[Matticus_Rex]

If arguing that the atomic bombs were not decisive is conspiratorial, then a lot of respected, mainstream subject matter experts are conspiracy theorists.

EDIT: For some reason I can't reply to the child comment, so I'll address it here. I'm not talking about pundits. I'm talking about historians and political scientists, arguing compellingly from evidence. This case is drastically overstated, and mainstream, pro-military sources have been on both sides of the debate since the beginning (including the US Strategic Bombing Survey, which affirmed the usefulness of most other US bombing plans, and yet found that "Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated."). The anti-bomb side also includes such obscure (/s) academic figures as Robert Pape, Gar Alperovitz, and Stephen Peter Rosen (certainly no anti-war/anti-government figure), as well as military contemporaries such as Douglas MacArthur, Chester Nimitz, William Leahy, William Halsey, Curtis LeMay, and Dwight Eisenhower.

The suggestion that this position is fringe or conspiracy-esque is simply not tenable.

[mezentius]

I'll put it this way: in popular culture, the controversy over the use of atomic weapons by America goes beyond purely what can be proved, and has instead become a proxy war for other contemporary political issues. (This is not a new phenomenon, and the history of Hiroshima discourse is a fascinating subject in itself.) You will no doubt find some 'mainstream' pundits—even NY Times op-ed authors—willing to carry water for the notion that the use of the atomic bomb was unnecessary, and furthermore known to be unnecessary by the principals at the time.

The evidence, however, does not support it.

EDIT: To respond to your edit, statements of opinion made by American military figures after the war are not considered convincing. A lot of our war scholarship used to be based on statements made by participants after the war, but we have become increasingly skeptical of this approach, for good reason. For example, our narrative about the German invasion of the Soviet Union used to be largely based on the postwar testimonies of Nazi generals. However new examination of primary sources—i.e. records, receipts, transcripts of meetings (some of which were previously buried in Soviet archives)—have shown that their testimonies were often extremely inaccurate, misleading, self-serving, or else calculated to produce a political effect in a Cold War context.

For the same reason, what figures like Halsey—and particularly Douglas MacArthur, an unreliable narrator worthy of "Pale Fire"—have to say about whether the atomic bombs were decisive should provoke skepticism at best. And this is not just because they were political figures and seasoned inter-service warriors, jockeying for position in a postwar hierarchy, but because they weren't there. The best way to determine whether the bomb was decisive is to examine primary sources, both in the American and Japanese context, some of which have been posted elsewhere in this thread.

By examining those sources you cannot really make the case that the bomb was not decisive, nor can you claim that the decision to use the bomb was primarily motivated by factors other than the urgent need to end the war quickly. While some people may indeed make that argument, they are relying on questionable postwar statements of opinion, with no convincing evidence to support them.