[bexsella]

There's a strong argument for the fact that Japan was effectively on its last legs by the time the bombs dropped. Indeed, the fire bombings that preceded dealt far more damage to Japan than both bombs, in real terms. Nuclear intervention wasn't a requirement for ending the war, the war was effectively over, and a sustained bombing campaign could have seen the end without the atomic bomb.

Now, the argument I find the most interesting in the case for using the bomb is public, and national and international political opinion. It is arguable that Truman saw very little options other than use the bomb at the time. They had spent millions on the project, politicians knew that something was being developed, and the Cold War was already on the horizon. Further if not used, and the existence of the bomb post war would come to light for the public, there might well be questions as to why it wasn't dropped, and, exactly the line you used, if it could have ended the war early.

Ultimately, I think a combination of international clout spun into national pride in total victory sealed the decision (although this conclusion was probably made by FDR before him as well). I will say, that the post-Hiroshima speech by Truman is also one of the most powerful speeches by a US president ever made.

> It is a harnessing of the basic power of the universe. The force from which the sun draws its power has been loosed against those who brought war to the Far East.

[AbrahamParangi]

This is a material argument that is totally undercut by the fact that the bomb gave japan an excuse to surrender that they needed and otherwise would not have had. The bomb was directly referenced in Hirohito's surrender speech:

Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.

It gave them an excuse to save face.

[howbadisthat]

Correct. The incentives for individuals rarely align with the incentives of polities (and it can be argued those entities only exist in history books).

The bombs and their awesome power served to align the incentives of individuals in decision making positions with the incentives ascribed to the polity named Japan to issue surrender.

[roenxi]

> Indeed, the fire bombings that preceded dealt far more damage to Japan than both bombs, in real terms.

That is a better anti-fire-bomb argument than an anti-nuclear one. But I suspect the feeling in Japanese command would have been something like "agh, we're becoming a weapons testing range and we don't like that" and that surely contributed to a quick surrnder.

[ioblomov]

American Prometheus revealed the most compelling reason for using both bombs I’ve seen to date:

  Truman had extracted a promise from Stalin that the Soviet Union would declare war on Japan by August 15—an event that he and many of his military planners thought would be decisive. “He’ll [Stalin] be in the Jap war on August 15,” Truman wrote in his diary on July 17. “Fini Japs when that comes about.”

Even more than avoiding a bloody island-hopping invasion of Japan, Truman wanted to avoid having to divide East Asia with the USSR as they had done Europe. The Soviets declared war on Japan and invaded Manchuria on August 7, one day after Hiroshima and two days before Nagasaki. By August 15, the Japanese had surrendered, largely limiting the Soviet sphere of influence to mainland Asia.

[himinlomax]

The German had just shown that they would fight to the last minute, enrolling cripples, children and old men in the Volkssturm. It was perfectly rational and reasonable for the US leadership to believe the Japanese were going to be just as stubborn and fanatical. In fact it would have been simply naive for them to believe otherwise.