What was the Allied plan to invade Japan’s home islands in 1945–46 called?

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

Downfall was a two-stage plan developed near the end of World War II. Operation Olympic would have invaded southern Kyūshū in November 1945, while Operation Coronet would have attacked the Kantō Plain near Tokyo in spring 1946.

The plan was never executed. Japan surrendered after the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the rapid deterioration of Japan’s military position. The cancellation avoided an invasion that planners expected to be exceptionally costly.

Operation Downfall is often confused with its two component operations. Olympic and Coronet were parts of Downfall, not alternative names for the entire invasion plan. Japan had prepared its own defensive strategy, Operation Ketsugō, to oppose the expected Allied landings.