During the final stages of World War II in 1945, the United States conducted two atomic bombings against the cities of Hiroshima and Nagasaki in Japan. These two events are the only use of nuclear weapons in war to date.
Following a firebombing campaign that destroyed many Japanese cities, the Allies prepared for a costly invasion of Japan. The war in Europe ended when Nazi Germany signed its instrument of surrender on 8 May, but the Pacific War continued. Together with the United Kingdom and the Republic of China, the United States called for a surrender of Japan in the Potsdam Declaration on 26 July 1945, threatening Japan with "prompt and utter destruction". The Japanese government ignored this ultimatum, and the United States deployed two nuclear weapons developed by the Manhattan Project. American soldiers dropped Little Boy on the city of Hiroshima on 6 August 1945, followed by Fat Man over Nagasaki on 9 August ...
Even more frightening
[Major General Leslie] Groves, of the U.S. Army Corps of Engineers, expected to have another atomic bomb ready for use on 19 August, with three more in September and a further three in October. On 10 August, he sent a memorandum to [General of the Army George] Marshall in which he wrote that "the next bomb . . should be ready for delivery on the first suitable weather after 17 or 18 August." On the same day, Marshall endorsed the memo with the comment, "It is not to be released over Japan without express authority from the President." ...
Two more Fat Man assemblies were readied. The third core was scheduled to leave Kirtland Field for Tinian on 12 August, and Tibbets was ordered by Major General Curtis LeMay to return to Utah to collect it. Robert Bacher was packaging it in Los Alamos when he received word from Groves that the shipment was suspended.
Japan surrendered on August 15.
(The Instrument of Surrender was signed on September 2, now known as V-J Day)