Fat Man was the U.S. codename of the atomic bomb dropped on Nagasaki.
The plutonium implosion weapon was dropped on 9 August 1945 by the B-29 bomber Bockscar. It exploded over Nagasaki and was the second nuclear weapon used in warfare. The first, Little Boy, had been dropped on Hiroshima three days earlier.
Fat Man was more complex in design than Little Boy because it used an implosion mechanism to compress a plutonium core. The bomb’s name belonged to a group of informal wartime design codenames; it was not Japan’s name for the weapon.
The Nagasaki bombing occurred on the same day that Soviet forces launched their offensive against Japan in Manchuria. Japan’s leadership faced both developments while debating whether to accept the Allied terms of the Potsdam Declaration.