Did the Soviet Union have a large impact on the surrender of Japan even though they declared war on Japan less than a month earlier?

That's not exactly true, though. To illustrate by example, Kōichi Kido recorded that on the morning of 9 August he was explicitly told by Hirohito that: "The Soviet Union declared war against us... because of this it is necessary to study and decide on the termination of war.

Great. Why didn't he mention that in the speech he gave to the Japanese people?

I'm not sure what you mean by this. The Japanese government was informed about the Hiroshima bomb on the evening of 6 August and was discussed at cabinet on 7 August. Whatever about official blustering or need for investigation, it was clear to all relevant parties (including the hawk Anami) that nuclear weapons had been used.

Except to the people of Japan. Hiroshima was blamed on an earthquake.

gain, none of this is to minimise the impact of the atomic bombs. There's always the risk that in trying to unpick the threads of those frantic few days we introduce distinctions that were never there in the minds of decision-makers. But that doesn't mean that we should ignore evidence that doesn't fit with our preferred narrative.

What evidence are you referring to? My reason for not finding the narrative that the USSR invasion was the primary reason for Japan's surrender revolves around the knowledge that the Soviet Union posed no threat to the way of life in Japan in the way that the American forces did. Understanding the logistical impossibility of a Soviet Invasion combined with the promise and demonstration of nuclear annhiliation by the Americans, which does anyone honestly believe, knowing all the conflicting reports and first hand accounts that come from sanitized reports after the war?

The fact is that Hasegawa wants the Soviet Union to be the reason for the surrender, because that means the bombs can be evil pointless wastes of life rather than a prime reason millions of people were spared the horrors of an invasion. The problem is that this relies on us believing that the Japanese thought that an invasion of Korea was more important than their cities being turned into a nuclear crater. I mean, I don't consider them that insane, but your mileage may vary.

/r/AskHistorians Thread