Steve, the Japanese were facing the Russians coming down, and would across soon. There were choices on who they would surrender to - the Russian, or to us? They knew the Emperor would not be accepted by the Russians, and it would be better overall to surrender to us. Some of the hard liners wanted to fight. They chose to surrender to us and a big influence was to avoid the negative impacts of the Russians.

That might have been a small factor, but not a major one.

Russia didn’t declare war on Japan until 2 days after Hiroshima and one day before Nagasaki. There was never an invasion threat posed by Russia to Japan; their entire army, minus a few garrison troops was still in Europe, consolidating it’s stranglehold over Eastern Europe and maintaining a threat faced toward the western allies.

Their only reason for declaring war on Japan was to position themselves as a belligerent for negotiations AFTER the war- divvying up the Japanese Empire.

And in that, they successfully obtained the Kuril Islands north of Japan- which had been contested territory between the two nations for years.

It was the bone thrown to Stalin by Truman that rewarded Stalin for declaring war on Japan.

But the USSR was exhausted in 1945 and never had the sealift capacity to invade Japan, let alone the troops.