The alternate to the bomb was called Operation Downfall
Operation Downfall
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Operation Downfall was the codename for the 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 operation had two parts: Operation Olympic and Operation Coronet. Set to begin in October 1945, Operation Olympic was intended to capture the southern third of the southernmost main Japanese island, Kyushu, with the recently captured island of Okinawa to be used as a staging area. Later, in spring 1946, Operation Coronet was the planned invasion of the Kanto Plain, near Tokyo, on the Japanese island of Honshu. Airbases on Kyushu 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 human history.[1]
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 Ketsugo, accordingly. The Japanese planned an all-out defense of Kyushu, 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 resisted the invasion, estimates ran into the millions for Allied casualties.
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Also, since the Japanese population was fiercely loyal to the Emperor and would be told to fight to the death it was thought that their population would be wiped out.