What was the planned Allied operation to invade Japan’s home islands called?

The planned Allied invasion of Japan’s home islands was called Operation Downfall.

Downfall was divided into two major phases. Operation Olympic was intended to land on southern Kyushu in November 1945. Operation Coronet was planned for the Kanto Plain near Tokyo in spring 1946. Japan’s surrender canceled both operations before they began.

The invasion plan was developed because Allied leaders expected Japan to continue fighting despite the destruction of its cities and naval forces. Planning included enormous troop movements, naval support, air operations, and preparations for very heavy casualties on both sides. The actual outcome made the proposed invasion unnecessary.

Downfall is sometimes mistaken for Operation Olympic, but Olympic was only the first phase. Operation Overlord, by contrast, was the Allied invasion of Normandy in June 1944. The planned invasion of Japan would have been larger than D-Day in some measures, but it never became an active operation.