Alex WellersteinVerified account

The US, it is worth emphasizing, knew about these efforts and these concerns. They had cracked Japanese diplomatic codes well before. They incorporated discussions between the head of the foreign minister and the Japanese ambassador to the USSR into their strategy.

This is important when considering the US choices of July 1945, esp. at the Potsdam Conference. Truman was lobbied by both the Department of War and Churchill to give some guarantee as to the Emperor's safety in the Potsdam Declaration, as they saw that this was a sticking point.

Truman, following the advise instead of his Secretary of State, James Byrnes, deliberately decided not to do this. It isn't entirely clear why, but the fact that by then he felt that Japan was likely to surrender without an invasion anyway played into it.

In one of the few remarks he made about this, he emphasized that "unconditional surrender" was essentially required to offset the Japanese perfidy of Pearl Harbor — that he wanted them to grovel. For whatever that is worth.

People have argued since the 1940s over whether the insistence on unconditional surrender, as opposed to granting the one condition the Japanese for sure wanted (Emperor preservation), prolonged the war. The answer, unsurprisingly, is "we don't know." But it's useful context.

Anyway. In July 1945, Truman knew for sure (because of the Trinity test) that the US had a workable atomic bomb. He also knew that Stalin was planning to renounce his neutrality with regards to Japan, and invade by mid-August 1945, because Stalin told him this.

The Americans AND the Japanese, as an aside, both considered a Soviet invasion to be a likely tipping-point for Japanese surrender. The Japanese army knew that they lacked the forces to repel an invasion by the USSR, and that Stalin didn't care about trading blood for territory.

(I only point this out because it was only about a year ago that I learned they had actually been analyzing this scenario for months before it happened. Some accounts make the Japanese military seem quite ignorant of the possibility, but it is very clear they were not.)