What was the planned Allied operation to invade Japan’s home islands in late 1945 and 1946?

Operation Downfall was the planned Allied operation to invade Japan’s home islands in late 1945 and 1946.

Downfall was divided into two major stages. Operation Olympic was scheduled to begin on 1 November 1945 with an invasion of southern Kyūshū. Operation Coronet was planned for 1 March 1946 against the Kantō Plain near Tokyo on Honshū. Neither stage occurred because Japan surrendered.

Military planners expected the operation to require enormous forces and to produce severe casualties. Japan prepared defensive plans that mobilized soldiers and civilians for resistance across the home islands. The projected campaign was considered potentially larger than the Normandy invasion because of the size of the forces and the anticipated fighting.

The atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and Japan’s internal decision to accept the Potsdam Declaration ended the need for Downfall. Its cancellation means that many casualty estimates associated with the invasion are projections, not deaths that actually occurred.