I think it was the atomic bomb and here’s why. Further down from this posting is a post by EternalVigilance from Truman’s diary-personal papers for this day that says:
“At 10:15 had the scientists, Bush and Conant and Geo. L. Harrison, Gen. Groves, Secs. State, War and Navy in to discuss the Atomic Bomb and how much could be published about it. A very interesting meeting. Ordered a press release for Sunday, covering its main features because so many fake scientists were telling crazy tales about it.”
So even among the United States, there were “fake scientists” telling crazy tales about the bombs.
Think how it was in Japan. There was no one there, really, who understood this weapon as the Americans do, and even among the Americans, a lot of BS was being discussed, said, speculated. Kind of like our news media is now in the first few hours of a breaking story.
So imagine how it is for Hirohito at this point. I’m sure he has his own personal scientists and friends telling him all kinds of stories. He is a marine biologist. Well, I’m not sure he took that role on until after the war, but let’s assume he’s very interested in science, and knows scientists.
Probably from his perspective this terrible weapon is going to kill A LOT of Japanese innocents, and it will be his reign that either must allow it, or, by ending the war now, stop it. And it is apparent to him that the government cannot make up its mind—they offer him a chance, and he takes it.
For the other “pro-surrender” men in the government, I think this is also on their minds. The military men want to fight it out to the death. Their focus is on honor, not the life of the little people. Well, maybe, but they think the people must fight to the end, too, just as they are. So the people aren’t being sacrificed—they themselves are also.
But the Emperor and Suzuki don’t really care about the Soviets. Sure, it’s a calamity, but it’s not here, right now, at home. Atomics bombs ARE.
But there's something else here. Halsey is famous for saying that by the end of the war the only place Japanese would be spoken was in hell. What he didn't understand is that virtually all Japanese would have been quite content with that. There was a significant portion of the leadership that expected an Allied invasion that would have killed off all of the Japanese, who then would have become kami or Japanese spirits, rising up and causing the invaders and occupiers to flee the sacred soil, so that it could once again be filled, not with living Japanese, but with the ancestral spirits of dead Japanese.
There were a few. Some were trained in the west. Japan had its own nuke program but not the resources to get it done