The essential argument in Alperovitz is that the atomic bombs were unnecessary, known to be unnecessary, and dropped primarily to scare the USSR. The exact who's and where's and why's have shifted a bit over the editions (Stimson was his initial villain, it then changed to Byrnes), but that's the basic argument.

The basic argument of Hasegawa is that the atomic bombs by themselves were insufficient to convince the Japanese to surrender when they did, that rather it was the additional factor of the Soviet invasion of Manchuria that caused them to fold.

As for their receptions — I am generalizing for a lot of scholarship here, but here it goes:

There are aspects of Alperovtiz's book that are very good scholarship. The guy has done a lot of research and I don't think anyone would fault him on his intentions. It is a well-documented book and while I don't quite find Alperovitz persuasive in his argument, I have found that if there is an obscure source that I think I'm the first to have found, Alperovitz probably found it first. In that sense he is a great historian. However, he falls into a number of "traps" regarding his reasoning and argument, essentially projecting too much foreknowledge on the people involved in the bomb planning, embracing a somewhat "conspiratorial" approach to historical figures that I don't find matches up with my assessments of them. There are aspects to his position that have been integrated into the more "middleground" scholarship, e.g. that diplomatic considerations were one of the reasons some of the people (like Byrnes) thought the atomic bombs ought to be dropped. But he has not convinced anyone but people who are pretty far "left" on the ideological spectrum (e.g., Oliver Stone) that the whole thing was an elaborate conspiracy. Historians have tended over time to see the "decision" to use the bomb as not really any single "decision," but rather a clustering of factors, and Alperovitz can contribute to that without one buying his whole argument. The later chapters of his book, which are dedicated to how Manhattan Project and Truman administration officials later sought to control the historical narrative of the bombs, are pretty good and mostly spot on — one can argue that was a conspiracy or not (I don't), but they definitely tried to make sure "their version of the facts" was front and center and felt their legacies were dependent on people thinking the bombings were a good idea.

As for Hasegawa, he has been pretty successful at convincing scholars that the Soviet invasion played a big role in the thinking of the Japanese high command. The book does a great job of contextualizing the Soviet role in the Pacific theatre and parsing over the positions of the various Japanese figures who mattered in the final decision. He manages to integrate aspects of Alperovitz's focus (e.g. US bomb strategy vis a vis the Soviet Union) without falling into the "conspiracy" traps. There are certainly historians who still think that the atomic bombs ended the war (and would have, without the Soviet intervention), but that has become a much more "hard-line" position. There are also historians who think that the atomic bombs had nothing to do with the end of the war, that it was only the Soviet intervention (kind of an extreme version of Hasegawa's argument), but that is a harder position to make compelling than the "mixed" one (because of the timing overlaps). The arguments against Hasegawa are that he does cherry-pick a bit, and I might add that he does a lot of the "perhaps" game whenever he is making a guess at what someone thought, without indicating as clearly as I would like about where he's diverging from an actual source. (I find this is a very common thing in diplomatic history, though, so I am not making this an attack on Hasegawa's character. If it were up to me — and as I argue in an article I am still trying to get out the door — historians of this subject should make it painfully clear when they are making a leaping interpretation, as opposed to reporting some kind of source evidence. The history can't be written without the interpretations, and there is no such thing as a "raw fact," but we should make the epistemological character of our various statements more clear when writing this sort of thing.)

J. Samuel Walker has written a number of articles summarizing the historiography of the bombing decision, the most recent in 2005, and he has suggested (though not without some pushback) that a "consensus view" has more or less emerged among scholars of the bombing which reflects the best parts of the Alperovitz and Hasegawa arguments (along with the more "orthodox" arguments) but jettisons the parts that might feel like a leap too far. I think this is more or less correct though there are still plenty of historians who take "hard" positions one way or the other (it might be uncharitable to suggest that many of them are still fighting ideological battles of decades' past, or have present-day political beliefs wrapped up in their views on this, but it might be true). Walker's essays are worth tracking down if you are curious about the ins and outs of the various positions on this:

• J. Samuel Walker, "The Decision to Use the Bomb: A Historiographical Update," Diplomatic History 14, no. 1 (January 1990), 97-114.

• J. Samuel Walker, "Recent Literature on Truman's Atomic Bomb Decision: A Search for Middle Ground," Diplomatic History 29, no. 2 (2005), 311-334.

Walker is the former official historian of the Nuclear Regulatory Commission, and writes very solid, careful histories that are not afraid to make broad conclusions but generally aren't about second-guessing the past or making a big, controversial splash. I have reviewed several of his books at this point (he has a very nice one on Three Mile Island, another on US nuclear waste disposal policies, and a monograph on Truman and the bomb) and respect him a lot. In my experience of knowing him professionally and communicating with him over e-mail, he is also willing to be persuaded with evidence, which is more than I can say about a lot of participants (on all sides) in this particular debate. He is a very "balanced" sort of reviewer of these things, acknowledging the positives and negatives of each source he approaches.

A brief sort of methodological conclusion: If you subscribe to the historical approach that people in the past couldn't predict the future, that they were complex human beings acting under a lot of different motives and influences, and that recovery of exact reasons for doing something complex is pretty much impossible, then it is quite possible to integrate a lot of different arguments into one view of things without subscribing to any one of them completely. If, on the other hand, you like your heroes heroic, your villains villainous, and like a smooth, "rational" narrative for your history, you are going to find it harder to do that sort of thing and have to take stronger accept/reject positions. You can tell by my condescension to the latter position that it is not the one I hold, but I am a squirrelly historian of science and we tend to prefer messy narratives (the diplomatic and political historians often like cleaner ones).