It is really a no-brainer.
The war was carried against Germany, only a holding action to protect Australia and New Zealand was waged against Japan.
That was certainly no secret.
Once Germany collapsed Japan was toast and knew it.
That the Russians were about to enter the war against her was a simple recognition of Germany’s final defeat.
Japan accepted surrender because it was finally offered to it on less than unconditional terms.
Because the US and england did not desire russia in the war.
The Mikado was allowed to remain, done deal, Japan surrendered.
Another interesting tidbit is that the USSR denied the US from using Soviet land bases from which to launch any airstrikes or assault upon Japan, while the US gave volumes of material supplies and weaponry to the Soviets to fight the Germans. That’s the same Russians who had also made a pact with Germany to oppose GB earlier in the war until Hitler also attacked the USSR.