Hell to Pay: Operation Downfall and the Invasion of Japan 1945-47, by D. M. Giangreco, examines the US plans for the invasion of Japan, had not the Japanese surrendered after the atomic explosions that destroyed Nagasaki and Hiroshima.
Giangreco uses both American and Japanese primary sources to show how this massive US invasion, and Japan's response, would have played out if it had actually taken place.
Operation Downfall was divided into two invasion phases. Operation Olympic, the invasion of the southern part of the southern island of Kyushu, which would then be used as a staging base for the next phase, Operation Coronet, the invasion of Tokyo.
This book was written, in part, as response to the Enola Gay controversy on the fiftieth anniversary of the dropping of the atomic bomb on Hiroshima. Giangreco wishes to show, with fine scholarship, that the alternative to the bombs would have been an invasion of Japan that would have caused over a million US casualties, and several million Japanese military and civilian casualties. The war would have not ended in 1945, but in 1947.
Operation Downfall would have been the largest military exercise in history. It is good that it did not take place. The cost on both sides would have been unbearable.