This is a popular re-interpretation of events among many Japanese historians and at least a few Russian historians that I've perused. My personal feeling is that it is a way to downplay the impact of the nuclear bombings of Hiroshima and Nagasaki, and by proxy make those attacks seem pointless and cruel.

The general gist of the argument is that Japan, even after the bombing of Hiroshima, was willing to fight it out. They still were fighting in China, and had strong forces in the form of the Kwangtung Army, a political force unto itself. This force had been one of the primary originators of the war with China, independent of the government in Tokyo as well. The Japanese had fought the Russians in a similar geographic area in 1904/05 and been sucessful with primarily defensive strategies, so there was some confidence there. Then, the Red Army invades Manchuria, and this seemingly strong force was rolled over with ease, and the Kwangtung Army's northern flank is basically shattered in 24 hours. By August 13 when the decision to surrender was being hotly debated, the fate of Manchuria and Korea were pretty clear and the hopes of the strength of the Kwangtung Army were pretty much obliterated. It was this loss (and the hope that getting peace now might preserve some remnants of the Empire) that motivated the Emperor's speech on August 15.

My biggest problem with this argument, however, is that the Soviet Union only posed a risk to forces engaged in combat outside of Japan. The Soviets were incapable of amphibious operations; their few attempts at them were near disasters and were only on a small scale. Thus, the home islands were safe from Soviet land power. The government in Tokyo was also aware that the Kwangtung Army was not as strong as it had been thanks to nearly a decade of engagement. With this in mind, the idea that the Soviet advance and victory in Manchuria was a shocking fact that suddenly made the Emperor surrender is a bit far-fetched to me. Let's think rationally here; what would you fear more, the obliteration of your home cities by air by super weapons, or the invasion of a colony across the sea by a force with no capability to attack you at home directly? The idea that the nuclear weapons were less important than the Soviet invasion pushes the realm of credibility for this reason, at least in my opinion. It demands that one believe the Emperor was more concerned with the fate of colonial holdings in Formosa and Korea than he was with the fate of the cities and people of Japan.

Now, all that said, I am sure that it did at least have some impact. The Japanese could have lied to themselves and thought that having the USSR remain neutral might give them a bit more of a bargaining chip, but what happens if the USSR doesn't invade, and the US drops another bomb, this time on Tokyo? Do you think the Emperor still puts off surrender? I don't.