Nuclear arms race
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The nuclear arms race was a competition for supremacy in nuclear weapons between the United States and Soviet Union during the Cold War. During the Cold War, in addition to the American and Soviet nuclear stockpiles, other countries also developed nuclear weapons as well, though none engaged in warhead production on the same size as the two superpowers. An additional nuclear arms race developed between India and Pakistan during the end of the 1990s.
 World War II
The first nuclear weapon was created by the American Manhattan Project during the Second World War and was developed for use against the Axis powers. Scientists in the Soviet Union, then an ally of the United States, were aware of the possibility of nuclear weapons and had been doing some work in that direction. Soviet scientists first became aware the Americans were almost certainly working on atomic weapons when all related articles disappeared from physics journals.
The Soviet Union, despite being an ally, was not informed of the American experiments until the Potsdam Conference in 1945. The Americans did not trust the Soviets to keep the information from German spies; there was also deep distrust of the Soviets and their intentions, despite the wartime partnership. Even during the war many government and military figures in the USA saw the USSR as a potential enemy in the future.
The Soviets were well aware of the program due to a spy ring operating within the American nuclear program. The atomic spies (including Klaus Fuchs  and Theodore Hall) kept Stalin well informed of American developments . When U.S. Vice President Harry S. Truman informed Stalin of the weapons, he was surprised at how calmly Stalin took the news and thought that Stalin had not understood what he had told him. In fact Stalin had long been aware of the program. The American program had been so secret that even Truman did not know about the weapons until he became president; Stalin had thus known about the Manhattan Project before Truman himself did.
In August of 1945 Truman ordered two bombs dropped on the Japanese cities of Hiroshima and Nagasaki, by the B-29 bombers Enola Gay and Bock's Car respectively, ostensibly to quickly end the war though questions have remained about additional motivations as well (see Atomic bombings of Hiroshima and Nagasaki).
 Early Cold War
The years immediately after the Second World War the Americans had a nuclear monopoly, on both specific knowledge and, most importantly, raw materials. Initially it was thought that uranium was relatively rare in the world , but this was discovered to be incorrect . While American leaders hoped the monopoly would be able to win concessions out of the Soviet Union, this proved ineffective. Stalin knew that he was at a disadvantage. However, he felt that the only solution was to bluff that he was certain the Americans would not use the weapons, and wouldn't care much if they did. Stalin guessed correctly that the American policy makers would not risk another massive war over the relatively removed issues like Berlin or Czechoslovakia, and additionally many felt compelled to give the Soviets concessions for their massive sacrifices in their front against Germany.
Behind the scenes the Soviet regime was working furiously to build their own atomic weapons . During the war Soviet efforts had been limited by a lack of uranium, but new supplies in Eastern Europe were taken and provided a steady supply while the Soviets developed a domestic source. Physicists were given massive funding and treated like royalty, but were also threatened with being shot if they did not make significant progress. The much feared NKVD head Lavrenty Beria was put in charge of the development process. The Soviet effort was aided by the information provided by their spies in the United States, however the information was not freely given to scientists and was instead used as an additional "check" on their progress (Beria trusted neither the scientists nor the espionage). While American thinkers had predicted that the USSR would not have nuclear weapons until the mid-1950s, the first Soviet bomb was detonated on August 29, 1949 , shocking the entire world. The weapon (called "Joe One" by the West) was more or less a copy of the weapon which the United States had dropped on Japan ("Fat Man").
Both governments devoted massive amounts of resources to increasing the quality and quantity of their nuclear arsenal. Both nations quickly began work on hydrogen bombs and the United States detonated the first such device on November 1, 1952 . Again the Soviets surprised the Americans by exploding a deployable thermonuclear device of their own the next August, though it was not actually a "true" multi-stage hydrogen bomb (that would wait until 1954) . The Soviet H-bomb was almost completely a product of domestic research, as their espionage sources in the USA had only worked on very preliminary (and incorrect) versions of the hydrogen bomb.
Delivery methods, such as the bomber fleets, were also expanded. The United States began with a considerable lead in this area, but the widespread introduction of jet powered interceptor aircraft upset this balance somewhat by reducing the effectiveness of the US bomber fleet. In 1949 Curtis LeMay was placed in command of the Strategic Air Command and started 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, giving the US the ability to convincingly penetrate the USSR.
The most important development in terms of delivery in the 1950s was the introduction of ICBMs. Missiles had long been seen as the ideal platform for nuclear weapons and in 1957 on the 4th of October with the launch of Sputnik the Soviet Union showed the world that they had missiles that could hit anywhere in the world. The United States launched their own on the 31 October 1959.
The period also saw attempts begin to defend against nuclear weapons. Both powers built large radar arrays to detect incoming bombers and missiles. Fighters to use against bombers and anti-ballistic missiles to use against ICBMs were also developed. Large underground bunkers were constructed to save the leadership of the superpowers, and individuals were told to build fallout shelters and taught how to react to a nuclear attack (civil defense).
 Mutually Assured Destruction (MAD)
All of these defensive measures were far from foolproof and by the 1950s both the United States and Soviet Union had the power to obliterate the other side. Both sides developed a "second-strike" capability , i.e. they could launch a devastating attack even after sustaining a full assault from the other side (especially by means of submarines). This policy was part of what became known as Mutually Assured Destruction: both sides knew that any attack upon the other would be suicide for themselves as well, and thus would (in theory) restrain from attacking one another.
Both Soviet and American thinkers hoped to use nuclear weapons to extract concessions from the other side, or from other powers such as China, but the risk of any use of these weapons were so large that both sides refrained from what John Foster Dulles referred to as brinkmanship. While some like General Douglas MacArthur argued nuclear weapons should be used during the Korean War both Truman and Eisenhower disagreed.
Both sides were also unaware of how their relative arsenals compared. The Americans tended to be lacking in confidence, earlier in the 1950s they believed in a non-existent "bomber gap" (aerial photography later discovered that the Soviets had been playing a sort of Potemkin village game with their bombers in their military parades, flying them in large circles to make it appear they had far more than they truly did), and the 1960 American presidential election saw accusations of a wholly spurious "missile gap" between the Soviets and the Americans. The Soviet government structure tended to exaggerate the power of Soviet weapons to the leadership and Nikita Khrushchev.
An additional controversy formed in the United States during the early-1960s over whether or not it was known if their weapons would work at all if it came down to it. All of the individual components of nuclear missiles had been tested separately (warheads, navigation systems, rockets), but it had been infeasible to test them all as a whole. Critics charged that it was not really known how a warhead would function in the gravity forces and temperature differences encountered in the upper atmosphere and outer space, and Kennedy was unwilling to run a risky test of an ICBM with a live warhead. The closest thing to an actual test, Operation Frigate Bird, which involved testing a live submarine launching a ballistic missile, was challenged by critics (including Curtis LeMay, who used doubt over missile accuracy to encourage the development of new bombers) on the grounds that it was a single test (and could therefore be an anomaly), was a lower-altitude SLBM (and therefore was subject to different conditions than an ICBM), and that significant modifications had been made to its warhead before testing (as that particular warhead was known to be potentially prone to predetonation).
 Initial nuclear proliferation
In addition to the United States and the Soviet Union, three other nations, the United Kingdom , People's Republic of China , and France  also developed far smaller nuclear stockpiles. In 1952, the United Kingdom became the third nation to possess nuclear weapons when it detonated an atomic bomb in Operation Hurricane on October 3, 1952. During the Cold War, British nuclear deterrence revolved around the Resolution class ballistic missile submarines armed with the American-built Polaris missile and the WE.177 gravity bomb.
France became the fourth nation to possess nuclear weapons on February 13, 1960, when the atomic bomb Gerboise Bleue was detonated in Algeria, then still a French colony. During the Cold War, the French nuclear deterrent was centered around the Force de frappe, a nuclear triad consisting of Dassault Mirage IV bombers carrying such nuclear weapons as the AN-22 gravity bomb and the ASMP stand-off attack missile, Pluton and Hades ballistic missiles, and the Redoutable class submarine armed with strategic nuclear missiles.
The People's Republic of China became the fifth nuclear power on October 16, 1964, when it detonated a uranium-235 bomb in a test codenamed 596. Due to Soviet/Chinese tensions, the Chinese may have used nuclear weapons against either the United States or the Soviet Union in the event of a US/USSR nuclear war. During the Cold War, the Chinese nuclear deterrent consisting of gravity bombs carried aboard H-6 bomber aircraft and within missile systems.
Economic problems caused by the arms race in both powers, combined with China's new role and the ability to verify disarmament led to a number of arms control agreements beginning in the 1970s. This period known as Détente allowed both states to reduce their spending on weapons systems. SALT I and SALT II and all limited the size of the states arsenals. Bans on nuclear testing, anti-ballistic missile systems, and weapons in space all attempted to limit the expansion of the arms race though the Partial Test Ban Treaty.
These treaties were only partially successful. Both states continued building massive numbers of nuclear weapons, and new technologies such as MIRVs limited the effectiveness of the treaties. Both superpowers retained the ability to destroy each other many times over.
 Reagan and Star Wars
Towards the end of Jimmy Carter's presidency, and continued strongly through the subsequent presidency of Ronald Reagan, the United States rejected disarmament and tried to restart the arms race through the production of new weapons and anti-weapons systems. The central part of this strategy was the Strategic Defense Initiative , a space based anti-ballistic missile system derided as "Star Wars" by its critics. During the second part of 1980's, the Soviet economy was teetering towards collapse and was unable to match American arms spending. Numerous negotiations by Mikhail Gorbachev attempted to come to agreements on reducing nuclear stockpiles, but the most radical were rejected by Reagan as they would also prohibit his SDI program.
 Post-Cold War
With the end of the Cold War the United States, and especially Russia, cut down on nuclear weapons spending. Fewer new systems were developed and both arsenals have shrunk. But both states still maintain stocks of nuclear missiles numbering in the thousands. In the USA, stockpile stewardship programs have taken over the role of maintaining the aging arsenal.
After the Cold War ended, a large amount of resources and money which was once spent on developing nuclear weapons was then spent on repairing the environmental damage produced by the nuclear arms race, and almost all former production sites are now major cleanup sites. In the USA, the plutonium production facility at Hanford, Washington and the uranium molding facility at Rocky Flats, Colorado are among the most polluted sites.
United States policy and strategy regarding nuclear proliferation was outlined in 1995 in the document "Essentials of Post-Cold War Deterrence".
 India and Pakistan
The South-Asian states of India and Pakistan have also engaged in a nuclear arms race. India detonated what it called a "peaceful nuclear device" in 1974 ("Smiling Buddha")  in response primarily to the development of a weapon by its neighbor China a decade before. In the last few decades of the 20th century, however, both Pakistan and India began to develop nuclear-capable rockets, and Pakistan had its own covert bomb program which extended over many years since the first Indian weapon was detonated. In 1998, both India and Pakistan tested their nuclear weapons in a tit-for-tat fashion (Operation Shakti for India), with India claiming to have tested a hydrogen bomb as well (though the validity of this is disputed). Their arms race is somewhat analogous to the US/USSR race, except that both the amount of resources which each can devote to weapons and the distances to be traversed are far less.
 Milestone nuclear explosions
The following list is of milestone nuclear explosions. In addition to the atomic bombings of Hiroshima and Nagasaki, the first nuclear test of a given weapon type for a country is included, and tests which were otherwise notable (such as the largest test ever). All yields (explosive power) are given in their estimated energy equivalents in kilotons of TNT (see megaton).
"Deployable" refers to whether the device tested could be hypothetically used in actual combat (in contrast with a proof-of-concept device). "Staging" refers to whether it was a "true" hydrogen bomb of the so-called Teller-Ulam configuration or simply a form of a boosted fission weapon. For a more complete list of nuclear test series, see List of nuclear tests. Some exact yield estimates, such as that of the Tsar Bomba and the tests by India and Pakistan in 1998, are somewhat contested among specialists.
 See also
- Nuclear warfare
- Essentials of Post-Cold War Deterrence
- Deterrence theory
- Nuclear disarmament
- Nuclear-free zone
- Space race