[mpyne]

Manchuria was across the Sea of Japan from the Japanese home islands. The Soviet invasion of Manchuria did nothing substantive to directly threaten Japan.

Japanese strategists wanted to be in Manchuria and Korea because of their proximity to imperial Japan, but that was also why they had invaded the Philippines (to defend seaborne lines of communication to their oil supplies in Borneo).

By the time the Soviets invaded, the Japanese had been ejected from much of their outlying empire, yet they had not surrendered, because Japan itself had the capability to fight.

The Soviet invasion of Manchuria did not change that, an invasion of Japan proper could only have come with involvement from the rest of the allies, including the sealift used for the invasions of Normandy and southern France.