Operation DOWNFALL was the overall planning U.S. planning term for the invasion of the home islands of Japan, should forcible entry and ground combat be necessary. Air, naval, and submarine operations, including the nuclear attacks against Japan, led to a Japanese surrender, making DOWNFALL unnecessary.
DOWNFALL had two main subsidiary operations, Operation OLYMPIC against the southern island of Kyushu, and Operation CORONET, which would move against the Kanto Plain containing Tokyo. Only a few DOWNFALL planners were aware of the Manhattan Project, the U.S. nuclear weapons development program, but plans were evolving to use nuclear weapons in tactical support.
Tentatively scheduled for 1 November 1945, the first phase of DOWNFALL would involve multiple landings in Kyushu, each by a corps-sized formation. The invasion would have massive air support, and tactical use of nuclear weapons was being consivered.
The Japanese expected that they would throw all possible resources into the defense, as they were uncertain they could stop an attack on the Kanto Plain. Regional command for the defense was under Second General Army, equivalent to a Western army group, headquartered in Hiroshima Castle before August 1945.