There is also a faction of historians that claimed Japan surrendered when they did due to the Soviet Union entering the war against Japan.
Everything I learned about WWII history reinforces that fact. The atomic bomb was dropped at the same time as the USSR was set to invade the home islands of Japan. The USA dropped the bombs as a warning to the Soviets not to invade.
At the same time, Japanese officials ignored the death tally from the Hiroshima bomb and stated they would continue the war no matter how many bombs were dropped on Japanese cities. Only the threat of falling under Soviet occupation, did they relent in favor of American occupation.
For starters, the USSR was only as effective as it was in WWII was Lend-Lease from the other Allied nations. They couldn't even have stopped the Nazis at Stalingrad without it. If the US and the UK stopped supplying them because they were engaging in an invasion of Japan, that invasion would have collapsed.
At war's end the Soviets Pacific fleet was a brown-water-only operation, ZERO blue water naval assets. And the Japanese already had handed their ass to them once in a naval battle earlier in the 20th Century.
So it's not as certain as all that that a Russian invasion even would have reached Japan.
Plus, the Japanese Army still was relatively intact because all the previous land battles had involved primarily naval infantry, not army. They were guarding the homeland and spoiling for a fight.
Plus, as the atomic bombings took place, the Japanese were training children as young as pre-schoolers to be ammunition bearers to help repulse the invasion.
The Japanese clearly were gearing up for a fight to the last man, and I don't think they would have cared whether the star on the invader's caps was silver or red.
The Japanese only stopped fighting because the Emperor took the extraordinary measure of telling them to in a radio address. Absent this, it's virtually certain the military commanders would have ordered their forces to keep fighting and tens of thousands of more Japanese would have died. Hirohito knew this and I think he ordered them to stop fighting out of compassion for his people, not out of fear of the Russians.
The only certainy is that we nuked them and they surrendered. Everything else is hindsight and speculation.