I actually read a few books about this for a seminar, there's a revisionist argument that the Japanese could have been induced to surrender before but that's gone out of style a bit, newer books that have done research in Japanese archives don't agree with that, the Japanese thought they could actually successfully repel an invasion and the Americans weren't extraordinarily confident, that and the Japanese didn't agree to peace terms that would have let them keep the Emperor.
Granted I haven't read everything, its not my field.
But the newer revisionist argument (that I've read) refutes that Japan would have surrendered but argues that Truman would have wanted to use the nukes anyway, and do it quickly before the Soviets entered the war so they could get what they were promised to enter the war, the Sakhalin Islands, the Kurile Islands, I don't know if that's the right spelling, keep them out of Mongolia, etc.
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