Fat Man was the atomic bomb dropped on Nagasaki on August 9, 1945.
The United States dropped Fat Man from the B-29 bomber Bockscar. It was a plutonium implosion weapon, unlike Little Boy, the uranium gun-type bomb dropped on Hiroshima three days earlier.
The Nagasaki bombing occurred amid a rapidly changing military and diplomatic crisis. The Soviet Union declared war on Japan in the early hours of August 9, and the United States used Fat Man later that day. Emperor Hirohito's surrender announcement followed on August 15.
Fat Man was named as part of an informal series of bomb nicknames. The Trinity test device, detonated in New Mexico on July 16, used the same general implosion principle but was not the bomb dropped on Nagasaki. The two atomic attacks, together with the Soviet entry into the war and Japan's internal political crisis, formed the immediate background to the surrender decision.