http://www.japanfocus.org/-Tsuyoshi-Hasegawa/2501
It was the fact that the Japanese had not expected the Soviet attack, and did not feel they could hold off both the Americans and Soviets simultaneously, that ended the war. Especially since prior to the Soviet invasion Japan had thought the Soviets were going to mediate a peace settlement between Japan and the Allies.
"Political Calculations
Soviet entry into the war was indeed a shock to the Japanese ruling elite, both civilian and military alike. Politically and diplomatically, it dashed any hope of ending the war through Soviet mediation. But Soviet entry meant more than merely precluding the option of Soviet mediation for peace. Here, we must consider the political calculations and psychological factors apparent in dealing with Japan’s two enemies. Before the invasion of Manchuria, the Soviet Union had been Japan’s best hope for peace, while the Japanese ruling elite felt bitter resentment toward the United States, which had demanded unconditional surrender. After August 9, this relationship was reversed. The small opening that the United States had intentionally left ajar in the Potsdam terms, which Japanese foreign ministry officials had astutely noticed as soon as the Potsdam Proclamation was issued, suddenly looked inviting, providing the only room in which the Japanese could maneuver. They concluded that suing for peace with the United States would confer a better chance of preserving the imperial house, if not the kokutai as it was envisaged by ultranationalists. No sooner had the marriage of convenience uniting right-wing Japan and the communist Soviet Union broken down than the Japanese ruling elite’s fear of communism sweeping away the emperor system was reawakened. To preserve the imperial house, it would be better to surrender before the USSR was able to dictate terms. On August 13, rejecting Anami’s request that the decision to accept U.S. Secretary of State James Byrnes’s counteroffer (the “Byrnes note”), which rejected Japan’s conditional acceptance of the Potsdal terms, be postponed, Suzuki explained: “If we miss today, the Soviet Union would take not only Manchuria, Korea, [and] Karafuto [Sakhalin Island], but also Hokkaido. This would destroy the foundation of Japan. We must end the war when we can deal with the United States.”[68] Furthermore, when Shigemitsu had a crucial meeting with Kido on the afternoon of August 9 at Prince Konoe’s request, which eventually led to Kido’s meeting with Hirohito that persuaded the emperor to accept the “sacred decision” scenario, Shigemitsu stressed the negative effect of further Soviet expansion on the fate of the imperial household.[69]"