The American policy on nuclear weapons was first set in 1942 when President Franklin D. Roosevelt made the decision to proceed with the Manhattan Project in order to develop an atomic weapon before the end of World War II. His motivation was to foreclose the possibility that Nazi Germany might develop and monopolize such a weapon in time to affect the outcome of the war. But before the Manhattan Project could come to fruition with a successful test of the nuclear device Trinity in July 1945, Roosevelt had died, Germany had surrendered, and President Harry S. Truman was faced with the decision as to how the atomic bomb should be used to shorten the war with Japan. Advised by a committee of soldiers, scientists, and statesmen that, for maximum shock effect, the bomb should be used against a major Japanese population center, Truman concurred with the recommendation in hopes of eliminating the need for a costly and prolonged invasion of the Japanese home islands.
The sequel is well known. A plutonium bomb and an untested uranium fission bomb were shipped to a B-29 bomber base in the Pacific. On 6 August 1945, the B-29 “Enola Gay” dropped the uranium bomb on Hiroshima, and on 9 August another B-29 destroyed Nagasaki with the plutonium bomb. Each of the two bombs exploded with the equivalent force of approximately twenty thousand tons of TNT (20 KT), and between them they probably killed 125,000 people outright and caused the eventual deaths of perhaps as many as 300,000 people. The destruction of the two cities, combined with a Soviet declaration of war on Japan between the two atomic bombings, had the desired effect. On 14 August, Japan accepted Allied terms for an armistice, and 2 September the Imperial Japanese government formally surrendered.