What codename did the Allies give their planned invasion of Japan’s home islands in World War II?

The Allies gave their planned invasion of Japan’s home islands the codename Operation Downfall.

Downfall was a proposed United States and British Empire operation for invading mainland Japan near the end of World War II. It was never carried out because Japan accepted the Allied demand for surrender in August 1945, before the invasion could begin.

The plan had two major stages. Operation Olympic was scheduled to begin on 1 November 1945 with landings in southern Kyūshū. Operation Coronet was planned for spring 1946 and would have attacked the Kantō Plain near Tokyo on Honshu.

Operation Downfall is often confused with its two component operations. Olympic and Coronet were parts of Downfall, not alternative names for the entire invasion. Operation Ketsugō was different again: it was Japan’s defensive plan for resisting an Allied invasion. If executed, Downfall would have been one of history’s largest amphibious operations, involving enormous forces and projected casualties on both sides.