Fat Man was the code name of the atomic bomb dropped on Nagasaki on August 9, 1945.
The plutonium implosion weapon was carried by the B-29 bomber Bockscar and detonated over Nagasaki. Its target had originally been Kokura, but cloud cover and smoke led the crew to select Nagasaki instead.
Fat Man followed Little Boy, the uranium bomb used against Hiroshima three days earlier. The two weapons used different designs, reflecting the rapid and experimental nature of the wartime atomic-bomb program. Fat Man produced a powerful blast despite detonating away from the city’s intended aiming point.
The Nagasaki attack came on the same day that Soviet forces began invading Japanese-held Manchuria. Japan’s leaders faced both developments as they debated ending the war. Hirohito announced acceptance of the Allied terms on August 15, while the formal surrender document was signed on September 2.