The Japanese military plan to defend the home islands against an Allied invasion was called Operation Ketsugō.
Ketsugō was Japan’s planned all-out defense of the home islands. Japanese commanders expected the Allies to land in southern Kyushu and prepared to concentrate troops, aircraft, naval forces, and civilians for resistance. The plan was developed in response to the anticipated invasion represented by the Allied Operation Downfall concept.
Ketsugō was never tested in combat because Japan accepted surrender terms before Operation Olympic could begin. Its existence helps explain why Allied planners expected extremely heavy casualties from an invasion. Japanese preparations included mobilizing civilians, organizing local defense units, and positioning forces near likely landing areas. Ketsugō is sometimes confused with Downfall, but the two names described opposing plans: Downfall was the Allied invasion plan, while Ketsugō was Japan’s defensive strategy. The surrender made both plans unnecessary.