The Manhattan Project, begun in 1942, was a secret collaboration between the United States, Canada, and the United Kingdom intended to deny Nazi Germany an atomic weapon. Under Robert Oppenheimer's direction at Los Alamos a fission bomb was designed; the first test was carried out at the Trinity site in New Mexico on 16 July 1945. Germany had already been defeated. The weapon was used instead against Japan, which had not yet surrendered.

At 8:15 on the morning of 6 August the uranium bomb 'Little Boy' detonated over Hiroshima. More than 70,000 people died at once; by year's end, with deaths from radiation and burns, the total reached around 140,000. Three days later the plutonium device 'Fat Man' was dropped on Nagasaki; the death toll there was about 74,000. Japan surrendered on 15 August and the Second World War ended.

Whether the bombs ought to have been used remains historically and ethically debated: their defenders argue that they ended the war without a land invasion; opponents stress the predominantly civilian targets and the evidence that Japan was already moving toward surrender. Whatever the assessment, after those two days the world stood in a different place. A nuclear arms race opened between the superpowers; the Cold War proceeded under the shadow of these weapons; humanity now held the means to destroy itself within a short span of time. The 1968 Treaty on the Non-Proliferation of Nuclear Weapons and the verification regimes that followed are products of this new condition.