Manhattan Project (Manhattan Engineer District)

The Manhattan Project, formally designated the Manhattan Engineer District, was the U.S. Army Corps of Engineers program established in 1942 to develop nuclear weapons before Nazi Germany could do so. Under Major General Leslie R. Groves as military director and physicist J. Robert Oppenheimer as scientific director of the Los Alamos Laboratory, the project eventually employed several hundred thousand workers across sites including Oak Ridge, Tennessee, and Hanford, Washington, and conducted the first successful test of a nuclear device at the Trinity site in New Mexico on July 16, 1945.

Role in Documented Cases

A Target Committee convened by the project in May 1945 drew up and refined a list of Japanese cities as candidates for atomic attack, weighing military and industrial significance alongside each city’s suitability for demonstrating the bomb’s destructive power to a wide area. Kyoto was removed from the list at the insistence of Secretary of War Henry Stimson because of its cultural and historic importance, leaving Hiroshima as the primary target for the first weapon. On August 6, 1945, a Manhattan Project bomb, “Little Boy,” was dropped on Hiroshima. Three days later, after the intended secondary target of Kokura was obscured by cloud cover, a second weapon, “Fat Man,” was dropped on Nagasaki. In the weeks before the bombings, a group of project scientists led by James Franck circulated a report urging a demonstration of the weapon before its use on a populated city; the recommendation was considered and rejected by the interim committee advising President Truman.

Accountability

No wartime or postwar tribunal examined the decision to use the bomb on Japanese cities, and the project’s leadership faced no legal proceedings over the target selection. In January 1947, under the Atomic Energy Act of 1946, control of the U.S. nuclear program passed from the Manhattan Engineer District to the civilian Atomic Energy Commission, ending direct Army administration. Oppenheimer himself later expressed public ambivalence about the weapon’s use and, in 1954, had his security clearance revoked following a hearing rooted in Cold War-era political suspicion rather than in any review of the target decisions. The necessity and morality of the bombings has remained a subject of ongoing historical and public debate rather than one resolved by any formal accountability process.

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