By August 1945 the war in Europe had ended and the Allied campaign against Japan had reached the Japanese home islands. On 6 August the United States dropped 'Little Boy' on Hiroshima. Three days later the second mission set out for Kokura, but cloud and smoke obscured the aiming point, and the crew of Bockscar diverted to the secondary target, Nagasaki. Fat Man detonated above the city at about 11:02 a.m. Nagasaki's hilly terrain absorbed part of the blast, which is why the physical destruction, though immense, was less uniformly spread than at Hiroshima despite the higher yield of the weapon.

Nagasaki remains the last occasion on which a nuclear weapon has been used in war, which is why the date anchors the global disarmament calendar rather than functioning only as a war memorial. Casualties came in waves — deaths in the blast itself, then from burns and injuries, then over years from radiation exposure. The long-term medical record of the survivors, the hibakusha, became the principal body of evidence used to argue for nuclear disarmament in international forums, and it underlies much of what is scientifically known about radiation effects on human populations. For Indian examinations the day connects to the wider disarmament architecture: the Non-Proliferation Treaty, the Comprehensive Nuclear-Test-Ban Treaty and India's stated doctrine of no-first-use.