[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.