenous unit in operation since 1985. Both use local uranium, as India does not import any nuclear fuel. It is estimated that India may have built up enough weapons-grade plutonium for a hundred nuclear warheads.

It is widely believed that the nuclear programs of India and Pakistan used CANDU reactors to produce fissionable materials for their weapons; however, this is not accurate. Both Canada (by supplying the 40 MW research reactor) and the United States (by supplying 21 tons of heavy water) supplied India with the technology necessary to create a nuclear weapons program, dubbed CIRUS (Canada-India Reactor, United States). Canada sold India the reactor on the condition that the reactor and any by-products would be "employed for peaceful purposes only.". Similarly, the United States sold India heavy water for use in the reactor "only... in connection with research into and the use of atomic energy for peaceful purposes". India, in violation of these agreements, used the Canadian-supplied reactor and American-supplied heavy water to produce plutonium for their first nuclear explosion, Smiling Buddha. The Indian government controversially justified this, however, by claiming that Smiling Buddha was a "peaceful nuclear explosion."

The country has at least three other research reactors including the tiny one which is exploring the use of thorium as a nuclear fuel, by breeding fissile U-233. In addition, an advanced heavy-water thorium cycle is under development.

India exploded a nuclear device in 1974, the so-called Smiling Buddha test, which it has consistently claimed was for peaceful purposes. Others saw it as a response to China's nuclear weapons capability. It was then universally perceived, notwithstanding official denials, to possess, or to be able to quickly assemble, nuclear weapons. In 1999 it deployed its own medium-range missile and has developed an intermediate-range missile capable of reaching targets in China's industrial heartland.

In 1995 the United States quietly intervened to head off a proposed nuclear test. However, in 1998 there were five more tests in Operation Shakti. These were unambiguously military, including one claimed to be of a sophisticated thermonuclear device, and their declared purpose was "to help in the design of nuclear weapons of different yields and different delivery systems".

Indian security policies are driven by:

It perceives nuclear weapons as a cost-effective political counter to China's nuclear and conventional weaponry, and the effects of its nuclear weapons policy in provoking Pakistan is, by some accounts, considered incidental.
India has had an unhappy relationship with China. After an uneasy ceasefire ended the 1962 war, relations between the two nations were frozen until 1998. Since then a degree of high-level contact has been established and a few elementary confidence-building measures put in place. China still occupies some territory which it captured during the aforementioned war, claimed by India, and India still occupies some territory claimed by China. Its nuclear weapon and missile support for Pakistan is a major bone of contention.

American President George W. Bush met with India Prime Minister Manmohan Singh to discuss India's involvement with nuclear weapons. The two countries agreed that the United States would give nuclear power assistance to India.

Over the several years, the Nuclear power infrastructure has been well established by Pakistan which is dedicated for the industrial and economic development of the country. Its current nuclear policy is directed and aimed to promote the socio-economic development of the people as a "foremost priority"; and to fulfill the energy, economic, and industrial needs from the nuclear sources. Currently, there are three operational mega-commercial nuclear power plants while three larger ones are under construction. The nuclear power supplies 787 MW (roughly ~3.6%) of electricity as of 2012, and the country has projected to produce 8800 MW electricity by 2030. Infrastructure established by the IAEA and the U.S. in the 1950s–1960s were based on peaceful research and development and economic prosperity of the country.

Although the civil-sector nuclear power was established in the 1950s, the country has an active nuclear weapons program which was started in the 1970s. The bomb program has its roots after East-Pakistan gained its independence as Bangladesh after India's successful intervention led to a decisive victory on Pakistan in 1971. This large-scale but clandestine atomic bomb project was directed towards the development of ingenious development of reactor and military-grade plutonium. In 1974, when India surprised the outer world with its successful detonation of its own bomb, codename "Smiling Buddha", it became "imperative for Pakistan" to pursue the weapons research. According to leading scientist in the program, it became clear once India detonated the bomb, "Newton's third law" came into "operation", from then on it was a classic case of "action and reaction". Earlier efforts were directed towards mastering the plutonium technology from France, but plutonium route was partially slowed down when the plan was failed after the U.S. intervention to cancel the project. Contrary to popular perception, Pakistan did not forego the "plutonium" route and covertly continued its indegenious research under Munir Khan and it succeeded with plutonium route in the early 1980s. Reacting on India's nuclear test (Smiling Buddha), Bhutto and the country's elite political and military science circle sensed this test as final and dangerous anticipation to Pakistan's "moral and physical existence." With Aziz Ahmed on his side, Bhutto launched a serious diplomatic offense and aggressively maintained at the session of the United Nations Security Council:

After 1974, Bhutto's government redoubled its effort, this time equally focused on uranium and plutonium. Pakistan had established science directorates in almost all of her embassies in the important countries of the world, with theoretical physicist S.A. Butt being the director. Abdul Qadeer Khan then established a network through Dubai to smuggle URENCO technology to Engineering Research Laboratories. Earlier, he worked with "Physics Dynamics Research Laboratories" (FDO), a subsidiary of the Dutch firm VMF-Stork based in Amsterdam. Later after joining, the Urenco, he had access through photographs and documents of the technology. Against the popular perception, the technology that A.Q. Khan had brought from Urenco was based on first generation civil rector technology, filled with many serious technical errors, though it was authentic and vital link for centrifuge project of the country. After the British Government stopped the British subsidiary of the American Emerson Electric Co. from shipping the components to Pakistan, he describes his frustration with a supplier from Germany as: "That man from the German team was unethical. When he did not get the order from us, he wrote a letter to a Labour Party member and questions were asked in [British] Parliament." By 1978, his efforts were paid off and made him into a national hero. In 1981, as a tribute, President General Muhammad Zia-ul-Haq, renamed the research institute after his name.

In early 1996, Prime minister Benazir Bhutto made it clear that "if India conducts a nuclear test, Pakistan could be forced to "follow suit". In 1997, her statement was echoed by Prime minister Nawaz Sharif who maintained to the fact that "since 1972, [P]akistan had progressed significantly, and we have left that stage (developmental) far behind. Pakistan will not be made a "hostage" to India by signing the CTBT, before (India).!" In May 1998, within weeks of India's nuclear tests, Pakistan announced that it had conducted six underground tests in the Chagai Hills, five on the 28th and one on the 30th of that month. Seismic events consistent with these claims were recorded.

In 2004, the revelation of A.Q. Khan's efforts led to the exposure of many defunct European consortiums which had defied export restrictions in the 1970s, and of many defunct Dutch companies that exported thousands of centrifuges to Pakistan as early as 1976. Many centrifuge components were apparently manufactured in Malaysian Scomi Precision Engineering with the assistance of South Asian and German companies, and used a UAE-based computer company as a false front.

It was widely believed to have direct involvement of the government of Pakistan. This claim could not be verified due to the refusal of the government of Pakistan to allow IAEA to interview the alleged head of the nuclear black market, who happened to be no other than A.Q. Khan. Confessing his crimes later a month on national television, he bailed out the government by taking full responsibility. Independent investigation conducted by IISS confirmed that he had control over the import-export deals, and his acquisition activities were largely unsupervised by Pakistan governmental authorities. All of his activities went undetected for several years. He duly confessed of running the atomic proliferation ring from Pakistan to Iran and North Korea. He was immediately given presidential immunity. Exact nature of the involvement at the governmental level is still unclear, but the manner in which the government acted cast doubt on the sincerity of Pakistan.

The Democratic People's Republic of Korea (or better known as North Korea), joined the NPT in 1985 and had subsequently signed a safeguards agreement with the IAEA. However, it was believed that North Korea was diverting plutonium extracted from the fuel of its reactor at Yongbyon, for use in nuclear weapons. The subsequent confrontation with IAEA on the issue of inspections and suspected violations, resulted in North Korea threatening to withdraw from the NPT in 1993. This eventually led to negotiations with the United States resulting in the Agreed Framework of 1994, which provided for IAEA safeguards being applied to its reactors and spent fuel rods. These spent fuel rods were sealed in canisters by the United States to prevent North Korea from extracting plutonium from them. North Korea had to therefore freeze its plutonium programme.

During this period, Pakistan-North Korea cooperation in missile technology transfer was being established. A high level delegation of Pakistan military visited North Korea in August–September 1992, reportedly to discuss the supply of missile technology to Pakistan. In 1993, PM Benazir Bhutto repeatedly traveled to China, and the paid state visit to North Korea. The visits are believed to be related to the subsequent acquisition technology to developed its Ghauri system by Pakistan. During the period 1992–1994, A.Q. Khan was reported to have visited North Korea thirteen times. The missile cooperation program with North Korea was under Dr. A. Q. Khan Research Laboratories. At this time China was under U.S. pressure not to supply the M Dongfeng series of missiles to Pakistan. It is believed by experts that possibly with Chinese connivance and facilitation, the latter was forced to approach North Korea for missile transfers. Reports indicate that North Korea was willing to supply missile sub-systems including rocket motors, inertial guidance systems, control and testing equipment for US$50 million.

It is not clear what North Korea got in return. Joseph S. Bermudez Jr. in "Jane's Defence Weekly" (27 November 2002) reports that Western analysts had begun to question what North Korea received in payment for the missiles; many suspected it was the nuclear technology. The KRL was in charge of both uranium program and also of the missile program with North Korea. It is therefore likely during this period that cooperation in nuclear technology between Pakistan and North Korea was initiated. Western intelligence agencies began to notice exchange of personnel, technology and components between KRL and entities of the North Korean 2nd Economic Committee (responsible for weapons production).

A "New York Times" report on 18 October 2002 quoted U.S. intelligence officials having stated that Pakistan was a major supplier of critical equipment to North Korea. The report added that equipment such as gas centrifuges appeared to have been "part of a barter deal" in which North Korea supplied Pakistan with missiles. Separate reports indicate ("The Washington Times", 22 November 2002) that U.S. intelligence had as early as 1999 picked up signs that North Korea was continuing to develop nuclear arms. Other reports also indicate that North Korea had been working covertly to develop an enrichment capability for nuclear weapons for at least five years and had used technology obtained from Pakistan ("The Washington Times", 18 October 2002).

Israel is also thought to possess an arsenal of potentially up to several hundred nuclear warheads based on estimates of the amount of fissile material produced by Israel. This has never been openly confirmed or denied however, due to Israel's policy of deliberate ambiguity.

An Israeli nuclear installation is located about ten kilometers to the south of Dimona, the Negev Nuclear Research Center. Its construction commenced in 1958, with French assistance. The official reason given by the Israeli and French governments was to build a nuclear reactor to power a "desalination plant", in order to "green the Negev". The purpose of the Dimona plant is widely assumed to be the manufacturing of nuclear weapons, and the majority of defense experts have concluded that it does in fact do that. However, the Israeli government refuses to confirm or deny this publicly, a policy it refers to as "ambiguity".

Norway sold 20 tonnes of heavy water needed for the reactor to Israel in 1959 and 1960 in a secret deal. There were no "safeguards" required in this deal to prevent usage of the heavy water for non-peaceful purposes. The British newspaper "Daily Express" accused Israel of working on a bomb in 1960.
When the United States intelligence community discovered the purpose of the Dimona plant in the early 1960s, it demanded that Israel agree to international inspections. Israel agreed, but on a condition that U.S., rather than IAEA, inspectors were used, and that Israel would receive advanced notice of all inspections.
Some claim that because Israel knew the schedule of the inspectors' visits, it was able to hide the alleged purpose of the site from the inspectors by installing temporary false walls and other devices before each inspection. The inspectors eventually informed the U.S. government that their inspections were useless due to Israeli restrictions on what areas of the facility they could inspect. In 1969, the United States terminated the inspections.

In 1986, Mordechai Vanunu, a former technician at the Dimona plant, revealed to the media some evidence of Israel's nuclear program. Israeli agents arrested him from Italy, drugged him and transported him to Israel, and an Israeli court then tried him in secret on charges of treason and espionage, and sentenced him to eighteen years imprisonment. He was freed on 21 April 2004, but was severely limited by the Israeli government. He was arrested again on 11 November 2004, though formal charges were not immediately filed.

Comments on photographs taken by Mordechai Vanunu inside the Negev Nuclear Research Center have been made by prominent scientists. British nuclear weapons scientist Frank Barnaby, who questioned Vanunu over several days, estimated Israel had enough plutonium for about 150 weapons.

According to Lieutenant Colonel Warner D. Farr in a report to the USAF Counterproliferation Center while France was previously a leader in nuclear research "Israel and France were at a similar level of expertise after WWII, and Israeli scientists could make significant contributions to the French effort." In 1986 Francis Perrin, French high-commissioner for atomic energy from 1951 to 1970 stated that in 1949 Israeli scientists were invited to the Saclay nuclear research facility, this cooperation leading to a joint effort including sharing of knowledge between French and Israeli scientists especially those with knowledge from the Manhattan Project.

The public stance of the two states on non-proliferation differs markedly. Pakistan has initiated a series of regional security proposals. It has repeatedly proposed a nuclear free zone in South Asia and has proclaimed its willingness to engage in nuclear disarmament and to sign the Non-Proliferation Treaty if India would do so. It has endorsed a United States proposal for a regional five power conference to consider non-proliferation in South Asia.

India has taken the view that solutions to regional security issues should be found at the international rather than the regional level, since its chief concern is with China. It therefore rejects Pakistan's proposals.

Instead, the 'Gandhi Plan', put forward in 1988, proposed the revision of the Non-Proliferation Treaty, which it regards as inherently discriminatory in favor of the nuclear-weapon States, and a timetable for complete nuclear weapons disarmament. It endorsed early proposals for a Comprehensive Test Ban Treaty and for an international convention to ban the production of highly enriched uranium and plutonium for weapons purposes, known as the 'cut-off' convention.

The United States for some years, especially under the Clinton administration, pursued a variety of initiatives to persuade India and Pakistan to abandon their nuclear weapons programs and to accept comprehensive international safeguards on all their nuclear activities. To this end, the Clinton administration proposed a conference of the five nuclear-weapon states, Japan, Germany, India and Pakistan.

India refused this and similar previous proposals, and countered with demands that other potential weapons states, such as Iran and North Korea, should be invited, and that regional limitations would only be acceptable if they were accepted equally by China. The United States would not accept the participation of Iran and North Korea and these initiatives have lapsed.

Another, more recent approach, centers on 'capping' the production of fissile material for weapons purposes, which would hopefully be followed by 'roll back'. To this end, India and the United States jointly sponsored a UN General Assembly resolution in 1993 calling for negotiations for a 'cut-off' convention. Should India and Pakistan join such a convention, they would have to agree to halt the production of fissile materials for weapons and to accept international verification on their relevant nuclear facilities (enrichment and reprocessing plants). It appears that India is now prepared to join negotiations regarding such a Cut-off Treaty, under the UN Conference on Disarmament.

Bilateral confidence-building measures between India and Pakistan to reduce the prospects of confrontation have been limited. In 1990 each side ratified a treaty not to attack the other's nuclear installations, and at the end of 1991 they provided one another with a list showing the location of all their nuclear plants, even though the respective lists were regarded as not being wholly accurate. Early in 1994 India proposed a bilateral agreement for a 'no first use' of nuclear weapons and an extension of the 'no attack' treaty to cover civilian and industrial targets as well as nuclear installations.

Having promoted the Comprehensive Test Ban Treaty since 1954, India dropped its support in 1995 and in 1996 attempted to block the Treaty. Following the 1998 tests the question has been reopened and both Pakistan and India have indicated their intention to sign the CTBT. Indian ratification may be conditional upon the five weapons states agreeing to specific reductions in nuclear arsenals. The UN Conference on Disarmament has also called upon both countries "to accede without delay to the Non-Proliferation Treaty", presumably as non-weapons states.

In 2004 and 2005, Egypt disclosed past undeclared nuclear activities and material to the IAEA. In 2007 and 2008, high enriched and low enriched uranium particles were found in environmental samples taken in Egypt. In 2008, the IAEA states Egypt's statements were consistent with its own findings. In May 2009, "Reuters" reported that the IAEA was conducting further investigation in Egypt.

In 2003, the IAEA reported that Iran had been in breach of its obligations to comply with provisions of its safeguard agreement. In 2005, the IAEA Board of Governors voted in a rare non-consensus decision to find Iran in non-compliance with its NPT Safeguards Agreement and to report that non-compliance to the UN Security Council. In response, the UN Security Council passed a series of resolutions citing concerns about the program. Iran's representative to the UN argues sanctions compel Iran to abandon its rights under the Nuclear Nonproliferation Treaty to peaceful nuclear technology. Iran says its uranium enrichment program is exclusively for peaceful purposes and has enriched uranium to "less than 5 percent," consistent with fuel for a nuclear power plant and significantly below the purity of WEU (around 90%) typically used in a weapons program. The director general of the International Atomic Energy Agency, Yukiya Amano, said in 2009 he had not seen any evidence in IAEA official documents that Iran was developing nuclear weapons.

Up to the late 1980s it was generally assumed that any undeclared nuclear activities would have to be based on the diversion of nuclear material from safeguards. States acknowledged the possibility of nuclear activities entirely separate from those covered by safeguards, but it was assumed they would be detected by national intelligence activities. There was no particular effort by IAEA to attempt to detect them.

Iraq had been making efforts to secure a nuclear potential since the 1960s. In the late 1970s a specialised plant, Osiraq, was constructed near Baghdad. The plant was attacked during the Iran–Iraq War and was destroyed by Israeli bombers in June 1981.

Not until the 1990 NPT Review Conference did some states raise the possibility of making more use of (for example) provisions for "special inspections" in existing NPT Safeguards Agreements. Special inspections can be undertaken at locations other than those where safeguards routinely apply, if there is reason to believe there may be undeclared material or activities.

After inspections in Iraq following the UN Gulf War cease-fire resolution showed the extent of Iraq's clandestine nuclear weapons program, it became clear that the IAEA would have to broaden the scope of its activities. Iraq was an NPT Party, and had thus agreed to place all its nuclear material under IAEA safeguards. But the inspections revealed that it had been pursuing an extensive clandestine uranium enrichment programme, as well as a nuclear weapons design programme.

The main thrust of Iraq's uranium enrichment program was the development of technology for electromagnetic isotope separation (EMIS) of indigenous uranium. This uses the same principles as a mass spectrometer (albeit on a much larger scale). Ions of uranium-238 and uranium-235 are separated because they describe arcs of different radii when they move through a magnetic field. This process was used in the Manhattan Project to make the highly enriched uranium used in the Hiroshima bomb, but was abandoned soon afterwards.

The Iraqis did the basic research work at their nuclear research establishment at Tuwaitha, near Baghdad, and were building two full-scale facilities at Tarmiya and Ash Sharqat, north of Baghdad. However, when the war broke out, only a few separators had been installed at Tarmiya, and none at Ash Sharqat.

The Iraqis were also very interested in centrifuge enrichment, and had been able to acquire some components including some carbon-fibre rotors, which they were at an early stage of testing. In May 1998, "Newsweek" reported that Abdul Qadeer Khan had sent Iraq centrifuge designs, which were apparently confiscated by the UNMOVIC officials. Iraqi officials said "the documents were authentic but that they had not agreed to work with A. Q. Khan, fearing an ISI sting operation, due to strained relations between two countries. The Government of Pakistan and A. Q. Khan strongly denied this allegation whilst the government declared the evidence to be "fraudulent".

They were clearly in violation of their NPT and safeguards obligations, and the IAEA Board of Governors ruled to that effect. The UN Security Council then ordered the IAEA to remove, destroy or render harmless Iraq's nuclear weapons capability. This was done by mid-1998, but Iraq then ceased all cooperation with the UN, so the IAEA withdrew from this work.

The revelations from Iraq provided the impetus for a very far-reaching reconsideration of what safeguards are intended to achieve.

Libya possesses ballistic missiles and previously pursued nuclear weapons under the leadership of Muammar Gaddafi. On 19 December 2003, Gaddafi announced that Libya would voluntarily eliminate all materials, equipment and programs that could lead to internationally proscribed weapons, including weapons of mass destruction and long-range ballistic missiles. Libya signed the Nuclear Non-Proliferation Treaty (NPT) in 1968 and ratified it in 1975, and concluded a safeguards agreement with the International Atomic Energy Agency (IAEA) in 1980. In March 2004, the IAEA Board of Governors welcomed Libya's decision to eliminate its formerly undeclared nuclear program, which it found had violated Libya's safeguards agreement, and approved Libya's Additional Protocol. The United States and the United Kingdom assisted Libya in removing equipment and material from its nuclear weapons program, with independent verification by the IAEA.

A report in the "Sydney Morning Herald" and "Searchina", a Japanese newspaper, report that two Myanma defectors saying that the Myanmar junta was secretly building a nuclear reactor and plutonium extraction facility with North Korea's help, with the aim of acquiring its first nuclear bomb in five years. According to the report, "The secret complex, much of it in caves tunnelled into a mountain at Naung Laing in northern Burma, runs parallel to a civilian reactor being built at another site by Russia that both the Russians and Burmese say will be put under international safeguards." In 2002, Myanmar had notified IAEA of its intention to pursue a civilian nuclear programme. Later, Russia announced that it would build a nuclear reactor in Myanmar. There have also been reports that two Pakistani scientists, from the AQ Khan stable, had been dispatched to Myanmar where they had settled down, to help Myanmar's project. Recently, the David Albright-led Institute for Science and International Security (ISIS) rang alarm bells about Myanmar attempting a nuclear project with North Korean help. If true, the full weight of international pressure will be brought against Myanmar, said officials familiar with developments. But equally, the information that has been peddled by the defectors is also "preliminary" and could be used by the west to turn the screws on Myanmar—on democracy and human rights issues—in the run-up to the elections in the country in 2010. During an ASEAN meeting in Thailand in July 2009, US secretary of state Hillary Clinton highlighted concerns of the North Korean link. "We know there are also growing concerns about military cooperation between North Korea and Burma which we take very seriously," Clinton said. However, in 2012, after contact with the American president, Barack Obama, the Burmese leader, Thein Sein, renounced military ties with DPRK (North Korea).

The Democratic People's Republic of Korea (DPRK) acceded to the NPT in 1985 as a condition for the supply of a nuclear power station by the USSR. However, it delayed concluding its NPT Safeguards Agreement with the IAEA, a process which should take only 18 months, until April 1992.

During that period, it brought into operation a small gas-cooled, graphite-moderated, natural-uranium (metal) fuelled "Experimental Power Reactor" of about 25 MWt (5 MWe), based on the UK Magnox design. While this was a well-suited design to start a wholly indigenous nuclear reactor development, it also exhibited all the features of a small plutonium production reactor for weapons purposes. North Korea also made substantial progress in the construction of two larger reactors designed on the same principles, a prototype of about 200 MWt (50 MWe), and a full-scale version of about 800 MWt (200 MWe). They made only slow progress; construction halted on both in 1994 and has not resumed. Both reactors have degraded considerably since that time and would take significant efforts to refurbish.

In addition it completed and commissioned a reprocessing plant that makes the Magnox spent nuclear fuel safe, recovering uranium and plutonium. That plutonium, if the fuel was only irradiated to a very low burn-up, would have been in a form very suitable for weapons. Although all these facilities at Yongbyon were to be under safeguards, there was always the risk that at some stage, the DPRK would withdraw from the NPT and use the plutonium for weapons.

One of the first steps in applying NPT safeguards is for the IAEA to verify the initial stocks of uranium and plutonium to ensure that all the nuclear materials in the country have been declared for safeguards purposes. While undertaking this work in 1992, IAEA inspectors found discrepancies which indicated that the reprocessing plant had been used more often than the DPRK had declared, which suggested that the DPRK could have weapons-grade plutonium which it had not declared to the IAEA. Information passed to the IAEA by a Member State (as required by the IAEA) supported that suggestion by indicating that the DPRK had two undeclared waste or other storage sites.

In February 1993 the IAEA called on the DPRK to allow special inspections of the two sites so that the initial stocks of nuclear material could be verified. The DPRK refused, and on 12 March announced its intention to withdraw from the NPT (three months' notice is required). In April 1993 the IAEA Board concluded that the DPRK was in non-compliance with its safeguards obligations and reported the matter to the UN Security Council. In June 1993 the DPRK announced that it had "suspended" its withdrawal from the NPT, but subsequently claimed a "special status" with respect to its safeguards obligations. This was rejected by IAEA.

Once the DPRK's non-compliance had been reported to the UN Security Council, the essential part of the IAEA's mission had been completed. Inspections in the DPRK continued, although inspectors were increasingly hampered in what they were permitted to do by the DPRK's claim of a "special status". However, some 8,000 corroding fuel rods associated with the experimental reactor have remained under close surveillance.

Following bilateral negotiations between the United States and the DPRK, and the conclusion of the Agreed Framework in October 1994, the IAEA has been given additional responsibilities. The agreement requires a freeze on the operation and construction of the DPRK's plutonium production reactors and their related facilities, and the IAEA is responsible for monitoring the freeze until the facilities are eventually dismantled. The DPRK remains uncooperative with the IAEA verification work and has yet to comply with its safeguards agreement.

While Iraq was defeated in a war, allowing the UN the opportunity to seek out and destroy its nuclear weapons programme as part of the cease-fire conditions, the DPRK was not defeated, nor was it vulnerable to other measures, such as trade sanctions. It can scarcely afford to import anything, and sanctions on vital commodities, such as oil, would either be ineffective or risk provoking war.

Ultimately, the DPRK was persuaded to stop what appeared to be its nuclear weapons programme in exchange, under the agreed framework, for about US$5 billion in energy-related assistance. This included two 1000 MWe light water nuclear power reactors based on an advanced U.S. System-80 design.

In January 2003 the DPRK withdrew from the NPT. In response, a series of discussions among the DPRK, the United States, and China, a series of six-party talks (the parties being the DPRK, the ROK, China, Japan, the United States and Russia) were held in Beijing; the first beginning in April 2004 concerning North Korea's weapons program.

On 10 January 2005, North Korea declared that it was in the possession of nuclear weapons. On 19 September 2005, the fourth round of the Six-Party Talks ended with a joint statement in which North Korea agreed to end its nuclear programs and return to the NPT in exchange for diplomatic, energy and economic assistance. However, by the end of 2005 the DPRK had halted all six-party talks because the United States froze certain DPRK international financial assets such as those in a bank in Macau.

On 9 October 2006, North Korea announced that it has performed its first-ever nuclear weapon test. On 18 December 2006, the six-party talks finally resumed. On 13 February 2007, the parties announced "Initial Actions" to implement the 2005 joint statement including shutdown and disablement of North Korean nuclear facilities in exchange for energy assistance. Reacting to UN sanctions imposed after missile tests in April 2009, North Korea withdrew from the six-party talks, restarted its nuclear facilities and conducted a second nuclear test on 25 May 2009.

On 12 February 2013, North Korea conducted an underground nuclear explosion with an estimated yield of 6 to 7 kilotonnes. The detonation registered a magnitude 4.9 disturbance in the area around the epicenter.

"See also: North Korea and weapons of mass destruction and Six-party talks"

Security of nuclear weapons in Russia remains a matter of concern. According to high-ranking Russian SVR defector Tretyakov, he had a meeting with two Russian businessman representing a state-created "C-W" corporation in 1991. They came up with a project of destroying large quantities of chemical wastes collected from Western countries at the island of Novaya Zemlya (a test place for Soviet nuclear weapons) using an underground nuclear blast. The project was rejected by Canadian representatives, but one of the businessmen told Tretyakov that he keeps his own nuclear bomb at his dacha outside Moscow. Tretyakov thought that man was insane, but the "businessmen" (Vladimir K. Dmitriev) replied: "Do not be so naive. With economic conditions the way they are in Russia today, anyone with enough money can buy a nuclear bomb. It's no big deal really".

In 1991, South Africa acceded to the NPT, concluded a comprehensive safeguards agreement with the IAEA, and submitted a report on its nuclear material subject to safeguards. At the time, the state had a nuclear power programme producing nearly 10% of the country's electricity, whereas Iraq and North Korea only had research reactors.

The IAEA's initial verification task was complicated by South Africa's announcement that between 1979 and 1989 it built and then dismantled a number of nuclear weapons. South Africa asked the IAEA to verify the conclusion of its weapons programme. In 1995 the IAEA declared that it was satisfied all materials were accounted for and the weapons programme had been terminated and dismantled.

South Africa has signed the NPT, and now holds the distinction of being the only known state to have indigenously produced nuclear weapons, and then verifiably dismantled them.

On 6 September 2007, Israel bombed an officially unidentified site in Syria which it later asserted was a nuclear reactor under construction ("see Operation Outside the Box"). The alleged reactor was not asserted to be operational and it was not asserted that nuclear material had been introduced into it. Syria said the site was a military site and was not involved in any nuclear activities. The IAEA requested Syria to provide further access to the site and any other locations where the debris and equipment from the building had been stored. Syria denounced what it called the Western "fabrication and forging of facts" in regards to the incident. IAEA Director General Mohamed ElBaradei criticized the strikes and deplored that information regarding the matter had not been shared with his agency earlier.

For a state that does not possess nuclear weapons, the capability to produce one or more weapons quickly and with little warning is called a breakout capability.There has been much debate in the academic study of International Security as to the advisability of proliferation. In the late 1950s and early 1960s, Gen. Pierre Marie Gallois of France, an adviser to Charles DeGaulle, argued in books like "The Balance of Terror: Strategy for the Nuclear Age" (1961) that mere possession of a nuclear arsenal, what the French called the "force de frappe", was enough to ensure deterrence, and thus concluded that the spread of nuclear weapons could increase international stability.

Some very prominent neo-realist scholars, such as Kenneth Waltz, Emeritus Professor of Political Science at UC Berkeley and Adjunct Senior Research Scholar at Columbia University, and John Mearsheimer, R. Wendell Harrison Distinguished Service Professor of Political Science at the University of Chicago, continue to argue along the lines of Gallois in a separate development. Specifically, these scholars advocate some forms of nuclear proliferation, arguing that it will decrease the likelihood of war, especially in troubled regions of the world. Aside from the majority opinion which opposes proliferation in any form, there are two schools of thought on the matter: those, like Mearsheimer, who favor selective proliferation, and those such as Waltz, who advocate a laissez-faire attitude to programs like North Korea's.

In embryo, Waltz argues that the logic of mutually assured destruction (MAD) should work in all security environments, regardless of historical tensions or recent hostility. He sees the Cold War as the ultimate proof of MAD logicthe only occasion when enmity between two Great Powers did not result in military conflict. This was, he argues, because nuclear weapons promote caution in decision-makers. Neither Washington nor Moscow would risk a nuclear apocalypse to advance territorial or power goals, hence a peaceful stalemate ensued (Waltz and Sagan (2003), p. 24). Waltz believes there to be no reason why this effect would not occur in all circumstances.

John Mearsheimer would not support Waltz's optimism in the majority of potential instances; however, he has argued for nuclear proliferation as policy in certain places, such as post–Cold War Europe. In two famous articles, Professor Mearsheimer opines that Europe is bound to return to its pre–Cold War environment of regular conflagration and suspicion at some point in the future. He advocates arming both Germany and Ukraine with nuclear weaponry in order to achieve a balance of power between these states in the east and France/UK in the west. If this does not occur, he is certain that war will eventually break out on the European continent.

Another separate argument against Waltz's open proliferation and in favor of Mearsheimer's selective distribution is the possibility of nuclear terrorism. Some countries included in the aforementioned laissez-faire distribution could predispose the transfer of nuclear materials or a bomb falling into the hands of groups not affiliated with any governments. Such countries would not have the political will or ability to safeguard attempts at devices being transferred to a third party. Not being deterred by self-annihilation, terrorism groups could push forth their own nuclear agendas or be used as shadow fronts to carry out the attack plans by mentioned unstable governments.

There are numerous arguments presented against both selective and total proliferation, generally targeting the very neorealist assumptions (such as the primacy of military security in state agendas, the weakness of international institutions, and the long-run unimportance of economic integration and globalization to state strategy) its proponents tend to make. With respect to Mearsheimer's specific example of Europe, many economists and neoliberals argue that the economic integration of Europe through the development of the European Union has made war in most of the European continent so disastrous economically so as to serve as an effective deterrent. Constructivists take this one step further, frequently arguing that the development of EU political institutions has led or will lead to the development of a nascent European identity, which most states on the European continent wish to partake in to some degree or another, and which makes all states within or aspiring to be within the EU regard war between them as unthinkable.

As for Waltz, the general opinion is that most states are not in a position to safely guard against nuclear use, that he underestimates the long-standing antipathy in many regions, and that weak states will be unable to prevent – or will actively provide for – the disastrous possibility of nuclear terrorism. Waltz has dealt with all of these objections at some point in his work; though to many, he has not adequately responded (Betts (2000)).

The Learning Channel documentary Doomsday: "On The Brink" illustrated 40 years of U.S. and Soviet nuclear weapons accidents. Even the 1995 Norwegian rocket incident demonstrated a potential scenario in which Russian democratization and military downsizing at the end of the Cold War did not eliminate the danger of accidental nuclear war through command and control errors. After asking: might a future Russian ruler or renegade Russian general be tempted to use nuclear weapons to make foreign policy? The documentary writers revealed a greater danger of Russian security over its nuclear stocks, but especially the ultimate danger of human nature to want the ultimate weapon of mass destruction to exercise political and military power. Future world leaders might not understand how close the Soviets, Russians, and Americans were to doomsday, how easy it all seemed because apocalypse was avoided for a mere 40 years between rivals, politicians not terrorists, who loved their children and did not want to die, against 30,000 years of human prehistory. History and military experts agree that proliferation can be slowed, but never stopped (technology cannot be uninvented).

Proliferation begets proliferation is a concept described by Scott Sagan in his article, "Why Do States Build Nuclear Weapons?". This concept can be described as a strategic chain reaction. If one state produces a nuclear weapon it creates almost a domino effect within the region. States in the region will seek to acquire nuclear weapons to balance or eliminate the security threat. Sagan describes this reaction best in his article when he states, “Every time one state develops nuclear weapons to balance against its main rival, it also creates a nuclear threat to another region, which then has to initiate its own nuclear weapons program to maintain its national security”. Going back through history we can see how this has taken place. When the United States demonstrated that it had nuclear power capabilities after the bombing of Hiroshima and Nagasaki, the Russians started to develop their program in preparation for the Cold War. With the Russian military buildup, France and the United Kingdom perceived this as a security threat and therefore they pursued nuclear weapons (Sagan, pg 71). Even though proliferation causes proliferation, this does not guarantee that other states will successfully develop nuclear weapons because the economic stability of a state plays an important role on whether the state will successfully be able to acquire nuclear weapons. The article written by Dong-Jong Joo and Erik Gartzke discusses how the economy of a country determines whether they will successfully acquire nuclear weapons.

Former Iranian President Mahmoud Ahmadinejad has been a frequent critic of the concept of "nuclear apartheid" as it has been put into practice by several countries, particularly the United States. In an interview with CNN's Christiane Amanpour, Ahmadinejad said that Iran was "against 'nuclear apartheid,' which means some have the right to possess it, use the fuel, and then sell it to another country for 10 times its value. We're against that. We say clean energy is the right of all countries. But also it is the duty and the responsibility of all countries, including ours, to set up frameworks to stop the proliferation of it." Hours after that interview, he spoke passionately in favor of Iran's right to develop nuclear technology, claiming the nation should have the same liberties.

Iran is a signatory of the Nuclear Non-Proliferation Treaty and claims that any work done in regards to nuclear technology is related only to civilian uses, which is acceptable under the treaty. Iran violated its safeguards obligations under the treaty by performing uranium-enrichment in secret, after which the United Nations Security Council ordered Iran to suspend all uranium-enrichment until July 2015.

India has also been discussed in the context of "nuclear apartheid". India has consistently attempted to pass measures that would call for full international disarmament, however they have not succeeded due to protests from those states that already have nuclear weapons. In light of this, India viewed nuclear weapons as a necessary right for all nations as long as certain states were still in possession of nuclear weapons. India stated that nuclear issues were directly related to national security.

Years before India's first underground nuclear test in 1998, the Comprehensive Nuclear-Test-Ban Treaty was passed. Some have argued that coercive language was used in an attempt to persuade India to sign the treaty, which was pushed for heavily by neighboring China. India viewed the treaty as a means for countries that already had nuclear weapons, primarily the five nations of the United Nations Security Council, to keep their weapons while ensuring that no other nations could develop them.

In their article, "The Correlates of Nuclear Proliferation," Sonali Singh and Christopher R. Way argue that states protected by a security guarantee from a great power, particularly if backed by the "nuclear umbrella" of extended deterrence, have less of an incentive to acquire their own nuclear weapons. States that lack such guarantees are more likely to feel their security threatened and so have greater incentives to bolster or assemble nuclear arsenals. As a result, it is then argued that bipolarity may prevent proliferation where as multipolarity may actually influence proliferation.

The nuclear weapons debate refers to the controversies surrounding the threat, use and stockpiling of nuclear weapons. Even before the first nuclear weapons had been developed, scientists involved with the Manhattan Project were divided over the use of the weapon. The only time nuclear weapons have been used in warfare was during the final stages of World War II when United States Army Air Forces B-29 Superfortress bombers dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki in early August 1945. The role of the bombings in Japan's surrender and the U.S.'s ethical justification for them have been the subject of scholarly and popular debate for decades.
Nuclear disarmament refers both to the act of reducing or eliminating nuclear weapons and to the end state of a nuclear-free world. Proponents of disarmament typically condemn a priori the threat or use of nuclear weapons as immoral and argue that only total disarmament can eliminate the possibility of nuclear war. Critics of nuclear disarmament say that it would undermine deterrence and make conventional wars more likely, more destructive, or both. The debate becomes considerably complex when considering various scenarios for example, total vs partial or unilateral vs multilateral disarmament.

Nuclear weapons debate

Even before the first nuclear weapons had been developed, scientists involved with the Manhattan Project were divided over the use of the weapon. Some—notably a number at the University of Chicago Metallurgical Laboratory, represented in part by Leó Szilárd—lobbied early on that the atomic bomb should only be built as a deterrent against Nazi Germany getting a bomb, and should not be used against populated cities. The Franck Report argued in June 1945 that instead of being used against a city, the first atomic bomb should be "demonstrated" to the Japanese on an uninhabited area. This recommendation was not agreed with by the military commanders, the Los Alamos Target Committee (made up of other scientists), or the politicians who had input into the use of the weapon. Because the Manhattan Project was considered to be "top secret", there was no public discussion of the use of nuclear arms, and even within the U.S. government, knowledge of the bomb was extremely limited.

The Little Boy atomic bomb was detonated over the Japanese city of Hiroshima on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings (including the headquarters of the 2nd General Army and Fifth Division) and killed approximately 75,000 people, among them 20,000 Japanese soldiers and 20,000 Koreans. Detonation of the "Fat Man" atomic bomb exploded over the Japanese city of Nagasaki three days later on 9 August 1945, destroying 60% of the city and killing approximately 35,000 people, among them 23,200-28,200 Japanese civilian munitions workers and 150 Japanese soldiers. The role of the bombings in Japan's surrender and the U.S.'s ethical justification for them has been the subject of scholarly and popular debate for decades. J. Samuel Walker suggests that "the controversy over the use of the bomb seems certain to continue".

After the bombings of Hiroshima and Nagasaki, the world’s nuclear weapons stockpiles grew, and nuclear weapons have been detonated on over two thousand occasions for testing and demonstration purposes. Countries known to have detonated nuclear weapons—and that acknowledge possessing such weapons—are (chronologically): the United States, the Soviet Union (succeeded as a nuclear power by Russia), the United Kingdom, France, the People's Republic of China, India, Pakistan, and North Korea.

In the early 1980s, following a revival of the nuclear arms race, a popular nuclear disarmament movement emerged. In October 1981 half a million people took to the streets in several cities in Italy, more than 250,000 people protested in Bonn, 250,000 demonstrated in London, and 100,000 marched in Brussels. The largest anti-nuclear protest was held on June 12, 1982, when one million people demonstrated in New York City against nuclear weapons. In October 1983, nearly 3 million people across western Europe protested nuclear missile deployments and demanded an end to the arms race.

Under the scenario of total multilateral disarmament, there is no possibility of nuclear war. Under scenarios of partial disarmament, there is disagreement as to how the probability of nuclear war would change. Critics of nuclear disarmament say that it would undermine the ability of governments to threaten sufficient retaliation upon an attack to deter aggression against them. Application of game theory to questions of strategic nuclear warfare during the Cold War resulted in the doctrine of mutually assured destruction (MAD), a concept developed, by Robert S. McNamara, among others, in the mid-1960s. The success of MAD in averting nuclear war was theorized to depend upon the “readiness at any time before, during, or after an attack to destroy the adversary as a functioning society." Those who believe governments should develop or maintain nuclear-strike capability, usually justify their position with reference to MAD and the Cold War, claiming that a "nuclear peace" was the result of both the U.S. and the U.S.S.R. possessing mutual second-strike retaliation capability. Since the end of the cold war, theories of deterrence in international relations have been further developed and generalized in the concept of the stability–instability paradox Proponents of disarmament call into question the assumption that political leaders are rational actors who place the protection of their citizens above other considerations, and highlight, as McNamara himself later acknowledged with the benefit of hindsight, the non-rational choices, chance and contingency which played a significant role in averting nuclear war, for example during the Cuban Missile Crisis of 1962 and the Able Archer 83 crisis of 1983, thus, they argue, evidence trumps theory and deterrence theories cannot be reconciled with the historical record.

Kenneth Waltz argues in favor of the continued proliferation of nuclear weapons. In the July 2012 issue of "Foreign Affairs" Waltz took issue with the view of most U.S., European, and Israeli, commentators and policymakers that a nuclear-armed Iran would be unacceptable. Instead Waltz argues that it would probably be the best possible outcome, as it would restore stability to the Middle East by balancing Israel's regional monopoly on nuclear weapons. Professor John Mueller of Ohio State University, author of "Atomic Obsession" has also dismissed the need to interfere with Iran's nuclear program and expressed that arms control measures are counterproductive. During a 2010 lecture at the University of Missouri, which was broadcast by C-Span, Dr. Mueller has also argued that the threat from nuclear weapons, including that from terrorists, has been exaggerated, both in the popular media, and by officials.

In contrast, various American government officials, including Henry Kissinger, George Shultz, Sam Nunn, and William Perry. who were in office during the Cold War period, are now advocating the elimination of nuclear weapons in the belief that the doctrine of mutual Soviet-American deterrence is obsolete, and that reliance on nuclear weapons for deterrence is becoming increasingly hazardous and decreasingly effective in the post cold war era A 2011 article in "The Economist" argues along similar lines, that risks are more acute in rivalries between relatively new nuclear states that lack the "security safeguards" developed by America and the Soviet Union and that additional risks are posed by the emergence of pariah states, such as North Korea (possibly soon to be joined by Iran), armed with nuclear weapons as well as the declared ambition of terrorists to steal, buy or build a nuclear device.

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.

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.

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.

The United States developed the first nuclear weapons during World War II in cooperation with the United Kingdom and Canada as part of the Manhattan Project, out of the fear that Nazi Germany would develop them first. It tested the first nuclear weapon on July 16, 1945 ("Trinity") at 5:30 am, and remains the only country to have used nuclear weapons in war, devastating the Japanese cities of Hiroshima and Nagasaki. It was the first nation to develop the hydrogen bomb, testing an experimental prototype in 1952 ("Ivy Mike") and a deployable weapon in 1954 ("Castle Bravo"). Throughout the Cold War it continued to modernize and enlarge its nuclear arsenal, but from 1992 on has been involved primarily in a program of Stockpile stewardship. The U.S. nuclear arsenal contained 31,175 warheads at its Cold War height (in 1966). During the Cold War, the United States built approximately 70,000 nuclear warheads, more than all other nuclear-weapon states combined.

List of states with nuclear weapons

There are eight sovereign states that have successfully detonated nuclear weapons. Five are considered to be nuclear-weapon states (NWS) under the terms of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). In order of acquisition of nuclear weapons these are: the United States, Russia (the successor state to the Soviet Union), the United Kingdom, France, and China.

Since the NPT entered into force in 1970, three states that were not parties to the Treaty have conducted nuclear tests, namely India, Pakistan, and North Korea. North Korea had been a party to the NPT but withdrew in 2003.

Israel is also generally understood to have nuclear weapons, but does not acknowledge it, maintaining a policy of deliberate ambiguity, and is not known definitively to have conducted a nuclear test. Israel is estimated to possess somewhere between 75 and 400 nuclear warheads. One possible motivation for "nuclear ambiguity" is deterrence with minimum political cost.

States that formerly possessed nuclear weapons are South Africa (developed nuclear weapons but then disassembled its arsenal before joining the NPT) and the former Soviet republics Belarus, Kazakhstan, and Ukraine.

According to Bulletin of the Atomic Scientists Nuclear Notebook, the total number of nuclear weapons worldwide is estimated at 9,220 in 2017.

The following is a list of states that have admitted the possession of nuclear weapons or are presumed to possess them, the approximate number of warheads under their control, and the year they tested their first weapon and their force configuration. This list is informally known in global politics as the "Nuclear Club". With the exception of Russia and the United States (which have subjected their nuclear forces to independent verification under various treaties) these figures are estimates, in some cases quite unreliable estimates. In particular, under the Strategic Offensive Reductions Treaty thousands of Russian and U.S. nuclear warheads are inactive in stockpiles awaiting processing. The fissile material contained in the warheads can then be recycled for use in nuclear reactors.

From a high of 70,300 active weapons in 1986, there are approximately 3,750 active nuclear warheads and 14,485 total nuclear warheads in the world. Many of the decommissioned weapons were simply stored or partially dismantled, not destroyed.

It is also noteworthy that since the dawn of the Atomic Age, the delivery methods of most states with nuclear weapons has evolved with some achieving a nuclear triad, while others have consolidated away from land and air deterrents to submarine-based forces.

These five states are known to have detonated a nuclear explosive before 1 January 1967 and are thus nuclear weapons states under the Treaty on the Non-Proliferation of Nuclear Weapons. They also happen to be the UN Security Council's permanent members with veto power on UNSC resolutions.

The Soviet Union tested its first nuclear weapon ("RDS-1") in 1949. This crash project was developed partially with information obtained via espionage during and after World War II. The Soviet Union was the second nation to have developed and tested a nuclear weapon. The direct motivation for Soviet weapons development was to achieve a balance of power during the Cold War. It tested its first megaton-range hydrogen bomb ("RDS-37") in 1955. The Soviet Union also tested the most powerful explosive ever detonated by humans, ("Tsar Bomba"), with a theoretical yield of 100 megatons, intentionally reduced to 50 when detonated. After its dissolution in 1991, the Soviet weapons entered officially into the possession of the Russian Federation. The Soviet nuclear arsenal contained some 45,000 warheads at its peak (in 1986); the Soviet Union built about 55,000 nuclear warheads since 1949.

The United Kingdom tested its first nuclear weapon ("Hurricane") in 1952. The UK had provided considerable impetus and initial research for the early conception of the atomic bomb, aided by Austrian, German and Polish physicists working at British universities who had either fled or decided not to return to Nazi Germany or Nazi controlled territories. The UK collaborated closely with the United States and Canada during the Manhattan Project, but had to develop its own method for manufacturing and detonating a bomb as U.S. secrecy grew after 1945. The United Kingdom was the third country in the world, after the United States and Soviet Union, to develop and test a nuclear weapon. Its programme was motivated to have an independent deterrent against the Soviet Union, while also maintaining its status as a great power. It tested its first hydrogen bomb in 1957 (Operation Grapple), making it the third country to do so after the United States and Soviet Union. The UK maintained a fleet of V bomber strategic bombers and ballistic missile submarines (SSBNs) equipped with nuclear weapons during the Cold War. It currently maintains a fleet of four 'Vanguard' class ballistic missile submarines equipped with Trident II missiles. In 2016, the UK House of Commons voted to renew the British nuclear weapons system with the "Dreadnought"-class submarine, without setting a date for the commencement of service of a replacement to the current system.

France tested its first nuclear weapon in 1960 ("Gerboise Bleue"), based mostly on its own research. It was motivated by the Suez Crisis diplomatic tension "vis-à-vis" both the Soviet Union and the Free World allies United States and United Kingdom. It was also relevant to retain great power status, alongside the United Kingdom, during the post-colonial Cold War (see: Force de frappe). France tested its first hydrogen bomb in 1968 ("Opération Canopus"). After the Cold War, France has disarmed 175 warheads with the reduction and modernization of its arsenal that has now evolved to a dual system based on submarine-launched ballistic missiles (SLBMs) and medium-range air-to-surface missiles (Rafale fighter-bombers). However new nuclear weapons are in development and reformed nuclear squadrons were trained during Enduring Freedom operations in Afghanistan. France signed the Nuclear Non-Proliferation Treaty in 1992. In January 2006, President Jacques Chirac stated a terrorist act or the use of weapons of mass destruction against France would result in a nuclear counterattack. In February 2015, President Francois Hollande stressed the need for a nuclear deterrent in "a dangerous world". He also detailed the French deterrent as "less than 300" nuclear warheads, three sets of 16 submarine-launched ballistic missiles and 54 medium-range air-to-surface missiles" and urged other states to show similar transparency.

China tested its first nuclear weapon device ("596") in 1964 at the Lop Nur test site. The weapon was developed as a deterrent against both the United States and the Soviet Union. Two years later, China had a fission bomb capable of being put onto a nuclear missile. It tested its first hydrogen bomb ("Test No. 6") in 1967, a mere 32 months after testing its first nuclear weapon (the shortest fission-to-fusion development known in history). The country is currently thought to have had a stockpile of around 240 warheads, though because of the limited information available, estimates range from 100 to 400. China is the only NPT nuclear-weapon state to give an unqualified negative security assurance with its "no first use" policy. China signed the Nuclear Non-Proliferation Treaty in 1992. On February 25, 2015 U.S. Vice Admiral Joseph Mulloy stated to the House Armed Services Committee's seapower subcommittee that the U.S. does not believe the PLAN currently deploys SLBMs on their submarine fleet.

India is not a party to the Nuclear Non-Proliferation Treaty. India tested what it called a "peaceful nuclear explosive" in 1974 (which became known as "Smiling Buddha"). The test was the first test developed after the creation of the NPT, and created new questions about how civilian nuclear technology could be diverted secretly to weapons purposes (dual-use technology). India's secret development caused great concern and anger particularly from nations that had supplied its nuclear reactors for peaceful and power generating needs, such as Canada.

Indian officials rejected the NPT in the 1960s on the grounds that it created a world of nuclear "haves" and "have-nots", arguing that it unnecessarily restricted "peaceful activity" (including "peaceful nuclear explosives"), and that India would not accede to international control of their nuclear facilities unless all other countries engaged in unilateral disarmament of their own nuclear weapons. The Indian position has also asserted that the NPT is in many ways a neo-colonial regime designed to deny security to post-colonial powers. Even after its 1974 test, India maintained that its nuclear capability was primarily "peaceful", but between 1988 and 1990 it apparently weaponized two dozen nuclear weapons for delivery by air. In 1998 India tested weaponized nuclear warheads ("Operation Shakti"), including a thermonuclear device.

In July 2005, U.S. President George W. Bush and Indian Prime Minister Manmohan Singh announced plans to conclude an Indo-US civilian nuclear agreement. This came to fruition through a series of steps that included India's announced plan to separate its civil and military nuclear programs in March 2006, the passage of the India–United States Civil Nuclear Agreement by the U.S. Congress in December 2006, the conclusion of a U.S.–India nuclear cooperation agreement in July 2007, approval by the IAEA of an India-specific safeguards agreement, agreement by the Nuclear Suppliers Group to a waiver of export restrictions for India, approval by the U.S. Congress and culminating in the signature of U.S.–India agreement for civil nuclear cooperation in October 2008. The U.S. State Department said it made it "very clear that we will not recognize India as a nuclear-weapon state". The United States is bound by the Hyde Act with India and may cease all cooperation with India if India detonates a nuclear explosive device. The US had further said it is not its intention to assist India in the design, construction or operation of sensitive nuclear technologies through the transfer of dual-use items. In establishing an exemption for India, the Nuclear Suppliers Group reserved the right to consult on any future issues which might trouble it. As of December 2017, India was estimated to have a stockpile of around 130 warheads.

Pakistan also is not a party to the Nuclear Non-Proliferation Treaty. Pakistan covertly developed nuclear weapons over decades, beginning in the late 1970s. Pakistan first delved into nuclear power after the establishment of its first nuclear power plant near Karachi with equipment and materials supplied mainly by western nations in the early 1970s. Pakistani President Zulfiqar Ali Bhutto promised in 1971 that if India could build nuclear weapons then Pakistan would too, according to him: "We will develop Nuclear stockpiles, even if we have to eat grass."

It is believed that Pakistan has possessed nuclear weapons since the mid-1980s. The United States continued to certify that Pakistan did not possess such weapons until 1990, when sanctions were imposed under the Pressler Amendment, requiring a cutoff of U.S. economic and military assistance to Pakistan. In 1998, Pakistan conducted its first six nuclear tests at the Ras Koh Hills in response to the five tests conducted by India a few weeks before.

In 2004, the Pakistani metallurgist Abdul Qadeer Khan, a key figure in Pakistan's nuclear weapons program, confessed to heading an international black market ring involved in selling nuclear weapons technology. In particular, Khan had been selling gas centrifuge technology to North Korea, Iran, and Libya. Khan denied complicity by the Pakistani government or Army, but this has been called into question by journalists and IAEA officials, and was later contradicted by statements from Khan himself.

As of early 2013, Pakistan was estimated to have had a stockpile of around 140 warheads, and in November 2014 it was projected that by 2020 Pakistan would have enough fissile material for 200 warheads.

North Korea was a party to the Nuclear Non-Proliferation Treaty, but announced a withdrawal on January 10, 2003, after the United States accused it of having a secret uranium enrichment program and cut off energy assistance under the 1994 Agreed Framework. In February 2005, North Korea claimed to possess functional nuclear weapons, though their lack of a test at the time led many experts to doubt the claim. In October 2006, North Korea stated that, in response to growing intimidation by the United States, it would conduct a nuclear test to confirm its nuclear status. North Korea reported a successful nuclear test on October 9, 2006 (see 2006 North Korean nuclear test). Most U.S. intelligence officials believed that the test was probably only partially successful with a yield of less than a kiloton. North Korea conducted a second, higher yield test on 25 May 2009 (see 2009 North Korean nuclear test) and a third test with still higher yield on 12 February 2013 (see 2013 North Korean nuclear test). North Korea claimed to have conducted its first hydrogen-bomb test on 5 January 2016, though measurements of seismic disturbances indicate that the detonation was not consistent with a hydrogen bomb. On 3 September 2017, North Korea detonated a device which caused a magnitude 6.1 tremor, consistent with a low-powered thermonuclear detonation; NORSAR estimates the yield at 250 kilotons of TNT. In 2018, North Korea announced a halt in nuclear weapons tests and made a conditional commitment to denuclearisation of the Korean Peninsula.

Israel is widely believed to have been the sixth country in the world to develop nuclear weapons, but has not acknowledged its nuclear forces. It had "rudimentary, but deliverable," nuclear weapons available as early as 1966. Israel is not a party to the NPT. Israel engages in strategic ambiguity, saying it would not be the first country to "introduce" nuclear weapons into the region, but refusing to otherwise confirm or deny a nuclear weapons program or arsenal. This policy of "nuclear opacity" has been interpreted as an attempt to get the benefits of deterrence with a minimum political cost.

According to the Natural Resources Defense Council and the Federation of American Scientists, Israel likely possesses around 75–200 nuclear weapons. The Stockholm International Peace Research Institute estimates that Israel has approximately 80 intact nuclear weapons, of which 50 are for delivery by Jericho II medium-range ballistic missiles and 30 are gravity bombs for delivery by aircraft. SIPRI also reports that there was renewed speculation in 2012 that Israel may also have developed nuclear-capable submarine-launched cruise missiles.Under NATO nuclear weapons sharing, the United States has provided nuclear weapons for Belgium, Germany, Italy, the Netherlands, and Turkey to deploy and store. This involves pilots and other staff of the "non-nuclear" NATO states practicing, handling, and delivering the U.S. nuclear bombs, and adapting non-U.S. warplanes to deliver U.S. nuclear bombs. However, since all U.S. nuclear weapons are protected with Permissive Action Links, the host states cannot easily arm the bombs without authorization codes from the U.S. Department of Defense. Former Italian President Francesco Cossiga acknowledged the presence of U.S. nuclear weapons in Italy. U.S. nuclear weapons were also deployed in Canada as well as Greece from 1963 to 1984. However, Canada withdrew three of the four nuclear-capable weapons systems by 1972. The single system retained, the AIR-2 Genie, had a yield of 1.5 kilotons, was designed to strike enemy aircraft as opposed to ground targets, and might not have qualified as a weapon of mass destruction given its limited yield.

Members of the Non-Aligned Movement have called on all countries to "refrain from nuclear sharing for military purposes under any kind of security arrangements." The Institute of Strategic Studies Islamabad (ISSI) has criticized the arrangement for allegedly violating Articles I and II of the NPT, arguing that "these Articles do not permit the NWS to delegate the control of their nuclear weapons directly or indirectly to others." NATO has argued that the weapons' sharing is compliant with the NPT because "the U.S. nuclear weapons based in Europe are in the sole possession and under constant and complete custody and control of the United States."

Nuclear weapons have been present in many nations, often as staging grounds under control of other powers. However, in only one instance has a nation given up nuclear weapons after being in full control of them. The fall of the Soviet Union left several former Soviet republics in physical possession of nuclear weapons, though not operational control which was dependent on Russian-controlled electronic Permissive Action Links and the Russian command and control system.

South Africa produced six nuclear weapons in the 1980s, but dismantled them in the early 1990s.

In 1979, there was a detection of a putative covert nuclear test in the Indian Ocean, called the Vela incident. It has long been speculated that it was a test by Israel, in collaboration with and support of South Africa, though this has never been confirmed. South Africa could not have constructed such a nuclear bomb until November 1979, two months after the "double flash" incident.
South Africa signed the Nuclear Non-Proliferation Treaty in 1991.

The primary application to date has been military (i.e. nuclear weapons), and the remainder of explosions include the following:Only two nuclear weapons have been deployed in combat—both by the United States against Japan in World War II. The first event occurred on the morning of 6 August 1945, when the United States Army Air Forces dropped a uranium gun-type device, code-named "Little Boy", on the city of Hiroshima, killing 70,000 people, including 20,000 Japanese combatants and 20,000 Korean slave laborers. The second event occurred three days later when the United States Army Air Forces dropped a plutonium implosion-type device, code-named "Fat Man", on the city of Nagasaki. It killed 39,000 people, including 27,778 Japanese munitions employees, 2,000 Korean slave laborers, and 150 Japanese combatants. In total, around 109,000 people were killed in these bombings. (See "Atomic bombings of Hiroshima and Nagasaki" for a full discussion). Nuclear weapons are largely seen as a 'deterrent' by most governments; the sheer scale of the destruction caused by a nuclear weapon has prevented serious consideration of their use in warfare, rendering the concept of total war completely useless.

Nuclear explosion

A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, though to date all fusion-based weapons have used a fission device to initiate fusion, and a pure fusion weapon remains a hypothetical device.

Atmospheric nuclear explosions are associated with mushroom clouds, although mushroom clouds can occur with large chemical explosions. It is possible to have an air-burst nuclear explosion without those clouds. Nuclear explosions produce radiation and radioactive debris.

The first man made nuclear explosion occurred on July 16, 1945 at 5:50 am on the Trinity Test Site near Alamogordo, New Mexico in the United States, an area now known as the White Sands Missile Range. The event involved the full-scale testing of an implosion-type fission atomic bomb. In a memorandum to the U.S. Secretary of War, General Leslie Groves describes the yield as equivalent to 15,000 to 20,000 tons of TNT. Following this test, a uranium-gun type nuclear bomb (Little Boy) was dropped on the Japanese city of Hiroshima on August 6, 1945, with a blast yield of 15 kilotons; and a plutonium implosion-type bomb (Fat Man) on Nagasaki on August 9, 1945, with a blast yield of 21 kilotons. In the years following World War II, eight countries have conducted nuclear tests with 2475 devices fired in 2120 tests.

In 1963, the United States, Soviet Union, and United Kingdom signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground tests. Many other non-nuclear nations acceded to the Treaty following its entry into force; however, two nuclear weapons states have not acceded: France, China

Since the Trinity test and excluding the combat use of nuclear weapons, mankind (those few nations with capability) has detonated roughly 1,700 nuclear explosions, all but 6 as tests. Of these, six were peaceful nuclear explosions. Nuclear tests are experiments carried out to determine the effectiveness, yield and explosive capability of nuclear weapons. Throughout the 20th century, most nations that have developed nuclear weapons had a staged test of them. Testing nuclear weapons can yield information about how the weapons work, as well as how the weapons behave under various conditions and how structures behave when subjected to a nuclear explosion. Additionally, nuclear testing has often been used as an indicator of scientific and military strength, and many tests have been overtly political in their intention; most nuclear weapons states publicly declared their nuclear status by means of a nuclear test.

The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical damage mechanisms as conventional explosives, but the energy produced by a nuclear explosive is millions of times more per gram and the temperatures reached are in the tens of megakelvin. Nuclear weapons are quite different from conventional weapons because of the huge amount of explosive energy they can put out and the different kinds of effects they make, like high temperatures and nuclear radiation.

The devastating impact of the explosion does not stop after the initial blast, as with conventional explosives. A cloud of nuclear radiation travels from the epicenter of the explosion, causing an impact to life forms even after the heat waves have ceased.

Any nuclear explosion (or nuclear war) would have wide-ranging, long-term, catastrophic effects. Radioactive contamination would cause genetic mutations and cancer across many generations.

So far, two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. On August 6, 1945, a uranium gun-type device (code name "Little Boy") was detonated over the Japanese city of Hiroshima. Three days later, on August 9, a plutonium implosion-type device (code name "Fat Man") was detonated over the Japanese city of Nagasaki. These two bombings resulted in the deaths of approximately 120,000 people.

Nuclear warfare

Nuclear warfare (sometimes atomic warfare or thermonuclear warfare) is a military conflict or political strategy in which nuclear weaponry is used to inflict damage on the enemy. Nuclear weapons are weapons of mass destruction; in contrast to conventional warfare, nuclear warfare can produce destruction in a much shorter time and can have a long-lasting radiological warfare result. A major nuclear exchange would have long-term effects, primarily from the fallout released, and could also lead to a "nuclear winter" that could last for decades, centuries, or even millennia after the initial attack. Some analysts dismiss the nuclear winter hypothesis, and calculate that even with nuclear weapon stockpiles at Cold War highs, although there would be billions of casualties, billions more rural people would nevertheless survive. However, others have argued that secondary effects of a nuclear holocaust, such as nuclear famine and societal collapse, would cause almost every human on Earth to starve to death.

After World War II, nuclear weapons were also developed by the Soviet Union (1949), the United Kingdom (1952), France (1960), and the People's Republic of China (1964), which contributed to the state of conflict and extreme tension that became known as the Cold War. In 1974, India, and in 1998, Pakistan, two countries that were openly hostile toward each other, developed nuclear weapons. Israel (1960s) and North Korea (2006) are also thought to have developed stocks of nuclear weapons, though it is not known how many. The Israeli government has never admitted or denied to having nuclear weapons, although it is known to have constructed the reactor and reprocessing plant necessary for building nuclear weapons. South Africa also manufactured several complete nuclear weapons in the 1980s, but subsequently became the first country to voluntarily destroy their domestically made weapons stocks and abandon further production (1990s). Nuclear weapons have been detonated on over 2,000 occasions for testing purposes and demonstrations.

After the collapse of the Soviet Union in 1991 and the resultant end of the Cold War, the threat of a major nuclear war between the two nuclear superpowers was generally thought to have declined. Since then, concern over nuclear weapons has shifted to the prevention of localized nuclear conflicts resulting from nuclear proliferation, and the threat of nuclear terrorism.

The possibility of using nuclear weapons in war is usually divided into two subgroups, each with different effects and potentially fought with different types of nuclear armaments.

The first, a limited nuclear war (sometimes "attack" or "exchange"), refers to a small-scale use of nuclear weapons by two (or more) belligerents. A "limited nuclear war" could include targeting military facilities—either as an attempt to pre-emptively cripple the enemy's ability to attack as a defensive measure, or as a prelude to an invasion by conventional forces, as an offensive measure. This term could apply to "any" small-scale use of nuclear weapons that may involve military or civilian targets (or both).

The second, a full-scale nuclear war, could consist of large numbers of nuclear weapons used in an attack aimed at an entire country, including military, economic, and civilian targets. Such an attack would almost certainly destroy the entire economic, social, and military infrastructure of the target nation, and would probably have a devastating effect on Earth's biosphere.

Some Cold War strategists such as Henry Kissinger argued that a limited nuclear war could be possible between two heavily armed superpowers (such as the United States and the Soviet Union). Some predict, however, that a limited war could potentially "escalate" into a full-scale nuclear war. Others have called limited nuclear war "global nuclear holocaust in slow motion", arguing that—once such a war took place—others would be sure to follow over a period of decades, effectively rendering the planet uninhabitable in the same way that a "full-scale nuclear war" between superpowers would, only taking a much longer (and arguably more agonizing) path to the same result.

Even the most optimistic predictions of the effects of a major nuclear exchange foresee the death of many millions of victims within a very short period of time. More pessimistic predictions argue that a full-scale nuclear war could potentially bring about the extinction of the human race, or at least its "near" extinction, with only a relatively small number of survivors (mainly in remote areas) and a reduced quality of life and life expectancy for centuries afterward. However, such predictions, assuming total war with nuclear arsenals at Cold War highs, have not been without criticism. Such a horrific catastrophe as global nuclear warfare would almost certainly cause permanent damage to most complex life on the planet, its ecosystems, and the global climate. If predictions about the production of a nuclear winter are accurate, it would also change the balance of global power, with countries such as Australia, New Zealand, India, China, Argentina and Brazil predicted to become world superpowers if the Cold War ever led to a large-scale nuclear attack.

A study presented at the annual meeting of the American Geophysical Union in December 2006 asserted that even a small-scale regional nuclear war could produce as many direct fatalities as all of World War II and disrupt the global climate for a decade or more. In a regional nuclear conflict scenario in which two opposing nations in the subtropics each used 50 Hiroshima-sized nuclear weapons (c. 15 kiloton each) on major population centers, the researchers predicted fatalities ranging from 2.6 million to 16.7 million per country. The authors of the study estimated that as much as five million tons of soot could be released, producing a cooling of several degrees over large areas of North America and Eurasia (including most of the grain-growing regions). The cooling would last for years and could be "catastrophic", according to the researchers.

Either a limited or full-scale nuclear exchange could occur during an "accidental nuclear war", in which the use of nuclear weapons is triggered unintentionally. Postulated triggers for this scenario have included malfunctioning early warning devices and/or targeting computers, deliberate malfeasance by rogue military commanders, consequences of an accidental straying of warplanes into enemy airspace, reactions to unannounced missile tests during tense diplomatic periods, reactions to military exercises, mistranslated or miscommunicated messages, and others. A number of these scenarios actually occurred during the Cold War, though none resulted in the use of nuclear weapons. Many such scenarios have been depicted in popular culture, such as in the 1959 film "On the Beach", the 1962 novel "Fail-Safe" (released as a film in 1964); the film "WarGames", released in 1983; and the film "", also released in 1964.

During the final stages of World War II in 1945, the United States conducted atomic raids on the Japanese cities of Hiroshima and Nagasaki, the first on August 6, 1945, and the second on August 9, 1945. These two events were the only times nuclear weapons have been used in combat.

For six months before the atomic bombings, the U.S. 20th Air Force under General Curtis LeMay executed low-level incendiary raids against Japanese cities. The worst air raid to occur during the process was not the nuclear attacks, but the "Operation Meetinghouse" raid on Tokyo. On the night of March 9–10, 1945, "Operation Meetinghouse" commenced and 334 Boeing B-29 Superfortress bombers took off to raid, with 279 of them dropping 1,665 tons of incendiaries and explosives on Tokyo. The bombing was meant to burn wooden buildings and indeed the bombing caused fire that created a 50 m/s wind, which is comparable to tornadoes. Each bomber carried 6 tons of bombs. A total of 381,300 bombs, which amount to 1,783 tons of bombs, were used in the bombing. Within a few hours of the raid, it had killed an estimated 100,000 people and destroyed of the city and 267,000 buildings in a single night — the deadliest bombing raid in military aviation history other than the atomic raids on Hiroshima and Nagasaki. By early August 1945, an estimated 450,000 people had died as the U.S. had intensely firebombed a total of 67 Japanese cities.

In late June 1945, as the U.S. wrapped up the two-and-a-half-month Battle of Okinawa (which cost the lives of 260,000 people, including 150,000 civilians), it was faced with the prospect of invading the Japanese home islands in an operation codenamed Operation Downfall. Based on the U.S. casualties from the preceding island-hopping campaigns, American commanders estimated that between 50,000 and 500,000 U.S. troops would die and at least 600,000–1,000,000 others would be injured while invading the Japanese home islands. The U.S. manufacture of 500,000 Purple Hearts from the anticipated high level of casualties during the U.S. invasion of Japan gave a demonstration of how deadly and costly it would be. President Harry S. Truman realized he could not afford such a horrendous casualty rate, especially since over 400,000 American combatants had already died fighting in both the European and the Pacific theaters of the war.

On July 26, 1945, the United States, the United Kingdom, and the Republic of China issued a Potsdam Declaration that called for the unconditional surrender of Japan. It stated that if Japan did not surrender, it would face "prompt and utter destruction". The Japanese government ignored this ultimatum, sending a message that they were not going to surrender. In response to the rejection, President Truman authorized the dropping of the atomic bombs. At the time of its use, there were only two atomic bombs available, and despite the fact that more were in production back in mainland U.S., the third bomb wouldn't be available for combat until September.

On August 6, 1945, the uranium-type nuclear weapon codenamed "Little Boy" was detonated over the Japanese city of Hiroshima with an energy of about, destroying nearly 50,000 buildings (including the headquarters of the 2nd General Army and Fifth Division) and killing approximately 70,000 people, including 20,000 Japanese combatants and 20,000 Korean slave laborers. Three days later, on August 9, a plutonium-type nuclear weapon codenamed "Fat Man" was used against the Japanese city of Nagasaki, with the explosion equivalent to about, destroying 60% of the city and killing approximately 35,000 people, including 23,200–28,200 Japanese munitions workers, 2,000 Korean slave laborers, and 150 Japanese combatants. The industrial damage in Nagasaki was high, partly owing to the inadvertent targeting of the industrial zone, leaving 68–80 percent of the non-dock industrial production destroyed.

Six days after the detonation over Nagasaki, Japan announced its surrender to the Allied Powers on August 15, 1945, signing the Instrument of Surrender on September 2, 1945, officially ending the Pacific War and, therefore, World War II, as Germany had already signed its Instrument of Surrender on May 8, 1945, ending the war in Europe. The two atomic bombings led, in part, to post-war Japan's adopting of the Three Non-Nuclear Principles, which forbade the nation from developing nuclear armaments.

After the successful Trinity nuclear test July 16, 1945, which was the very first nuclear detonation, the Manhattan project lead manager J. Robert Oppenheimer recalled:
Immediately after the atomic bombings of Japan, the status of atomic weapons in international and military relations was unclear. Presumably, the United States hoped atomic weapons could offset the Soviet Union's larger conventional ground forces in Eastern Europe, and possibly be used to pressure Soviet leader Joseph Stalin into making concessions. Under Stalin, the Soviet Union pursued its own atomic capabilities through a combination of scientific research and espionage directed against the American program. The Soviets believed that the Americans, with their limited nuclear arsenal, were unlikely to engage in any new world wars, while the Americans were not confident they could prevent a Soviet takeover of Europe, despite their atomic advantage.

Within the United States the authority to produce and develop nuclear weapons was removed from military control and put instead under the civilian control of the United States Atomic Energy Commission. This decision reflected an understanding that nuclear weapons had unique risks and benefits that were separate from other military technology known at the time.

For several years after World War II, the United States developed and maintained a strategic force based on the Convair B-36 bomber that would be able to attack any potential enemy from bomber bases in the United States. It deployed atomic bombs around the world for potential use in conflicts. Over a period of a few years, many in the American defense community became increasingly convinced of the invincibility of the United States to a nuclear attack. Indeed, it became generally believed that the threat of nuclear war would deter any strike against the United States.

Many proposals were suggested to put all American nuclear weapons under international control (by the newly formed United Nations, for example) as an effort to deter both their usage and an arms race. However, no terms could be arrived at that would be agreed upon by both the United States and the Soviet Union.

On August 29, 1949, the Soviet Union tested its first nuclear weapon at Semipalatinsk in Kazakhstan (see also Soviet atomic bomb project). Scientists in the United States from the Manhattan Project had warned that, in time, the Soviet Union would certainly develop nuclear capabilities of its own. Nevertheless, the effect upon military thinking and planning in the United States was dramatic, primarily because American military strategists had not anticipated the Soviets would "catch up" so soon. However, at this time, they had not discovered that the Soviets had conducted significant nuclear espionage of the project from spies at Los Alamos, the most significant of which was done by the theoretical physicist Klaus Fuchs. The first Soviet bomb was more or less a deliberate copy of the Fat Man plutonium device. In the same year the first US-Soviet nuclear war plan was penned in the US with Operation Dropshot.

With the monopoly over nuclear technology broken, worldwide nuclear proliferation accelerated. The United Kingdom tested its first independent atomic bomb in 1952, followed by France in 1960 and then China in 1964. While much smaller than the arsenals of the United States and the Soviet Union, Western Europe's nuclear reserves were nevertheless a significant factor in strategic planning during the Cold War. A top-secret White Paper, compiled by the Royal Air Force and produced for the British Government in 1959, estimated that British bombers carrying nuclear weapons were capable of destroying key cities and military targets in the Soviet Union, with an estimated 16 million deaths in the Soviet Union (half of whom were estimated to be killed on impact and the rest fatally injured) "before" bomber aircraft from the U.S. Strategic Air Command reached their targets.

Although the Soviet Union had nuclear weapon capabilities in the beginning of the Cold War, the United States still had an advantage in terms of bombers and weapons. In any exchange of hostilities, the United States would have been capable of bombing the Soviet Union, whereas the Soviet Union would have more difficulty carrying out the reverse mission.

The widespread introduction of jet-powered interceptor aircraft upset this imbalance somewhat by reducing the effectiveness of the American bomber fleet. In 1949 Curtis LeMay was placed in command of the Strategic Air Command and instituted a program to update the bomber fleet to one that was all-jet. During the early 1950s the B-47 and B-52 were introduced, providing the ability to bomb the Soviet Union more easily. Before the development of a capable strategic missile force in the Soviet Union, much of the war-fighting doctrine held by western nations revolved around using a large number of smaller nuclear weapons in a tactical role. It is debatable whether such use could be considered "limited" however, because it was believed that the United States would use its own strategic weapons (mainly bombers at the time) should the Soviet Union deploy any kind of nuclear weapon against civilian targets. Douglas MacArthur, an American general, was fired by President Harry Truman, partially because he persistently requested permission to use his own discretion in deciding whether to utilize atomic weapons on the People's Republic of China in 1951 during the Korean War. Mao Zedong, China's communist leader, gave the impression that he would welcome a nuclear war with the capitalists because it would annihilate what he viewed as their "imperialist" system.

The concept of a "Fortress North America" emerged during the Second World War and persisted into the Cold War to refer to the option of defending Canada and the United States against their enemies if the rest of the world were lost to them. This option was rejected with the formation of NATO and the decision to permanently station troops in Europe.

In the summer of 1951 Project Vista started, in which project analysts such as Robert F. Christy looked at how to defend Western Europe from a Soviet invasion. The emerging development of tactical nuclear weapons were looked upon as a means to give Western forces a qualitative advantage over the Soviet numerical supremacy in conventional weapons.

Several scares about the increasing ability of the Soviet Union's strategic bomber forces surfaced during the 1950s. The defensive response by the United States was to deploy a fairly strong "layered defense" consisting of interceptor aircraft and anti-aircraft missiles, like the Nike, and guns, like the Skysweeper, near larger cities. However, this was a small response compared to the construction of a huge fleet of nuclear bombers. The principal nuclear strategy was to massively penetrate the Soviet Union. Because such a large area could not be defended against this overwhelming attack in any credible way, the Soviet Union would lose any exchange.

This logic became ingrained in American nuclear doctrine and persisted for much of the duration of the Cold War. As long as the strategic American nuclear forces could overwhelm their Soviet counterparts, a Soviet pre-emptive strike could be averted. Moreover, the Soviet Union could not afford to build any reasonable counterforce, as the economic output of the United States was far larger than that of the Soviets, and they would be unable to achieve "nuclear parity".

Soviet nuclear doctrine, however, did not match American nuclear doctrine. Soviet military planners assumed they could win a nuclear war. Therefore, they "expected" a large-scale nuclear exchange, followed by a "conventional war" which itself would involve heavy use of tactical nuclear weapons. American doctrine rather assumed that Soviet doctrine was similar, with the "mutual" in Mutually Assured Destruction necessarily requiring that the other side see things in much the same way, rather than believing—as the Soviets did—that they could fight a large-scale, "combined nuclear and conventional" war.

In accordance with their doctrine, the Soviet Union conducted large-scale military exercises to explore the possibility of defensive and offensive warfare during a nuclear war. The exercise, under the code name of "Snowball", involved the detonation of a nuclear bomb about twice as powerful as that which fell on Nagasaki and an army of approximately 45,000 soldiers on maneuvers through the hypocenter immediately after the blast. The exercise was conducted on September 14, 1954, under command of Marshal Georgy Zhukov to the north of Totskoye village in Orenburg Oblast, Russia.

A revolution in nuclear strategic thought occurred with the introduction of the intercontinental ballistic missile (ICBM), which the Soviet Union first successfully tested in August 1957. In order to deliver a warhead to a target, a missile was much faster and more cost-effective than a bomber, and enjoyed a higher survivability due to the enormous difficulty of interception of the ICBMs (due to their high altitude and extreme speed). The Soviet Union could now afford to achieve nuclear parity with the United States in raw numbers, although for a time, they appeared to have chosen not to.

Photos of Soviet missile sites set off a wave of panic in the U.S. military, something the launch of Sputnik would do for the American public a few months later. Politicians, notably then-U.S. Senator John F. Kennedy suggested that a "missile gap" existed between the Soviet Union and the United States. The US military gave missile development programs the highest national priority, and several spy aircraft and reconnaissance satellites were designed and deployed to observe Soviet progress.

Early ICBMs and bombers were relatively inaccurate, which led to the concept of countervalue strikes — attacks directly on the enemy population, which would theoretically lead to a collapse of the enemy's will to fight. During the Cold War, the Soviet Union invested in extensive protected civilian infrastructure, such as large "nuclear-proof" bunkers and non-perishable food stores. By comparison, smaller scale civil defense programs were instituted in the United States starting in the 1950s, where schools and other public buildings had basements stocked with non-perishable food supplies, canned water, first aid, and dosimeter and Geiger counter radiation-measuring devices. Many of the locations were given "Fallout shelter" designation signs. CONELRAD radio information systems were adopted, whereby the commercial radio sector (later supplemented by the National Emergency Alarm Repeaters) would broadcast on two AM frequencies in the event of a Civil Defense (CD) emergency. These two frequencies, 640 and 1240 kHz, were marked with small CD triangles on the tuning dial of radios of the period, as can still be seen on 1950s-vintage radios on online auction sites and museums. A few backyard fallout shelters were built by private individuals.

Henry Kissinger's view on tactical nuclear war in his controversial 1957 book Nuclear Weapons and Foreign Policy was that any nuclear weapon exploded in air burst mode that was below 500 kiloton in yield and thus averting serious fallout, may be more decisive and less costly in human lives than a protracted conventional war.

A list of targets made by the U.S.A. was released sometime during December 2015 by the U.S. National Archives and Records Administration. The language used to describe targets is "designated ground zeros". The list was released after a request was made during 2006 by William Burr who belongs to a research group at George Washington University, and belongs to a previously top-secret 800-page document. The list is entitled "Atomic Weapons Requirements Study for 1959" and was produced by U.S. Strategic Air Command during the year 1956.

In 1960, the United States developed its first Single Integrated Operational Plan, a range of targeting options, and described launch procedures and target sets against which nuclear weapons would be launched, variants of which were in use from 1961 to 2003. That year also saw the start of the Missile Defense Alarm System, an American system of 12 early-warning satellites that provided limited notice of Soviet intercontinental ballistic missile launches between 1960 and 1966. The Ballistic Missile Early Warning System was completed in 1964.

A complex and worrisome situation developed in 1962, in what is called the Cuban Missile Crisis. The Soviet Union placed medium-range ballistic missiles from the United States, possibly as a direct response to American Jupiter missiles placed in Turkey. After intense negotiations, the Soviets ended up removing the missiles from Cuba and decided to institute a massive weapons-building program of their own. In exchange, the United States dismantled its launch sites in Turkey, although this was done secretly and not publicly revealed for over two decades. Khrushchev did not even reveal this part of the agreement when he came under fire by political opponents for mishandling the crisis. Communication delays during the crisis led to the establishment of the Moscow–Washington hotline to allow reliable, direct communications between the two nuclear powers.

By the late 1960s, the number of ICBMs and warheads was so high on both sides that it was believed that both the United States and the Soviet Union were capable of completely destroying the infrastructure and a large proportion of the population of the other country. Thus, by some western game theorists, a balance of power system known as mutually assured destruction (or "MAD") came into being. It was thought that no full-scale exchange between the powers would result in an outright winner, with at best one side emerging the pyrrhic victor. Thus both sides were deterred from risking the initiation of a direct confrontation, instead being forced to engage in lower intensity proxy wars.

During this decade the People's Republic of China began to build subterranean infrastructure such as the Underground Project 131 following the Sino-Soviet split.
One drawback of the MAD doctrine was the possibility of a nuclear war occurring without either side intentionally striking first. Early Warning Systems (EWS) were notoriously error-prone. For example, on 78 occasions in 1979 alone, a "missile display conference" was called to evaluate detections that were "potentially threatening to the North American continent". Some of these were trivial errors and were spotted quickly, but several went to more serious levels. On September 26, 1983, Stanislav Petrov received convincing indications of an American first strike launch against the Soviet Union, but positively identified the warning as a false alarm. Though it is unclear what role Petrov's actions played in preventing a nuclear war during this incident, he has been honored by the United Nations for his actions.

Similar incidents happened many times in the United States, due to failed computer chips, misidentifications of large flights of geese, test programs, and bureaucratic failures to notify early warning military personnel of legitimate launches of test or weather missiles. For many years, the U.S. Air Force's strategic bombers were kept airborne on a daily rotating basis "around the clock" (see Operation Chrome Dome), until the number and severity of accidents, the 1968 Thule Air Base B-52 crash in particular, persuaded policymakers it was not worthwhile.

Israel responded to the Arab Yom Kippur War attack on 6 October 1973 by assembling 13 nuclear weapons in a tunnel under the Negev desert when Syrian tanks were sweeping in across the Golan Heights. On 8 October 1973, Israeli Prime Minister Mrs Golda Meir authorized Defense Minister Moshe Dayan to activate the 13 Israeli nuclear warheads and distribute them to Israeli air force units, with the intent that they be used if Israel began to be overrun.

On 24 October 1973, as US President Nixon was preoccupied with the Watergate scandal, Henry Kissinger ordered a DEFCON-3 alert preparing American B-52 nuclear bombers for war. Intelligence reports indicated that the USSR was preparing to defend Egypt in its Yom Kippur war with Israel. It had become apparent that if Israel had dropped nuclear weapons on Egypt or Syria, as it prepared to do, then the USSR would have retaliated against Israel, with the US then committed to providing Israeli assistance, possibly escalating to a general nuclear war.

By the late 1970s, people in both the United States and the Soviet Union, along with the rest of the world, had been living with the concept of mutual assured destruction (MAD) for about a decade, and it became deeply ingrained into the psyche and popular culture of those countries.

On May 18, 1974, India conducted its first nuclear test in the Pokhran test range. The name of the operation was Smiling Buddha, and India termed the test as a "peaceful nuclear explosion".

The Soviet Duga early warning over-the-horizon radar system was made operational in 1976. The extremely powerful radio transmissions needed for such a system led to much disruption of civilian shortwave broadcasts, earning it the nickname "Russian Woodpecker".

The idea that any nuclear conflict would eventually escalate was a challenge for military strategists. This challenge was particularly severe for the United States and its NATO allies. It was believed (until the 1970s) that a Soviet tank invasion of Western Europe would quickly overwhelm NATO conventional forces, leading to the necessity of the West escalating to the use of tactical nuclear weapons, one of which was the W-70.

This strategy had one major (and possibly critical) flaw, which was soon realized by military analysts but highly underplayed by the U.S. military: conventional NATO forces in the European theatre of war were far outnumbered by similar Soviet and Warsaw Pact forces, and it was assumed that in case of a major Soviet attack (commonly envisioned as the "Red tanks rolling towards the North Sea" scenario) that NATO—in the face of quick conventional defeat—would soon have no other choice but to resort to tactical nuclear strikes against these forces. Most analysts agreed that once the first nuclear exchange had occurred, escalation to global nuclear war would likely become inevitable. The Soviet bloc's vision of an atomic war between NATO and Warsaw Pact forces was simulated in the top secret exercise Seven Days to the River Rhine in 1979. The British government exercised their vision of Soviet nuclear attack with Square Leg in early 1980.

Large hardened nuclear weapon storage areas were built across European countries in anticipation of local US and European forces falling back as the conventional NATO defense from the Soviet Union, named REFORGER, was believed to only be capable of stalling the Soviets for a short time.

In the late 1970s and, particularly, during the early 1980s under U.S. President Ronald Reagan, the United States renewed its commitment to a more powerful military, which required a large increase in spending on U.S. military programs. These programs, which were originally part of the defense budget of U.S. President Jimmy Carter, included spending on conventional and nuclear weapons systems. Under Reagan, defensive systems like the Strategic Defense Initiative were emphasized as well.

Another major shift in nuclear doctrine was the development and the improvement of the submarine-launched, nuclear-armed, ballistic missile, or SLBM. It was hailed by many military theorists as a weapon that would make nuclear war less likely. SLBMs—which can move with "stealth" (greatly lessened detectability) virtually anywhere in the world—give a nation a "second strike" capability (i.e., after absorbing a "first strike"). Before the advent of the SLBM, thinkers feared that a nation might be tempted to initiate a first strike if it felt confident that such a strike would incapacitate the nuclear arsenal of its enemy, making retaliation impossible. With the advent of SLBMs, no nation could be certain that a first strike would incapacitate its enemy's entire nuclear arsenal. To the contrary, it would have to fear a near certain retaliatory second strike from SLBMs. Thus, a first strike was a much less feasible (or desirable) option, and a deliberately initiated nuclear war was thought to be less likely to start.

However, it was soon realized that submarines could approach enemy coastlines undetected and decrease the warning time (the time between detection of the missile launch and the impact of the missile) from as much as half an hour to possibly under three minutes. This effect was especially significant to the United States, Britain and China, whose capitals all lay within 100 miles (160 km) of their coasts. Moscow was much more secure from this type of threat, due to its considerable distance from the sea. This greatly increased the credibility of a "surprise first strike" by one faction and (theoretically) made it possible to knock out or disrupt the chain of command of a target nation before any counterstrike could be ordered (known as a "decapitation strike"). It strengthened the notion that a nuclear war could possibly be "won", resulting not only in greatly increased tensions and increasing calls for fail-deadly control systems, but also in a dramatic increase in military spending. The submarines and their missile systems were very expensive, and one fully equipped nuclear-powered and nuclear-armed missile submarine could cost more than the entire GNP of a developing country. It was also calculated, however, that the greatest cost came in the development of "both" sea- and land-based anti-submarine defenses and in improving and strengthening the "chain of command", and as a result, military spending skyrocketed.

South Africa developed a nuclear weapon capability during the 1970s and early 1980s. It was operational for a brief period before being dismantled in the early 1990s.

According to the 1980 United Nations report "General and Complete Disarmament: Comprehensive Study on Nuclear Weapons: Report of the Secretary-General", it was estimated that there were a total of about 40,000 nuclear warheads in existence at that time, with a potential combined explosive yield of approximately 13,000 megatons. By comparison, when the volcano Mount Tambora erupted in 1815—turning 1816 into the Year Without A Summer due to the levels of global dimming sulfate aerosols and ash expelled—it exploded with a force of roughly 800 to 1,000 megatons, and ejected of mostly rock/tephra, that included 120 million tonnes of sulfur dioxide as an upper estimate. A larger eruption, approximately 74,000 years ago, in Mount Toba produced of tephra, forming lake Toba, and produced an estimated of sulfur dioxide. The explosive energy of the eruption may have been as high as equivalent to 20,000,000 megatons (Mt) of TNT, while the asteroid created Chicxulub impact, that is connected with the extinction of the dinosaurs corresponds to at least 70,000,000 Mt of energy, which is roughly 7000 times the maximum arsenal of the US and Soviet Union.

However, comparisons with supervolcanoes are more misleading than helpful due to the different aerosols released, the likely air burst fuzing height of nuclear weapons and the globally scattered location of these potential nuclear detonations all being in contrast to the singular and subterranean nature of a supervolcanic eruption. Moreover, assuming the entire world stockpile of weapons were grouped together, it would be difficult, due to the nuclear fratricide effect, to ensure the individual weapons would go off all at once. Nonetheless, many people believe that a full-scale nuclear war would result, through the nuclear winter effect, in the extinction of the human species, though not all analysts agree on the assumptions that underpin these nuclear winter models.

On Sept. 1, 1983, Korean Air Lines Flight 007 was shot down by Soviet jet fighters. On the 26th, a Soviet early warning station under the command of Stanislav Petrov falsely detected 5 inbound intercontinental ballistic missiles from the US. Petrov correctly assessed the situation as a false alarm, and hence did not report his finding to his superiors. It is quite possible that his actions prevented "World War III", as the Soviet policy at that time was immediate nuclear response upon discovering inbound ballistic missiles.

The world came unusually close to nuclear war when the Soviet Union thought that the NATO military exercise Able Archer 83 was a ruse or "cover-up" to begin a nuclear first strike. The Soviets responded by raising readiness and preparing their nuclear arsenal for immediate use. Soviet fears of an attack ceased once the exercise concluded without incident.

Although the dissolution of the Soviet Union ended the Cold War and greatly reduced tensions between the United States and the Russian Federation, the Soviet Union's formal successor state, both countries remained in a "nuclear stand-off" due to the continuing presence of a very large number of deliverable nuclear warheads on both sides. Additionally, the end of the Cold War led the United States to become increasingly concerned with the development of nuclear technology by other nations outside of the former Soviet Union. In 1995, a branch of the U.S. Strategic Command produced an outline of forward-thinking strategies in the document "Essentials of Post–Cold War Deterrence".

In 1996, a Russian continuity of government facility, Kosvinsky Mountain, which is believed to be a counterpart to the US Cheyenne Mountain Complex, was completed. It was designed to resist US earth-penetrating nuclear warheads, and is believed to host the Russian Strategic Rocket Forces alternate command post, a post for the general staff built to compensate for the vulnerability of older Soviet era command posts in the Moscow region. In spite of this, the primary command posts for the Strategic Rocket Forces remains Kuntsevo in Moscow and the secondary is the Kosvinsky Mountain in the Urals. The timing of the Kosvinsky facilities completion date is regarded as one explanation for U.S. interest in a new nuclear "bunker buster" Earth-penetrating warhead and the declaration of the deployment of the B-61 mod 11 in 1997; Kosvinsky is protected by about 1000 feet of granite.

As a consequence of the 9/11 attacks, American forces immediately increased their readiness to the highest level in 28 years, closing the blast doors of the NORAD's Cheyenne Mountain Operations Center for the first time due to a non-exercise event. But unlike similar increases during the Cold War, Russia immediately decided to stand down a large military exercise in the Arctic region, in order to minimize the risk of incidents, rather than following suit.

The former chair of the United Nations disarmament committee stated that there are more than 16,000 strategic and tactical nuclear weapons ready for deployment and another 14,000 in storage, with the U.S. having nearly 7,000 ready for use and 3,000 in storage, and Russia having about 8,500 ready for use and 11,000 in storage. In addition, China is thought to possess about 400 nuclear weapons, Britain about 200, France about 350, India about 80–100, and Pakistan 100–110. North Korea is confirmed as having nuclear weapons, though it is not known how many, with most estimates between 1 and 10. Israel is also widely believed to possess usable nuclear weapons. NATO has stationed about 480 American nuclear weapons in Belgium, the Netherlands, Italy, Germany, and Turkey, and several other nations are thought to be in pursuit of an arsenal of their own.

Pakistan's nuclear policy was significantly affected by the 1965 war with India. The 1971 war and India's nuclear program played a role in Pakistan's decision to go nuclear. India and Pakistan both decided not to participate in the NPT. Pakistan's nuclear policy became fixated on India because India refused to join the NPT and remain opened to nuclear weapons. Impetus by Indian actions spurred Pakistan's nuclear research. After nuclear weapons construction was started by Bhutto's command, the chair of Pakistan Atomic Energy Commission Usmani quit in objection. The 1999 war between Pakistan and India occurred after both acquired nuclear weapons. It is believed by some that nuclear weapons are the reason a big war has not broken out in the subcontinent. India and Pakistan still have a risk of nuclear conflict on the issue of war over Kashmir. Nuclear capability deliverable by sea were claimed by Pakistan in 2012. The aim was to achieve a "minimum credible deterrence". Pakistan's nuclear program culminated in the tests at Chagai. One of the aims of Pakistan's programs is fending off potential annexation and maintaining independence.

A key development in nuclear warfare throughout the 2000s and early 2010s is the proliferation of nuclear weapons to the developing world, with India and Pakistan both publicly testing several nuclear devices, and North Korea conducting an underground nuclear test on October 9, 2006. The U.S. Geological Survey measured a 4.2 magnitude earthquake in the area where the North Korean test is said to have occurred. A further test was announced by the North Korean government on May 25, 2009. Iran, meanwhile, has embarked on a nuclear program which, while officially for civilian purposes, has come under close scrutiny by the United Nations and many individual states.

Recent studies undertaken by the CIA cite the enduring India-Pakistan conflict as the one "flash point" most likely to escalate into a nuclear war. During the Kargil War in 1999, Pakistan came close to using its nuclear weapons in case the conventional military situation underwent further deterioration. Pakistan's foreign minister had even warned that it would "use any weapon in our arsenal", hinting at a nuclear strike against India. The statement was condemned by the international community, with Pakistan denying it later on. This conflict remains the only war (of any sort) between two declared nuclear powers. The 2001-2002 India-Pakistan standoff again stoked fears of nuclear war between the two countries. Despite these very serious and relatively recent threats, relations between India and Pakistan have been improving somewhat over the last few years. However, with the November 26, 2008 Mumbai terror attacks, tensions again worsened.

Another potential geopolitical issue which is considered particularly worrisome by military analysts is a possible conflict between the United States and the People's Republic of China over Taiwan. Although economic forces are thought to have reduced the possibility of a military conflict, there remains concern about the increasing military buildup of China (China is rapidly increasing its naval capacity), and that any move toward Taiwan independence could potentially spin out of control.

Israel is thought to possess somewhere between one hundred and four hundred nuclear warheads. It has been asserted that the Dolphin-class submarines which Israel received from Germany have been adapted to carry Popeye cruise missiles with nuclear warheads, so as to give Israel a second strike capability. Israel has been involved in wars with its neighbors in the Middle East (and with other "non-state actors") on numerous prior occasions, and its small geographic size and population could mean that, in the event of future wars, the Israeli military might have very little time to react to an invasion or other major threat. Such a situation could escalate to nuclear warfare very quickly in some scenarios.

On March 7, 2013, North Korea threatened the United States with a pre-emptive nuclear strike. On April 9, North Korea urged foreigners to leave South Korea, stating that both countries were on the verge of nuclear war. On April 12, North Korea stated that a nuclear war was unavoidable. The country declared Japan as its first target.

In the year of 2014, when Russian relationships with the USA had become worse, it was stated by the Russian state-owned Russia 1 TV channel that ‘Russia is the only country in the world that is really capable of turning the USA into radioactive ash’.

The above examples envisage nuclear warfare at a strategic level, i.e., total war. However, nuclear powers have the ability to undertake more limited engagements.

"Sub-strategic use" includes the use of either "low-yield" tactical nuclear weapons, or of variable yield strategic nuclear weapons in a very limited role, as compared to battlefield exchanges of larger-yield strategic nuclear weapons. This was described by the UK Parliamentary Defence Select Committee as "the launch of one or a limited number of missiles against an adversary as a means of conveying a political message, warning or demonstration of resolve". It is believed that all current nuclear weapons states possess tactical nuclear weapons, with the exception of the United Kingdom, which decommissioned its tactical warheads in 1998. However, the UK does possess scalable-yield strategic warheads, and this technology tends to blur the difference between "strategic", "sub-strategic", and "tactical" use or weapons. American, French and British nuclear submarines are believed to carry at least "some" missiles with dial-a-yield warheads for this purpose, potentially allowing a strike as low as one kiloton (or less) against a single target. Only the People's Republic of China and the Republic of India have declarative, unqualified, unconditional "no first use" nuclear weapons policies. India and Pakistan maintain only a credible minimum deterrence.

Commodore Tim Hare, former Director of Nuclear Policy at the British Ministry of Defence, has described "sub-strategic use" as offering the Government "an extra option in the escalatory process before it goes for an all-out strategic strike which would deliver unacceptable damage". However, this sub-strategic capacity has been criticized as potentially increasing the "acceptability" of using nuclear weapons. Combined with the trend in the reduction in the worldwide nuclear arsenal as of 2007 is the warhead miniaturization and modernization of the remaining strategic weapons that is presently occurring in all the declared nuclear weapon states, into more "usable" configurations. The Stockholm International Peace Research Institute suggests that this is creating a culture where use of these weapons is more acceptable and therefore is increasing the risk of war, as these modern weapons do not possess the same psychological deterrent value as the large Cold-War era, multi-megaton warheads.

In many ways, this present change in the balance of terror can be seen as the complete embracement of the switch from the 1950s Eisenhower doctrine of "massive retaliation" to one of "flexible response", which has been growing in importance in the US nuclear war fighting plan/SIOP every decade since.

For example, the United States adopted a policy in 1996 of allowing the targeting of its nuclear weapons at non-state actors ("terrorists") armed with weapons of mass destruction.

Another dimension to the tactical use of nuclear weapons is that of such weapons deployed at sea for use against surface and submarine vessels. Until 1992, vessels of the United States Navy (and their aircraft) deployed various such weapons as bombs, rockets (guided and unguided), torpedoes, and depth charges. Such tactical naval nuclear weapons were considered more acceptable to use early in a conflict because there would be few civilian casualties. It was feared by many planners that such use would probably quickly have escalated into large-scale nuclear war. This situation was particularly exacerbated by the fact that such weapons at sea were not constrained by the safeguards provided by the Permissive Action Link attached to U.S. Air Force and Army nuclear weapons. It is unknown if the navies of the other nuclear powers yet today deploy tactical nuclear weapons at sea.

The 2018 US Nuclear Posture Review emphasised the need for the US to have sub-strategic nuclear weapons as additional layers for its nuclear deterrence.

Nuclear terrorism by non-state organizations or actors (even individuals) is a largely unknown and understudied factor in nuclear deterrence thinking, as states possessing nuclear weapons are susceptible to retaliation in kind, while sub- or trans-state actors may be less so. The collapse of the Soviet Union has given rise to the possibility that former Soviet nuclear weapons might become available on the black market (so-called 'loose nukes').

A number of other concerns have been expressed about the security of nuclear weapons in newer nuclear powers with relatively less stable governments, such as Pakistan, but in each case, the fears have been addressed to some extent by statements and evidence provided by those nations, as well as cooperative programs between nations. Worry remains, however, in many circles that a relative decrease in security of nuclear weapons has emerged in recent years, and that terrorists or others may attempt to exert control over (or use) nuclear weapons, militarily applicable technology, or nuclear materials and fuel.

Another possible nuclear terrorism threat are devices designed to disperse radioactive materials over a large area using conventional explosives, called dirty bombs. The detonation of a "dirty bomb" would not cause a nuclear explosion, nor would it release enough radiation to kill or injure a large number of people. However, it could cause severe disruption and require potentially very costly decontamination procedures and increased spending on security measures.

The predictions of the effects of a major countervalue nuclear exchange include millions of city dweller deaths within a short period of time. Some 1980s predictions had gone further and argued that a full-scale nuclear war could eventually bring about the extinction of the human race. Such predictions, sometimes but not always based on total war with nuclear arsenals at Cold War highs, received contemporary criticism. On the other hand, some 1980s governmental predictions, such as FEMA's CRP-2B and NATO's "Carte Blanche," have received criticism from groups like the Federation of American Scientists for being overly optimistic. CRP-2B, for instance, infamously predicted that 80% of Americans would survive a nuclear exchange with the Soviet Union, a figure that neglected nuclear war's impacts on healthcare infrastructure, the food supply, and the ecosystem and assumed that all major cities could be successfully evacuated within 3–5 days. A number of Cold War publications advocated preparations that could purportedly enable a large proportion of civilians to survive even a total nuclear war. Among the most famous of these is Nuclear War Survival Skills.

To avoid injury and death from a nuclear weapons heat flash and blast effects, the two most far ranging prompt effects of nuclear weapons, schoolchildren were taught to duck and cover by the early Cold War film of the same name. Such advice is once again being given in case of nuclear terrorist attacks.

Prussian blue, or "Radiogardase", is stockpiled in the US, along with potassium iodide and DPTA as pharmaceuticals useful in treating internal exposure to harmful radioisotopes in fallout.

Publications on adapting to a changing diet and supplying nutritional food sources following a nuclear war, with particular focus on agricultural radioecology, include "Nutrition in the postattack environment" by the RAND corporation.

The British government developed a public alert system for use during nuclear attack with the expectation of a four-minute warning before detonation. The United States expected a warning time of anywhere from half an hour (for land-based missiles) to less than three minutes (for submarine-based weapons). Many countries maintain plans for continuity of government and continuity of operations following a nuclear attack or similar disasters. These range from a designated survivor, intended to ensure survival of some form of government leadership, to the Soviet Dead Hand system, which allows for retaliation even if all Soviet leadership were destroyed. Nuclear submarines are given letters of last resort: orders on what action to take in the event that an enemy nuclear strike has destroyed the government.

A number of other countries around the world have taken significant efforts to maximize their survival prospects in the event of large calamities, both natural and manmade. For example, metro stations in Pyongyang, North Korea, were constructed below ground, and were designed to serve as nuclear shelters in the event of war, with each station entrance built with thick steel blast doors. An example of privately funded fallout shelters is the Ark Two Shelter in Ontario, Canada, and autonomous shelters have been constructed with an emphasis on post-war networking and reconstruction.
In Switzerland, the majority of homes have an underground blast and fallout shelter. The country has an overcapacity of such shelters and can accommodate slightly more than the nation's population size.

While the nuclear fallout shelters described above are the ideal long term protection methods against dangerous radiation exposure in the event of a nuclear catastrophe, it is also necessary to have mobile protection equipment for medical and security personnel to safely assist in containment, evacuation, and many other necessary public safety objectives which ensue as a result of nuclear detonation. There are many basic shielding strategies used to protect against the deposition of radio active material from external radiation environments. Respirators which protect against internal deposition are used to prevent the inhalation and ingestion of radioactive material and dermal protective equipment which is used to protect against the deposition of material on external structures like skin, hair, and clothing. While these protection strategies do slightly reduce the exposure, they provide almost no protection from externally penetrating gamma radiation, which is the cause of Acute Radiation Syndrome and can be extremely lethal in high dosages. Naturally, shielding the entire body from high energy gamma radiation is optimal, but the required mass to provide adequate attenuation makes functional movement nearly impossible.

Recent scientific studies have shown the feasibility of partial body shielding as a viable protection strategy against externally penetrating gamma radiation. The concept is based in providing sufficient attenuation to only the most radio-sensitive organs and tissues in efforts to defer the onset of Acute Radiation Syndrome, the most immediate threat to humans from high doses of gamma radiation. Acute Radiation Syndrome is a result of irreversible bone marrow damage from high energy radiation exposure. Due to the regenerative property of hematopoietic stem cells found in bone marrow, it is only necessary to protect enough bone marrow to repopulate the exposed areas of the body with the shielded supply. Because 50% of the body's supply of bone marrow is stored in the pelvic region which is also in close proximity to other radio-sensitive organs in the abdomen, the lower torso is a logical choice as the primary target for protection.

This research allows for the development of relatively lightweight mobile radiation protection equipment which can provide life saving protection from intense radioactive environments without hindering functional mobility. This technology has wide-ranging military, emergency personnel, private sector, and civilian application as the radiation risks of nuclear energy, warfare, and terrorism around the world continue to grow. One example of such technology is the 360 Gamma, a radiation protection belt designed on the principles of bone marrow shielding.

More information on bone marrow shielding can be found in the Health Physics Radiation Safety Journal article Selective Shielding of Bone Marrow: An Approach to Protecting Humans from External Gamma Radiation, or in the Organisation for Economic Co-operation and Development (OECD) and the Nuclear Energy Agency (NEA)'s 2015 report: Occupational Radiation Protection in Severe Accident Management.

Nuclear warfare and weapons are staple elements of speculative fiction.

On August 6, 1945, the uranium-type nuclear weapon codenamed "Little Boy" was detonated over the Japanese city of Hiroshima with an energy of about, destroying nearly 50,000 buildings (including the headquarters of the 2nd General Army and Fifth Division) and killing approximately 70,000 people, including 20,000 Japanese combatants and 20,000 Korean slave laborers. Three days later, on August 9, a plutonium-type nuclear weapon codenamed "Fat Man" was used against the Japanese city of Nagasaki, with the explosion equivalent to about, destroying 60% of the city and killing approximately 35,000 people, including 23,200–28,200 Japanese munitions workers, 2,000 Korean slave laborers, and 150 Japanese combatants. The industrial damage in Nagasaki was high, partly owing to the inadvertent targeting of the industrial zone, leaving 68–80 percent of the non-dock industrial production destroyed.

Nuclear warfare

So far, two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. On August 6, 1945, a uranium gun-type device (code name "Little Boy") was detonated over the Japanese city of Hiroshima. Three days later, on August 9, a plutonium implosion-type device (code name "Fat Man") was detonated over the Japanese city of Nagasaki. These two bombings resulted in the deaths of approximately 120,000 people.

After the collapse of the Soviet Union in 1991 and the resultant end of the Cold War, the threat of a major nuclear war between the two nuclear superpowers was generally thought to have declined. Since then, concern over nuclear weapons has shifted to the prevention of localized nuclear conflicts resulting from nuclear proliferation, and the threat of nuclear terrorism.

The possibility of using nuclear weapons in war is usually divided into two subgroups, each with different effects and potentially fought with different types of nuclear armaments.

The first, a limited nuclear war (sometimes "attack" or "exchange"), refers to a small-scale use of nuclear weapons by two (or more) belligerents. A "limited nuclear war" could include targeting military facilities—either as an attempt to pre-emptively cripple the enemy's ability to attack as a defensive measure, or as a prelude to an invasion by conventional forces, as an offensive measure. This term could apply to "any" small-scale use of nuclear weapons that may involve military or civilian targets (or both).

The second, a full-scale nuclear war, could consist of large numbers of nuclear weapons used in an attack aimed at an entire country, including military, economic, and civilian targets. Such an attack would almost certainly destroy the entire economic, social, and military infrastructure of the target nation, and would probably have a devastating effect on Earth's biosphere.

Some Cold War strategists such as Henry Kissinger argued that a limited nuclear war could be possible between two heavily armed superpowers (such as the United States and the Soviet Union). Some predict, however, that a limited war could potentially "escalate" into a full-scale nuclear war. Others have called limited nuclear war "global nuclear holocaust in slow motion", arguing that—once such a war took place—others would be sure to follow over a period of decades, effectively rendering the planet uninhabitable in the same way that a "full-scale nuclear war" between superpowers would, only taking a much longer (and arguably more agonizing) path to the same result.

A study presented at the annual meeting of the American Geophysical Union in December 2006 asserted that even a small-scale regional nuclear war could produce as many direct fatalities as all of World War II and disrupt the global climate for a decade or more. In a regional nuclear conflict scenario in which two opposing nations in the subtropics each used 50 Hiroshima-sized nuclear weapons (c. 15 kiloton each) on major population centers, the researchers predicted fatalities ranging from 2.6 million to 16.7 million per country. The authors of the study estimated that as much as five million tons of soot could be released, producing a cooling of several degrees over large areas of North America and Eurasia (including most of the grain-growing regions). The cooling would last for years and could be "catastrophic", according to the researchers.

Either a limited or full-scale nuclear exchange could occur during an "accidental nuclear war", in which the use of nuclear weapons is triggered unintentionally. Postulated triggers for this scenario have included malfunctioning early warning devices and/or targeting computers, deliberate malfeasance by rogue military commanders, consequences of an accidental straying of warplanes into enemy airspace, reactions to unannounced missile tests during tense diplomatic periods, reactions to military exercises, mistranslated or miscommunicated messages, and others. A number of these scenarios actually occurred during the Cold War, though none resulted in the use of nuclear weapons. Many such scenarios have been depicted in popular culture, such as in the 1959 film "On the Beach", the 1962 novel "Fail-Safe" (released as a film in 1964); the film "WarGames", released in 1983; and the film "", also released in 1964.

During the final stages of World War II in 1945, the United States conducted atomic raids on the Japanese cities of Hiroshima and Nagasaki, the first on August 6, 1945, and the second on August 9, 1945. These two events were the only times nuclear weapons have been used in combat.

In late June 1945, as the U.S. wrapped up the two-and-a-half-month Battle of Okinawa (which cost the lives of 260,000 people, including 150,000 civilians), it was faced with the prospect of invading the Japanese home islands in an operation codenamed Operation Downfall. Based on the U.S. casualties from the preceding island-hopping campaigns, American commanders estimated that between 50,000 and 500,000 U.S. troops would die and at least 600,000–1,000,000 others would be injured while invading the Japanese home islands. The U.S. manufacture of 500,000 Purple Hearts from the anticipated high level of casualties during the U.S. invasion of Japan gave a demonstration of how deadly and costly it would be. President Harry S. Truman realized he could not afford such a horrendous casualty rate, especially since over 400,000 American combatants had already died fighting in both the European and the Pacific theaters of the war.

On July 26, 1945, the United States, the United Kingdom, and the Republic of China issued a Potsdam Declaration that called for the unconditional surrender of Japan. It stated that if Japan did not surrender, it would face "prompt and utter destruction". The Japanese government ignored this ultimatum, sending a message that they were not going to surrender. In response to the rejection, President Truman authorized the dropping of the atomic bombs. At the time of its use, there were only two atomic bombs available, and despite the fact that more were in production back in mainland U.S., the third bomb wouldn't be available for combat until September.

Six days after the detonation over Nagasaki, Japan announced its surrender to the Allied Powers on August 15, 1945, signing the Instrument of Surrender on September 2, 1945, officially ending the Pacific War and, therefore, World War II, as Germany had already signed its Instrument of Surrender on May 8, 1945, ending the war in Europe. The two atomic bombings led, in part, to post-war Japan's adopting of the Three Non-Nuclear Principles, which forbade the nation from developing nuclear armaments.

After the successful Trinity nuclear test July 16, 1945, which was the very first nuclear detonation, the Manhattan project lead manager J. Robert Oppenheimer recalled:
Immediately after the atomic bombings of Japan, the status of atomic weapons in international and military relations was unclear. Presumably, the United States hoped atomic weapons could offset the Soviet Union's larger conventional ground forces in Eastern Europe, and possibly be used to pressure Soviet leader Joseph Stalin into making concessions. Under Stalin, the Soviet Union pursued its own atomic capabilities through a combination of scientific research and espionage directed against the American program. The Soviets believed that the Americans, with their limited nuclear arsenal, were unlikely to engage in any new world wars, while the Americans were not confident they could prevent a Soviet takeover of Europe, despite their atomic advantage.

Within the United States the authority to produce and develop nuclear weapons was removed from military control and put instead under the civilian control of the United States Atomic Energy Commission. This decision reflected an understanding that nuclear weapons had unique risks and benefits that were separate from other military technology known at the time.

For several years after World War II, the United States developed and maintained a strategic force based on the Convair B-36 bomber that would be able to attack any potential enemy from bomber bases in the United States. It deployed atomic bombs around the world for potential use in conflicts. Over a period of a few years, many in the American defense community became increasingly convinced of the invincibility of the United States to a nuclear attack. Indeed, it became generally believed that the threat of nuclear war would deter any strike against the United States.

Many proposals were suggested to put all American nuclear weapons under international control (by the newly formed United Nations, for example) as an effort to deter both their usage and an arms race. However, no terms could be arrived at that would be agreed upon by both the United States and the Soviet Union.

On August 29, 1949, the Soviet Union tested its first nuclear weapon at Semipalatinsk in Kazakhstan (see also Soviet atomic bomb project). Scientists in the United States from the Manhattan Project had warned that, in time, the Soviet Union would certainly develop nuclear capabilities of its own. Nevertheless, the effect upon military thinking and planning in the United States was dramatic, primarily because American military strategists had not anticipated the Soviets would "catch up" so soon. However, at this time, they had not discovered that the Soviets had conducted significant nuclear espionage of the project from spies at Los Alamos, the most significant of which was done by the theoretical physicist Klaus Fuchs. The first Soviet bomb was more or less a deliberate copy of the Fat Man plutonium device. In the same year the first US-Soviet nuclear war plan was penned in the US with Operation Dropshot.

With the monopoly over nuclear technology broken, worldwide nuclear proliferation accelerated. The United Kingdom tested its first independent atomic bomb in 1952, followed by France in 1960 and then China in 1964. While much smaller than the arsenals of the United States and the Soviet Union, Western Europe's nuclear reserves were nevertheless a significant factor in strategic planning during the Cold War. A top-secret White Paper, compiled by the Royal Air Force and produced for the British Government in 1959, estimated that British bombers carrying nuclear weapons were capable of destroying key cities and military targets in the Soviet Union, with an estimated 16 million deaths in the Soviet Union (half of whom were estimated to be killed on impact and the rest fatally injured) "before" bomber aircraft from the U.S. Strategic Air Command reached their targets.

Although the Soviet Union had nuclear weapon capabilities in the beginning of the Cold War, the United States still had an advantage in terms of bombers and weapons. In any exchange of hostilities, the United States would have been capable of bombing the Soviet Union, whereas the Soviet Union would have more difficulty carrying out the reverse mission.

The concept of a "Fortress North America" emerged during the Second World War and persisted into the Cold War to refer to the option of defending Canada and the United States against their enemies if the rest of the world were lost to them. This option was rejected with the formation of NATO and the decision to permanently station troops in Europe.

In the summer of 1951 Project Vista started, in which project analysts such as Robert F. Christy looked at how to defend Western Europe from a Soviet invasion. The emerging development of tactical nuclear weapons were looked upon as a means to give Western forces a qualitative advantage over the Soviet numerical supremacy in conventional weapons.

Several scares about the increasing ability of the Soviet Union's strategic bomber forces surfaced during the 1950s. The defensive response by the United States was to deploy a fairly strong "layered defense" consisting of interceptor aircraft and anti-aircraft missiles, like the Nike, and guns, like the Skysweeper, near larger cities. However, this was a small response compared to the construction of a huge fleet of nuclear bombers. The principal nuclear strategy was to massively penetrate the Soviet Union. Because such a large area could not be defended against this overwhelming attack in any credible way, the Soviet Union would lose any exchange.

This logic became ingrained in American nuclear doctrine and persisted for much of the duration of the Cold War. As long as the strategic American nuclear forces could overwhelm their Soviet counterparts, a Soviet pre-emptive strike could be averted. Moreover, the Soviet Union could not afford to build any reasonable counterforce, as the economic output of the United States was far larger than that of the Soviets, and they would be unable to achieve "nuclear parity".

Soviet nuclear doctrine, however, did not match American nuclear doctrine. Soviet military planners assumed they could win a nuclear war. Therefore, they "expected" a large-scale nuclear exchange, followed by a "conventional war" which itself would involve heavy use of tactical nuclear weapons. American doctrine rather assumed that Soviet doctrine was similar, with the "mutual" in Mutually Assured Destruction necessarily requiring that the other side see things in much the same way, rather than believing—as the Soviets did—that they could fight a large-scale, "combined nuclear and conventional" war.

In accordance with their doctrine, the Soviet Union conducted large-scale military exercises to explore the possibility of defensive and offensive warfare during a nuclear war. The exercise, under the code name of "Snowball", involved the detonation of a nuclear bomb about twice as powerful as that which fell on Nagasaki and an army of approximately 45,000 soldiers on maneuvers through the hypocenter immediately after the blast. The exercise was conducted on September 14, 1954, under command of Marshal Georgy Zhukov to the north of Totskoye village in Orenburg Oblast, Russia.

A revolution in nuclear strategic thought occurred with the introduction of the intercontinental ballistic missile (ICBM), which the Soviet Union first successfully tested in August 1957. In order to deliver a warhead to a target, a missile was much faster and more cost-effective than a bomber, and enjoyed a higher survivability due to the enormous difficulty of interception of the ICBMs (due to their high altitude and extreme speed). The Soviet Union could now afford to achieve nuclear parity with the United States in raw numbers, although for a time, they appeared to have chosen not to.

Photos of Soviet missile sites set off a wave of panic in the U.S. military, something the launch of Sputnik would do for the American public a few months later. Politicians, notably then-U.S. Senator John F. Kennedy suggested that a "missile gap" existed between the Soviet Union and the United States. The US military gave missile development programs the highest national priority, and several spy aircraft and reconnaissance satellites were designed and deployed to observe Soviet progress.

Henry Kissinger's view on tactical nuclear war in his controversial 1957 book Nuclear Weapons and Foreign Policy was that any nuclear weapon exploded in air burst mode that was below 500 kiloton in yield and thus averting serious fallout, may be more decisive and less costly in human lives than a protracted conventional war.

A list of targets made by the U.S.A. was released sometime during December 2015 by the U.S. National Archives and Records Administration. The language used to describe targets is "designated ground zeros". The list was released after a request was made during 2006 by William Burr who belongs to a research group at George Washington University, and belongs to a previously top-secret 800-page document. The list is entitled "Atomic Weapons Requirements Study for 1959" and was produced by U.S. Strategic Air Command during the year 1956.

In 1960, the United States developed its first Single Integrated Operational Plan, a range of targeting options, and described launch procedures and target sets against which nuclear weapons would be launched, variants of which were in use from 1961 to 2003. That year also saw the start of the Missile Defense Alarm System, an American system of 12 early-warning satellites that provided limited notice of Soviet intercontinental ballistic missile launches between 1960 and 1966. The Ballistic Missile Early Warning System was completed in 1964.

A complex and worrisome situation developed in 1962, in what is called the Cuban Missile Crisis. The Soviet Union placed medium-range ballistic missiles from the United States, possibly as a direct response to American Jupiter missiles placed in Turkey. After intense negotiations, the Soviets ended up removing the missiles from Cuba and decided to institute a massive weapons-building program of their own. In exchange, the United States dismantled its launch sites in Turkey, although this was done secretly and not publicly revealed for over two decades. Khrushchev did not even reveal this part of the agreement when he came under fire by political opponents for mishandling the crisis. Communication delays during the crisis led to the establishment of the Moscow–Washington hotline to allow reliable, direct communications between the two nuclear powers.

By the late 1960s, the number of ICBMs and warheads was so high on both sides that it was believed that both the United States and the Soviet Union were capable of completely destroying the infrastructure and a large proportion of the population of the other country. Thus, by some western game theorists, a balance of power system known as mutually assured destruction (or "MAD") came into being. It was thought that no full-scale exchange between the powers would result in an outright winner, with at best one side emerging the pyrrhic victor. Thus both sides were deterred from risking the initiation of a direct confrontation, instead being forced to engage in lower intensity proxy wars.

During this decade the People's Republic of China began to build subterranean infrastructure such as the Underground Project 131 following the Sino-Soviet split.
One drawback of the MAD doctrine was the possibility of a nuclear war occurring without either side intentionally striking first. Early Warning Systems (EWS) were notoriously error-prone. For example, on 78 occasions in 1979 alone, a "missile display conference" was called to evaluate detections that were "potentially threatening to the North American continent". Some of these were trivial errors and were spotted quickly, but several went to more serious levels. On September 26, 1983, Stanislav Petrov received convincing indications of an American first strike launch against the Soviet Union, but positively identified the warning as a false alarm. Though it is unclear what role Petrov's actions played in preventing a nuclear war during this incident, he has been honored by the United Nations for his actions.

Similar incidents happened many times in the United States, due to failed computer chips, misidentifications of large flights of geese, test programs, and bureaucratic failures to notify early warning military personnel of legitimate launches of test or weather missiles. For many years, the U.S. Air Force's strategic bombers were kept airborne on a daily rotating basis "around the clock" (see Operation Chrome Dome), until the number and severity of accidents, the 1968 Thule Air Base B-52 crash in particular, persuaded policymakers it was not worthwhile.

Israel responded to the Arab Yom Kippur War attack on 6 October 1973 by assembling 13 nuclear weapons in a tunnel under the Negev desert when Syrian tanks were sweeping in across the Golan Heights. On 8 October 1973, Israeli Prime Minister Mrs Golda Meir authorized Defense Minister Moshe Dayan to activate the 13 Israeli nuclear warheads and distribute them to Israeli air force units, with the intent that they be used if Israel began to be overrun.

On 24 October 1973, as US President Nixon was preoccupied with the Watergate scandal, Henry Kissinger ordered a DEFCON-3 alert preparing American B-52 nuclear bombers for war. Intelligence reports indicated that the USSR was preparing to defend Egypt in its Yom Kippur war with Israel. It had become apparent that if Israel had dropped nuclear weapons on Egypt or Syria, as it prepared to do, then the USSR would have retaliated against Israel, with the US then committed to providing Israeli assistance, possibly escalating to a general nuclear war.

By the late 1970s, people in both the United States and the Soviet Union, along with the rest of the world, had been living with the concept of mutual assured destruction (MAD) for about a decade, and it became deeply ingrained into the psyche and popular culture of those countries.

On May 18, 1974, India conducted its first nuclear test in the Pokhran test range. The name of the operation was Smiling Buddha, and India termed the test as a "peaceful nuclear explosion".

The Soviet Duga early warning over-the-horizon radar system was made operational in 1976. The extremely powerful radio transmissions needed for such a system led to much disruption of civilian shortwave broadcasts, earning it the nickname "Russian Woodpecker".

The idea that any nuclear conflict would eventually escalate was a challenge for military strategists. This challenge was particularly severe for the United States and its NATO allies. It was believed (until the 1970s) that a Soviet tank invasion of Western Europe would quickly overwhelm NATO conventional forces, leading to the necessity of the West escalating to the use of tactical nuclear weapons, one of which was the W-70.

This strategy had one major (and possibly critical) flaw, which was soon realized by military analysts but highly underplayed by the U.S. military: conventional NATO forces in the European theatre of war were far outnumbered by similar Soviet and Warsaw Pact forces, and it was assumed that in case of a major Soviet attack (commonly envisioned as the "Red tanks rolling towards the North Sea" scenario) that NATO—in the face of quick conventional defeat—would soon have no other choice but to resort to tactical nuclear strikes against these forces. Most analysts agreed that once the first nuclear exchange had occurred, escalation to global nuclear war would likely become inevitable. The Soviet bloc's vision of an atomic war between NATO and Warsaw Pact forces was simulated in the top secret exercise Seven Days to the River Rhine in 1979. The British government exercised their vision of Soviet nuclear attack with Square Leg in early 1980.

Large hardened nuclear weapon storage areas were built across European countries in anticipation of local US and European forces falling back as the conventional NATO defense from the Soviet Union, named REFORGER, was believed to only be capable of stalling the Soviets for a short time.

In the late 1970s and, particularly, during the early 1980s under U.S. President Ronald Reagan, the United States renewed its commitment to a more powerful military, which required a large increase in spending on U.S. military programs. These programs, which were originally part of the defense budget of U.S. President Jimmy Carter, included spending on conventional and nuclear weapons systems. Under Reagan, defensive systems like the Strategic Defense Initiative were emphasized as well.

South Africa developed a nuclear weapon capability during the 1970s and early 1980s. It was operational for a brief period before being dismantled in the early 1990s.

However, comparisons with supervolcanoes are more misleading than helpful due to the different aerosols released, the likely air burst fuzing height of nuclear weapons and the globally scattered location of these potential nuclear detonations all being in contrast to the singular and subterranean nature of a supervolcanic eruption. Moreover, assuming the entire world stockpile of weapons were grouped together, it would be difficult, due to the nuclear fratricide effect, to ensure the individual weapons would go off all at once. Nonetheless, many people believe that a full-scale nuclear war would result, through the nuclear winter effect, in the extinction of the human species, though not all analysts agree on the assumptions that underpin these nuclear winter models.

On Sept. 1, 1983, Korean Air Lines Flight 007 was shot down by Soviet jet fighters. On the 26th, a Soviet early warning station under the command of Stanislav Petrov falsely detected 5 inbound intercontinental ballistic missiles from the US. Petrov correctly assessed the situation as a false alarm, and hence did not report his finding to his superiors. It is quite possible that his actions prevented "World War III", as the Soviet policy at that time was immediate nuclear response upon discovering inbound ballistic missiles.

The world came unusually close to nuclear war when the Soviet Union thought that the NATO military exercise Able Archer 83 was a ruse or "cover-up" to begin a nuclear first strike. The Soviets responded by raising readiness and preparing their nuclear arsenal for immediate use. Soviet fears of an attack ceased once the exercise concluded without incident.

Although the dissolution of the Soviet Union ended the Cold War and greatly reduced tensions between the United States and the Russian Federation, the Soviet Union's formal successor state, both countries remained in a "nuclear stand-off" due to the continuing presence of a very large number of deliverable nuclear warheads on both sides. Additionally, the end of the Cold War led the United States to become increasingly concerned with the development of nuclear technology by other nations outside of the former Soviet Union. In 1995, a branch of the U.S. Strategic Command produced an outline of forward-thinking strategies in the document "Essentials of Post–Cold War Deterrence".

In 1996, a Russian continuity of government facility, Kosvinsky Mountain, which is believed to be a counterpart to the US Cheyenne Mountain Complex, was completed. It was designed to resist US earth-penetrating nuclear warheads, and is believed to host the Russian Strategic Rocket Forces alternate command post, a post for the general staff built to compensate for the vulnerability of older Soviet era command posts in the Moscow region. In spite of this, the primary command posts for the Strategic Rocket Forces remains Kuntsevo in Moscow and the secondary is the Kosvinsky Mountain in the Urals. The timing of the Kosvinsky facilities completion date is regarded as one explanation for U.S. interest in a new nuclear "bunker buster" Earth-penetrating warhead and the declaration of the deployment of the B-61 mod 11 in 1997; Kosvinsky is protected by about 1000 feet of granite.

As a consequence of the 9/11 attacks, American forces immediately increased their readiness to the highest level in 28 years, closing the blast doors of the NORAD's Cheyenne Mountain Operations Center for the first time due to a non-exercise event. But unlike similar increases during the Cold War, Russia immediately decided to stand down a large military exercise in the Arctic region, in order to minimize the risk of incidents, rather than following suit.

The former chair of the United Nations disarmament committee stated that there are more than 16,000 strategic and tactical nuclear weapons ready for deployment and another 14,000 in storage, with the U.S. having nearly 7,000 ready for use and 3,000 in storage, and Russia having about 8,500 ready for use and 11,000 in storage. In addition, China is thought to possess about 400 nuclear weapons, Britain about 200, France about 350, India about 80–100, and Pakistan 100–110. North Korea is confirmed as having nuclear weapons, though it is not known how many, with most estimates between 1 and 10. Israel is also widely believed to possess usable nuclear weapons. NATO has stationed about 480 American nuclear weapons in Belgium, the Netherlands, Italy, Germany, and Turkey, and several other nations are thought to be in pursuit of an arsenal of their own.

A key development in nuclear warfare throughout the 2000s and early 2010s is the proliferation of nuclear weapons to the developing world, with India and Pakistan both publicly testing several nuclear devices, and North Korea conducting an underground nuclear test on October 9, 2006. The U.S. Geological Survey measured a 4.2 magnitude earthquake in the area where the North Korean test is said to have occurred. A further test was announced by the North Korean government on May 25, 2009. Iran, meanwhile, has embarked on a nuclear program which, while officially for civilian purposes, has come under close scrutiny by the United Nations and many individual states.

Recent studies undertaken by the CIA cite the enduring India-Pakistan conflict as the one "flash point" most likely to escalate into a nuclear war. During the Kargil War in 1999, Pakistan came close to using its nuclear weapons in case the conventional military situation underwent further deterioration. Pakistan's foreign minister had even warned that it would "use any weapon in our arsenal", hinting at a nuclear strike against India. The statement was condemned by the international community, with Pakistan denying it later on. This conflict remains the only war (of any sort) between two declared nuclear powers. The 2001-2002 India-Pakistan standoff again stoked fears of nuclear war between the two countries. Despite these very serious and relatively recent threats, relations between India and Pakistan have been improving somewhat over the last few years. However, with the November 26, 2008 Mumbai terror attacks, tensions again worsened.

Another potential geopolitical issue which is considered particularly worrisome by military analysts is a possible conflict between the United States and the People's Republic of China over Taiwan. Although economic forces are thought to have reduced the possibility of a military conflict, there remains concern about the increasing military buildup of China (China is rapidly increasing its naval capacity), and that any move toward Taiwan independence could potentially spin out of control.

Israel is thought to possess somewhere between one hundred and four hundred nuclear warheads. It has been asserted that the Dolphin-class submarines which Israel received from Germany have been adapted to carry Popeye cruise missiles with nuclear warheads, so as to give Israel a second strike capability. Israel has been involved in wars with its neighbors in the Middle East (and with other "non-state actors") on numerous prior occasions, and its small geographic size and population could mean that, in the event of future wars, the Israeli military might have very little time to react to an invasion or other major threat. Such a situation could escalate to nuclear warfare very quickly in some scenarios.

On March 7, 2013, North Korea threatened the United States with a pre-emptive nuclear strike. On April 9, North Korea urged foreigners to leave South Korea, stating that both countries were on the verge of nuclear war. On April 12, North Korea stated that a nuclear war was unavoidable. The country declared Japan as its first target.

In the year of 2014, when Russian relationships with the USA had become worse, it was stated by the Russian state-owned Russia 1 TV channel that ‘Russia is the only country in the world that is really capable of turning the USA into radioactive ash’.

The above examples envisage nuclear warfare at a strategic level, i.e., total war. However, nuclear powers have the ability to undertake more limited engagements.

Commodore Tim Hare, former Director of Nuclear Policy at the British Ministry of Defence, has described "sub-strategic use" as offering the Government "an extra option in the escalatory process before it goes for an all-out strategic strike which would deliver unacceptable damage". However, this sub-strategic capacity has been criticized as potentially increasing the "acceptability" of using nuclear weapons. Combined with the trend in the reduction in the worldwide nuclear arsenal as of 2007 is the warhead miniaturization and modernization of the remaining strategic weapons that is presently occurring in all the declared nuclear weapon states, into more "usable" configurations. The Stockholm International Peace Research Institute suggests that this is creating a culture where use of these weapons is more acceptable and therefore is increasing the risk of war, as these modern weapons do not possess the same psychological deterrent value as the large Cold-War era, multi-megaton warheads.

In many ways, this present change in the balance of terror can be seen as the complete embracement of the switch from the 1950s Eisenhower doctrine of "massive retaliation" to one of "flexible response", which has been growing in importance in the US nuclear war fighting plan/SIOP every decade since.

For example, the United States adopted a policy in 1996 of allowing the targeting of its nuclear weapons at non-state actors ("terrorists") armed with weapons of mass destruction.

Another dimension to the tactical use of nuclear weapons is that of such weapons deployed at sea for use against surface and submarine vessels. Until 1992, vessels of the United States Navy (and their aircraft) deployed various such weapons as bombs, rockets (guided and unguided), torpedoes, and depth charges. Such tactical naval nuclear weapons were considered more acceptable to use early in a conflict because there would be few civilian casualties. It was feared by many planners that such use would probably quickly have escalated into large-scale nuclear war. This situation was particularly exacerbated by the fact that such weapons at sea were not constrained by the safeguards provided by the Permissive Action Link attached to U.S. Air Force and Army nuclear weapons. It is unknown if the navies of the other nuclear powers yet today deploy tactical nuclear weapons at sea.

The 2018 US Nuclear Posture Review emphasised the need for the US to have sub-strategic nuclear weapons as additional layers for its nuclear deterrence.

Nuclear terrorism by non-state organizations or actors (even individuals) is a largely unknown and understudied factor in nuclear deterrence thinking, as states possessing nuclear weapons are susceptible to retaliation in kind, while sub- or trans-state actors may be less so. The collapse of the Soviet Union has given rise to the possibility that former Soviet nuclear weapons might become available on the black market (so-called 'loose nukes').

A number of other concerns have been expressed about the security of nuclear weapons in newer nuclear powers with relatively less stable governments, such as Pakistan, but in each case, the fears have been addressed to some extent by statements and evidence provided by those nations, as well as cooperative programs between nations. Worry remains, however, in many circles that a relative decrease in security of nuclear weapons has emerged in recent years, and that terrorists or others may attempt to exert control over (or use) nuclear weapons, militarily applicable technology, or nuclear materials and fuel.

Another possible nuclear terrorism threat are devices designed to disperse radioactive materials over a large area using conventional explosives, called dirty bombs. The detonation of a "dirty bomb" would not cause a nuclear explosion, nor would it release enough radiation to kill or injure a large number of people. However, it could cause severe disruption and require potentially very costly decontamination procedures and increased spending on security measures.

To avoid injury and death from a nuclear weapons heat flash and blast effects, the two most far ranging prompt effects of nuclear weapons, schoolchildren were taught to duck and cover by the early Cold War film of the same name. Such advice is once again being given in case of nuclear terrorist attacks.

Prussian blue, or "Radiogardase", is stockpiled in the US, along with potassium iodide and DPTA as pharmaceuticals useful in treating internal exposure to harmful radioisotopes in fallout.

Publications on adapting to a changing diet and supplying nutritional food sources following a nuclear war, with particular focus on agricultural radioecology, include "Nutrition in the postattack environment" by the RAND corporation.

The British government developed a public alert system for use during nuclear attack with the expectation of a four-minute warning before detonation. The United States expected a warning time of anywhere from half an hour (for land-based missiles) to less than three minutes (for submarine-based weapons). Many countries maintain plans for continuity of government and continuity of operations following a nuclear attack or similar disasters. These range from a designated survivor, intended to ensure survival of some form of government leadership, to the Soviet Dead Hand system, which allows for retaliation even if all Soviet leadership were destroyed. Nuclear submarines are given letters of last resort: orders on what action to take in the event that an enemy nuclear strike has destroyed the government.

A number of other countries around the world have taken significant efforts to maximize their survival prospects in the event of large calamities, both natural and manmade. For example, metro stations in Pyongyang, North Korea, were constructed below ground, and were designed to serve as nuclear shelters in the event of war, with each station entrance built with thick steel blast doors. An example of privately funded fallout shelters is the Ark Two Shelter in Ontario, Canada, and autonomous shelters have been constructed with an emphasis on post-war networking and reconstruction.
In Switzerland, the majority of homes have an underground blast and fallout shelter. The country has an overcapacity of such shelters and can accommodate slightly more than the nation's population size.

Recent scientific studies have shown the feasibility of partial body shielding as a viable protection strategy against externally penetrating gamma radiation. The concept is based in providing sufficient attenuation to only the most radio-sensitive organs and tissues in efforts to defer the onset of Acute Radiation Syndrome, the most immediate threat to humans from high doses of gamma radiation. Acute Radiation Syndrome is a result of irreversible bone marrow damage from high energy radiation exposure. Due to the regenerative property of hematopoietic stem cells found in bone marrow, it is only necessary to protect enough bone marrow to repopulate the exposed areas of the body with the shielded supply. Because 50% of the body's supply of bone marrow is stored in the pelvic region which is also in close proximity to other radio-sensitive organs in the abdomen, the lower torso is a logical choice as the primary target for protection.

This research allows for the development of relatively lightweight mobile radiation protection equipment which can provide life saving protection from intense radioactive environments without hindering functional mobility. This technology has wide-ranging military, emergency personnel, private sector, and civilian application as the radiation risks of nuclear energy, warfare, and terrorism around the world continue to grow. One example of such technology is the 360 Gamma, a radiation protection belt designed on the principles of bone marrow shielding.

More information on bone marrow shielding can be found in the Health Physics Radiation Safety Journal article Selective Shielding of Bone Marrow: An Approach to Protecting Humans from External Gamma Radiation, or in the Organisation for Economic Co-operation and Development (OECD) and the Nuclear Energy Agency (NEA)'s 2015 report: Occupational Radiation Protection in Severe Accident Management.

Nuclear warfare and weapons are staple elements of speculative fiction.