Many US military experts and top generals believed it was unnecessary. Japan was defeated, particularly with the entry of the Soviet Union into the war, it was over for them.
Yes, Japan was defeated a year before the bombs were dropped. The problem was Japan decided not to surrender even though themselves they knew they lost.
That's the problem.
Japan not surrendering even when they knew.
Per Japanese documents even after one bomb was dropped they still would not surrender. It took the second bomb, and even then the war cabinet was split on 3-3 on whether to surrender.
Two bombs dropping only got them to the point of a tie. The Emperor had to be called in to break the tie after two bombings.
And the Japanese knew the Soviets were going to enter the war. That was already in their calculations and they still were for fighting.
"definitive"? What does that even mean in a field of study like history? You're telling me there has been zero new analysis on the subject since 1995? No new insights?
And even that assumes that Alperovitz's initial premises were valid, that he did not miss any evidence,or exclude or discount any because his own biases and such, and so that is conclusions followed logically from all of that.
How is it "definitive"?
It's exhaustive and extremely well documented.
Which is not a dissimilar thing to the US "wavering" over the commitment to an invasion of Japan. The nuclear bombs "resolved" that. We'll never know what would have happened otherwise.