[vkou]

> That it was the threat of Soviet invasion which really tipped the scales.

That's exactly what tipped the scales. The Soviet betrayal of the non-aggression pact, their march through the Inner Mongolia desert (Which Japanese planners considered impossible, given the lack of logistics infrastructure in it), their blitz through Manchuria, the complete collapse of Japan's positions in it, as well as the Soviet naval invasion of the Sakhalin and Kuril islands was what brought the war to a close. Japan could not continue the war without access to Manchuria, and was really, really not interested in getting occupied by the Soviets.