Operation Downfall, the proposed Allied plan for the invasion of Japan, would have been the largest amphibious operation in history. It was planned in two phases on the few beaches that were adequate for a massive landing force:
- Operation Olympic, to be launched from Okinawa in November 1945, would capture the southern third of the island Kyūshū, which could then be used as a base for air attacks on other targets in Japan.
The Japanese reportedly planned expending the majority of their defensive efforts on the island of Kyūshū, leaving little reserve for other operations.
Some American military planners advocated the use of chemical weapons in the invasion of japan – although it had been outlawed by the Geneva Convention, neither the U.S. nor Japan had signed the agreement.
Casualty predictions varied widely, but were extremely high and dependent on the level of resistance by Japanese civilians.
- The U.S. Joint Chiefs of Staff estimated that casualties from Operation Olympic would cost ~456,000 men, including 109,000 killed. With the addition of Operation Coronet, the estimate was 1.2 million casualties, with 267,000 deaths.
- Admiral Chester Nimitz staff calculated that America would suffer 49,000 in the first 30 days of the invasion
- General Douglas MacArthur’s staff estimated ~125,000 casualties after 120 days
- General George Marshall estimated 31,000 in 30 days after landing in Kyushu
- Admiral William Leahy estimated that the invasion would cost 268,000 casualties
- U.S. Navy Department estimated that the total losses to America would be between 1.7 and 4 million with 400,000 to 800,000 deaths. They also estimated ~10 million Japanese casualties