That is an often overlooked part of the equation. If I remember correctly, Japan was a week or two away from being split in two like germany.

And Japan and the Soviet Union had been at each others throats since before there was a Soviet Union and Japan thumped the tsar.

I am sure Japan did not want to surrender under a Soviet flag that was looking for 50 years of retribution.

In a strange way it was an American coup to get peace signed before Russia started stripping the place down to the bone.