Stalin's defeat of the Kwantung Army in Manchuria is what finally convinced them there was no way forward. That the Soviet Union actually defeated the Japanese is obviously not a popular opinion in the west.

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Even the second atomic bomb had not dissuaded them from continuing the war. But when reports from the Kwantung Army began to arrive, reporting significant Soviet penetration in Manchuria and the situation as “obscure,” objections to surrender were far less convincing.

The Soviets’ Manchurian Campaign, August Storm, destroyed the last vestige of Japanese military power outside Japan, and put the final nail in the coffin of those Japanese militarists who, even after suffering two atomic attacks, intended to continue the war to the death.

https://warfarehistorynetwork.com/article/the-soviet-invasio...