The question of the decision making process to drop the bomb is a very commonly asked one, and there is an extensive collection of posts related to the atomic bombs and the Japanese decision to accept the Potsdam Proclamation and the end of the war in our FAQ. While in no way trying to hinder future discussion, I would like to link you to some posts by our users who have talked extensively on this issue.

• /u/ScipioAsina discusses the changes in the Japanese political landscape in the days between the dropping of the bomb on Hiroshima and the Japanese surrender here. I also recommend the comments by /u/restricteddata and /u/t-o-k-u-m-e-i later in this same thread.

• /u/t-o-k-u-m-e-i discusses the question of whether the onus of deciding the end of the war lies with the atomic bomb or Soviet entry into the war here, with a particular reference to Tsuyoshi Hasegawa's work Racing the Enemy. A now deleted user also delves into the reaction to Hasegawa's work and its wider acceptance by historians here.

• /u/RestrictedData delves into many of the common questions in his response to whether the U.S. had to nuke Japan.

• /u/RestrictedData also highlights that the very issue of using such terminology as "decision to use the bomb" is a bit overly guided by our modern ideas of nuclear possibility and do not graft well onto the historical reality of American decision making in 1945 in a blog post he made here. This post also delves into several other misconceptions that seem to permeate pop culture discussion when it comes to the use of the atomic bomb.

Fundamentally, it seems as though the thread you've linked to is arguing a few things: 1) that the U.S. could've brought about Japanese unconditional surrender by either amending their terms of surrender or waiting for the Soviet Union to invade; 2) the U.S. decision to use the bomb was driven moreso by a desire to frighten the Soviet Union than to defeat Japan, coming to the conclusion that the use of the bombs was unnecessary. My focus is admittedly much more so on Tokyo than Washington, so I will not get too far into the second, but I would take some umbrage with some of the claims made in the thread regarding internal Japanese decision making.

In particular, the twitter thread links to other threads by the same user arguing that Japan was on the verge of surrender with the implication that U.S. insistence on unconditional surrender was unnecessary. They accurately point out that Japan was eagerly seeking Soviet intercession to come to some kind of negotiated settlement with the United States and the United Kingdom. Indeed, quite accurately, Japanese leadership was well aware that they were not winning the war and were actively looking for an end to the war. However, the claim that Japanese surrender was only one condition away is quite difficult to sustain. Namely, it presupposes that there was a unity of purpose within the Japanese leadership that simply did not exist. While the Japanese were seeking an end to the war, the precise terms that the Japanese would have accepted were quite flexible. Some key members of the War Party within the Supreme War Council, like War Minister General Anami Korechika wanted more extensive concessions for Japan, including--but not limited to--preventing an occupation, retaining the Japanese military, and enabling Japan to handle prosecution of war criminals. The War Party held that Japan could force better terms from the Allies by staging one last decisive battle for the Home Islands that would inflict so many casualties that the Allies would have to accept a negotiated settlement. Similarly, the Peace Party was similarly unclear. The Peace Party was willing to set aside most of the conditions the War Party desired, but even they insisted on a condition of "preserving the kokutai", and retaining the Emperor in some form. Of course, there was not a clear consensus on quite what that meant either. Did it mean that the Emperor would simply be retained as a constitutional figurehead? Would the Emperor have to retain supreme power to "preserve the kokutai". Up until the end, there was dispute over what would be acceptable.

It is true that Truman was unwilling to bend on the issue of Japanese unconditional surrender. He quite clearly felt that the Japanese did not deserve quarter and that he would insist on unconditional surrender. There were definitely U.S. advisors who argued that an adjustment of U.S. statements to offer assurances that the Emperor could be retained might cause the Japanese to be more willing to surrender. However, these pressures were ultimately not followed on it. Notably, Secretary of State Byrnes was quite insistent that offering clemency to the Emperor would be politically unacceptable among the American public.

This in turn leads to the key question: what would happen if the U.S. had made such an offer?

Ultimately, that is a question we cannot answer. It's entirely speculative and that's not something we do on r/AskHistorians. However, the political situation in Japan was absolutely not one where a Japanese surrender was available with an easy change.

This particular interpretation/retelling of events seems to be mostly influenced by the Oliver Stone and Gar Alperovitz accounts. If you search for those names on here with relation to the atomic bombing, you will see many responses. There is some truth but also some big swerves from what most historians would sign on to. And some very large exaggerations.

As just one example, the thread asserts that there were scores of objections to the use of the bomb by scientists, policymakers, and military leaders. This is an exaggeration. There was a fairly small group of scientists who objected strenuously to the use of the bomb on cities without a warning. There was maybe one policymaker type who voiced an opinion against its use during the time. There were military leaders who well after the events happened claimed that they weren't sure it was necessary at the time, but there is no reputable evidence that they actually voiced this opinions at the time (and it is not even clear that the held them). (Why did they voice them after the fact? In part because they were afraid that the conventional military effort was being overshadowed in the postwar, and they feared deep funding cuts. That doesn't make them right or wrong, it is just the sort of context you need to understand this sort of thing, and the sort of context that this Twitter thread does not provide.)

By far the overriding sentiment of those who knew about this planning was that of course you would use a new weapon you had, and of course it might help out with the war, and that there were multiple reasons to do so. They saw very few reasons not to do so (they were not concerned with Japanese casualties, for better or ill — even the scientists who objected couched their argument less in terms of humane treatment and in terms of long-term political consequences).

That there were some objections is hardly evidence that there was no reason to use them, or that there weren't strong arguments on the other side. This is what I mean by an exaggeration — the thread author is overstating the size of the objections, and also their importance to the decision-making process. And one could ask, in any event, whether all of these opinions voiced should be taken as being equally valid — the physicists were good at making the bombs, but does that make them experts on military and political strategy? One can see why the politicians and military leaders involved did not think that their objections were deal-killers.