A unified command was deemed necessary for an invasion of Japan. Allied command was divided into regions: by 1945, for example, Chester Nimitz was the Allied C-in-C Pacific Ocean Areas, while Douglas MacArthur was Supreme Allied Commander, South West Pacific Area, and Admiral Louis Mountbatten was the Supreme Allied Commander, South East Asia Command. The Pacific War was not under a single Allied commander-in-chief (C-in-C). Once the atomic bomb became available, General Marshall envisioned using it to support the invasion if sufficient numbers could be produced in time. Īt the time, the development of the atomic bomb was a very closely guarded secret (not even then-Vice President Harry Truman knew of its existence until he became President), known only to a few top officials outside the Manhattan Project, and the initial planning for the invasion of Japan did not take its existence into consideration. Leahy, and Generals of the Army George Marshall and Hap Arnold (the latter being the commander of the U.S. 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 Kyūshū, 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 Ketsugō, accordingly. If Downfall had taken place, it would have been the largest amphibious operation in history. Airbases on Kyūshū 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 Kantō 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, Kyūshū, 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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