https://en.wikipedia.org/wiki/Soviet–Japanese_War_(1945)#Importance_and_consequences
"In the 'Sixty years after Hiroshima' issue of the Weekly Standard, American historian
Richard B. Frank points out that there are a number of schools of thought with varying opinions of what caused the Japanese to surrender.
"He describes what he calls the 'traditionalist' view, which asserts that the Japanese surrendered because the Americans dropped the atomic bombs.
"He goes on to summarise other points of view.
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Tsuyoshi Hasegawa's research has led him to conclude that the atomic bombings were not the principal reason for Japan's capitulation.
"He argues that Japan's leaders were impacted more by the swift and devastating Soviet victories on the mainland in the week following
Joseph Stalin's August 8 declaration of war because the Japanese strategy to protect the home islands was designed to fend off a US invasion from the South, and left virtually no spare troops to counter a Soviet threat from the North.
There were many Japanese and Russians alive in 1945 who remembered the Russo-Japanese War which had ended in 1905 with a surprising Japanese victory.
Rumors of Russian atrocities in Nazi Germany were also reaching Japanese ears. Japan wanted no part of a Soviet occupation like the one pillaging Germany. As long as they were allowed to keep their Emperor, the Japanese would have surrendered without the destruction of Hiroshima and Nagasaki.