What was the Allied plan for invading Japan’s home islands before Japan surrendered in 1945?

Operation Downfall was the Allied plan for invading Japan’s home islands before Japan surrendered in 1945.

The proposed operation was designed as a two-stage invasion. Operation Olympic would have attacked southern Kyūshū, Japan’s southernmost main island, in November 1945. Operation Coronet would then have targeted the Kantō Plain near Tokyo on Honshū in spring 1946.

Planners expected an invasion to be extremely costly. Japan prepared its own defensive plan, Operation Ketsugō, and Allied estimates varied widely, with projected casualties reaching from hundreds of thousands to much higher totals. The proposed campaign would have required enormous naval, air and ground forces.

Downfall was canceled after Japan accepted the Potsdam Declaration and announced its surrender. The atomic bombings, the Soviet declaration of war and invasion of Manchuria, and the resulting political crisis all contributed to the decision that a direct invasion would not be necessary. Operation Downfall therefore remained a plan rather than an executed campaign.