<Ul> <Li> Japanese surrender </Li> </Ul> <P> Operation Downfall was the proposed Allied plan for the invasion of Japan near the end of World War II . The planned operation was abandoned when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war and the Soviet invasion of Manchuria . The operation had two parts: Operations Olympic and Coronet . Set to begin in November 1945, Operation Olympic was intended to capture the southern third of the southernmost main Japanese island, Kyūshū, with the recently captured island of Okinawa to be used as a staging area . Later, in the spring of 1946, Operation Coronet was the planned invasion of the Kantō Plain, near Tokyo, on the Japanese island of Honshu . Airbases on Kyūshū captured in Operation Olympic would allow land - based air support for Operation Coronet . If Downfall had taken place, it would have been the largest amphibious operation in history . </P> <P> Japan's geography made this invasion plan quite obvious to the Japanese as well; they were able to predict the Allied invasion plans accurately and thus adjust their defensive plan, Operation Ketsugō, accordingly . The Japanese planned an all - out defense of Kyūshū, with little left in reserve for any subsequent defense operations . Casualty predictions varied widely, but were extremely high . Depending on the degree to which Japanese civilians would have resisted the invasion, estimates ran up into the millions for Allied casualties . </P> <P> Responsibility for planning Operation Downfall fell to American commanders Fleet Admiral Chester Nimitz, General of the Army Douglas MacArthur and the Joint Chiefs of Staff--Fleet Admirals Ernest King and William D. Leahy, and Generals of the Army George Marshall and Hap Arnold (the latter being the commander of the US Army Air Forces). At the time, MacArthur was also being considered for promotion to a special "super rank" of General of the Armies, so as to be granted operational authority over other five - star officers . However, the proposal to promote MacArthur was only at the level of informal discussion by the time World War II ended . </P>

How many american casualties were predicted during the planning for the invasion of japan