On August 6 and 9, 1945, the United States dropped two atomic bombs on Japan, ending World War II and inaugurating the nuclear age. This chapter examines the US decision to use atomic weapons on Japan and the origins of a nuclear taboo in the period up to 1950. In 1945, the atomic bomb was widely accepted as a legitimate weapon of war. Most politicians and generals regarded it as completely natural to use the atomic weapons they had at hand to end the war against Japan as speedily as possible. Yet by the time of the Korean War, five years later, significant inhibitions on the use of atomic weapons were already evident, and these cannot be accounted for by deterrence.

The origins of the taboo lay especially in a set of policy precedents and categories established by President Truman and the United Nations which marked out nuclear weapons as different from other weapons. These efforts were propelled by fears of the destructive power of the new weapon, the shock and horror of the Japan bombings, and the moral qualms of key leaders, especially Truman.