I was reading some comments on /r/HistoricalWhatIf and I had not really read much about any soviet invasion. Was the Soviet threat that important that it actually forced the surrender?
Here is the topic I was reading: http://www.reddit.com/r/HistoricalWhatIf/comments/25vy0f/what_if_japan_didnt_surrender_after_the_atomic/
And the comment: http://www.reddit.com/r/HistoricalWhatIf/comments/25vy0f/what_if_japan_didnt_surrender_after_the_atomic/chlfz9g
This is the thesis of the scholar Tsuyoshi Hasegawa in his book Racing the Enemy. There was a thread here very recently on historians' perspectives on the book and the thesis.