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The United States, with assistance from the United Kingdom and Canada, designed and built the first atomic bombs under what was called the Manhattan Project. The project was initially started at the instigation of European refugee scientists (including Albert Einstein) and American scientists who feared that Nazi Germany would also be conducting a full-scale bomb development program (that program was later discovered to be much smaller and further behind). The project itself eventually employed over 130,000 people at its peak at over thirty institutions spread over the United States, and cost a total of nearly US$2 billion, making it one of the largest and most costly research and development programs of all time.

The first nuclear device, called "Gadget," was detonated during the "Trinity" test near Alamogordo, New Mexico on July 16, 1945. The Hiroshima and Nagasaki bombs were the second and third to be detonated and as of 2007 the only ones ever detonated in a military action. (See Weapons of Mass Destruction.)

During World War II both the Allies and Axis powers had previously pursued policies of strategic bombing and the targeting of civilian infrastructure. In numerous cases these had caused huge numbers of civilian casualties and were (or came to be) controversial. In Germany, the Allied firebombing of Dresden resulted in roughly 30,000 deaths. The March 1945 firebombing of Tokyo killed 72,489 people, according to the Japan War History office.[7] By August, about 60 Japanese cities had been destroyed through a massive aerial campaign, including massive firebombing raids on the cities of Tokyo and Kobe.

Over 3Ð... years of direct U.S. involvement in World War II, approximately 290,000 Americans had been killed in action and another 110,000 killed as a result of the war,[8] 90,000 of them incurred in the war against Japan.[9] In the months prior to the bombings, the Battle of Okinawa resulted in an estimated 50,000-150,000 civilian deaths, 100,000-125,000 Japanese or Okinawan military or conscript deaths and over 72,000 American casualties.[citation needed] Deathtoll was given as 107,539 counted dead plus an estimated 23,764 in the closed caves or buried by the Japanese. Since the number was far above the estimated Japanese force on the island the army intelligence supposed that about 42,000 was civilians.[10] A commonly provided justification for the bombings is that an invasion of the Japanese mainland was expected to result in casualties many times greater than in Okinawa.

U.S. President Harry S. Truman was unaware of the Manhattan Project until Franklin Roosevelt's death. Truman asked U.S. Secretary of War Henry L. Stimson to head a group of prominent citizens called the Interim Committee, which included three respected scientists and had been set up to advise the President on the military, political, and scientific questions raised by the possible use of the first atomic bomb. On May 31, Stimson put his conclusions to the committee and a four-man Scientific Panel. Stimson supported use of the bomb, stating "Our great task is to bring this war to a prompt and successful conclusion." But Dr. J. Robert Oppenheimer, one of the Scientific Panel members, stated that a single atomic bomb would probably kill twenty thousand people, and the target should be a military one, not civilian. Another scientist, Dr. Arthur Holly Compton, suggested dropping the bomb on an isolated