Responsibility for the planning of 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. Depending on the degree to which Japanese civilians would have resisted the invasion, estimates ran up into the millions for Allied casualties. Casualty predictions varied widely, but were extremely high. The Japanese planned an all-out defense of KyЕ«shЕ«, with little left in reserve for any subsequent defense operations. Japan's geography made this invasion plan quite obvious to the Japanese as well they were able to accurately predict the Allied invasion plans and thus adjust their defensive plan, Operation KetsugЕЌ, accordingly.

If Downfall had taken place, it would have been the largest amphibious operation in history. Airbases on KyЕ«shЕ« captured in Operation Olympic would allow land-based air support for Operation Coronet. In early 1946 would come Operation Coronet, the planned invasion of the KantЕЌ Plain, near Tokyo, on the main Japanese island of Honshu. Set to begin in November 1945, Operation Olympic was intended to capture the southern third of the southernmost main Japanese island, KyЕ«shЕ«, with the recently captured island of Okinawa to be used as a staging area. The operation had two parts: Operation Olympic and Operation Coronet. The planned operation was canceled when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war and the invasion of Manchuria. Operation Downfall was the proposed Allied plan for the invasion of the Japanese home islands near the end of World War II. (66 divisions, 36 brigades, and 45 regiments, not counting PCFC units.) (Up to 58 US and 3-5+ Commonwealth divisions)

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