The nuclear arms race was an arms race competition for supremacy in nuclear warfare between the United States, the Soviet Union, and their respective allies during the Cold War. During this same period, in addition to the American and Soviet nuclear stockpiles, other countries developed nuclear weapons, though no other country engaged in warhead production on nearly the same scale as the two superpowers. The race began during World War II, dominated by the Western Allies' Manhattan Project and Soviet atomic spies. Following the atomic bombings of Hiroshima and Nagasaki, the Soviet Union accelerated its atomic bomb project, resulting in the RDS-1 test in 1949. Both sides then pursued an all-out effort, realizing deployable thermonuclear weapons by the mid-1950s. The arms race in nuclear testing culminated with the 1961 Tsar Bomba. Atmospheric testing was ended in the 1963 Partial Nuclear Test Ban Treaty. Subsequent work focused on warhead miniaturization, more lightweight and lower fallout designs, and variants of neutron bombs, led by Los Alamos and Livermore in the US and VNIIEF and VNIITF in the USSR. Seven other countries developed nuclear weapons during the Cold War. The UK and France, both NATO members, developed fission and fusion weapons throughout the 1950s, and 1960s, respectively. China developed both against the backdrop of the Sino-Soviet split. Israel, India, Pakistan, and South Africa subsequently developed at least fission weapons. Nuclear weapons delivery vehicles were a major field of competition. Initially strategic bombers were the only option. By 1960, both sides had developed intercontinental ballistic missiles and submarine-launched ballistic missiles, resulting in the nuclear triad. Additionally, smaller systems for tactical nuclear weapons delivery were extensively developed and deployed, including tactical ballistic missile and cruise missiles, nuclear artillery, demolition munitions, depth bombs, and torpedoes. Key regions of nuclear build-up included the Eastern European Warsaw Pact, NATO members West Germany, Italy, Greece, and Turkey, and US-allied Japan, South Korea, Taiwan, and the Philippines. Confrontations with nuclear threats occurred during the Korean War, the First and Second Taiwan Strait Crises, the Berlin Crisis of 1961, and most significantly the Cuban Missile Crisis. Détente during the 1960s and 1970s limited the arms race, especially via the Non-Proliferation Treaty and Anti-Ballistic Missile Treaty. Tensions were renewed in the early 1980s, in the development and deployment to Europe of MRBMs, IRBMs, and supersonic strategic bombers, as well as the space-based Strategic Defense Initiative. Under the leadership of Mikhail Gorbachev, the USSR negotiated the Intermediate-Range Nuclear Forces Treaty and START I, until its dissolution in 1991 brought to an end the Cold War nuclear arms race. Russia and the US maintain the world's largest nuclear stockpiles. The 1993 START II, 1996 CTBT, and 2010 New START treaties further curtailed the arms race in the post-Cold War period. Tensions have resurged in what is sometimes called a Second Cold War. The US-Russian INF and New START treaties broke down in 2019 and 2023, against the backdrop of the Russia-Ukraine War, and Russia announced six "nuclear super weapons". In the Pacific, the US and China are in competition over hypersonic weapons.
World War II During World War II, four nuclear weapons research programs existed. The industrial Manhattan Project, directed by the US military and coordinated with the UK (Tube alloys) and Canada, developed the first nuclear weapons. More limited scientific research was carried out in the Soviet atomic bomb project, German nuclear program, and Japanese nuclear program.
Allied-German rivalry From 1934, the Vemork hydroelectric station of Norsk Hydro Rjukan entered operation as the world's only industrial-scale production site for heavy water, suitable as a moderator for atomic pile experiments leading to nuclear reactors. In early 1940, a French Deuxième Bureau agent arranged for the purchase of the entire Norwegian stock of heavy water, 187 liters. The Germans had offered the same purchase, but the French obtained the agreement of the Norwegian government after telling them of its military purpose. It was shipped to the Collège de France nuclear laboratory in Paris prior to the German invasion of Norway in April, but during the German invasion of France in June it was shipped to England. After various secret storages it was moved to the Cavendish Laboratory in Cambridge, for the work of refugee French nuclear physicists Hans von Halban and Lew Kowarski. In February 1941, the Luftwaffe carried out a raid on Cambridge, hitting the Cavendish Laboratory. Kowarski said in a postwar interview that his and von Halban's work there on a heavy water-moderated atomic pile was specifically targeted in retaliation for a prior British raid on Heidelberg, where Walther Bothe and Arnold Flammersfeld were working on similar pile experiments at what is now the Max Planck Institute for Medical Research. In February 1943, a Special Operations Executive-trained team of Norwegian commandos detonated explosive charges on the heavy-water electrolysis chambers at the Vemork hydroelectric plant during Operation Gunnerside. Along with other covert and bombing raids, the Allies successfully crippled heavy water moderator production for the German nuclear program. In 1943, a US Office of Strategic Services operation named Project AZUSA aimed to interview Italian physicists to learn what they knew about German nuclear physicists Werner Heisenberg and Carl Friedrich von Weizsäcker. In December 1944, OSS spy Moe Berg travelled to Zurich where Heisenberg was giving a lecture, with orders to shoot Heisenberg with a concealed pistol if he deemed the German atomic bomb program close to completion.
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