United States and weapons of mass destruction
The United States is known to have possessed three types of weapons of mass destruction: nuclear weapons, chemical weapons, and biological weapons. The U.S. is the only country to have used nuclear weapons in combat, when it detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki during World War II. It had secretly developed the earliest form of the atomic weapon during the 1940s under the title "Manhattan Project". The United States pioneered the development of both the nuclear fission and hydrogen bombs (the latter involving nuclear fusion). It was the world's first and only nuclear power for four years (1945–1949), until the Soviet Union managed to produce its own nuclear weapon. The United States has the second largest number of deployed nuclear weapons in the world, after Russia.
Atomic bombings of Hiroshima and Nagasaki
The American monopoly on nuclear weapons lasted only four years before the Soviet Union detonated an atomic bomb in September 1949. The United States responded with the development of the hydrogen bomb, a nuclear weapon a thousand times as powerful as the bombs that devastated Hiroshima and Nagasaki. Such ordinary fission bombs would henceforth be regarded as small tactical nuclear weapons. By 1986, the United States would have 23,317 nuclear weapons, while the Soviet Union had 40,159. By 2017, nine nations had nuclear weapons, but Japan was not one of them. Japan reluctantly signed the Treaty on the Non-Proliferation of Nuclear Weapons in February 1970, but it still sheltered under the American nuclear umbrella. American nuclear weapons were stored on Okinawa, and sometimes in Japan itself, albeit in contravention of agreements between the two nations. Lacking the resources to fight the Soviet Union using conventional forces, the Western Alliance came to depend on the use of nuclear weapons to defend itself during the Cold War, a policy that became known in the 1950s as the New Look. In the decades after Hiroshima and Nagasaki, the United States would threaten to use its nuclear weapons many times.