Atomic bombings of Hiroshima and Nagasaki
Legacy

By 30 June 1946, there were components for nine atomic bombs in the US arsenal, all Fat Man devices identical to the one used at Nagasaki. The nuclear weapons were handmade devices, and a great deal of work remained to improve their ease of assembly, safety, reliability and storage before they were ready for production. There were also many improvements to their performance that had been suggested or recommended, but that had not been possible under the pressure of wartime development. The Chairman of the Joint Chiefs of Staff, Fleet Admiral William D. Leahy, decried the use of the atomic bombs as adopting "an ethical standard common to the barbarians of the Dark Ages", but in October 1947 he reported a military requirement for 400 bombs.

The American monopoly on nuclear weapons lasted 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 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 had 23,317 nuclear weapons and the Soviet Union had 40,159. In early 2019, more than 90% of the world's 13,865 nuclear weapons were owned by the United States and Russia.

By 2020, 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 is 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, NATO 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 many times to use its nuclear weapons.

On 7 July 2017, more than 120 countries voted to adopt the UN Treaty on the Prohibition of Nuclear Weapons. Elayne Whyte Gómez, President of the UN negotiations, said, "the world has been waiting for this legal norm for 70 years". As of 2023, Japan has not signed the treaty.
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