What was the Allied plan for a full-scale invasion of Japan’s home islands in 1945–1946 called?

The Allied plan for a full-scale invasion of Japan’s home islands was called Operation Downfall.

Downfall was designed as a two-stage operation. Operation Olympic would have attacked southern Kyūshū in November 1945, creating airfields and staging areas. Operation Coronet was planned for spring 1946 and would have sent Allied forces into the Kantō Plain near Tokyo on Honshū.

The plan was canceled after Japan accepted surrender terms. Its planners faced difficult geography, strong Japanese defenses, and the possibility of enormous casualties. Allied estimates varied widely, but the invasion was expected to be one of the largest military operations ever attempted. Japan was also preparing its own defense plan, known as Operation Ketsugō.

Operation Downfall should not be confused with Operation Overlord, the Allied invasion of Normandy in 1944. Downfall never took place, so its casualty figures and likely outcome remain estimates rather than battlefield results. Its cancellation is one reason the final weeks of the Pacific War are often discussed alongside invasion planning.