Japan finally made up its mind to surrender when they heard that the USSR had invaded Manchuria. The Japanese army there crumbled like a rotting house of cards.
The paramount concern of the Japanese was to preserve the Emperor system which was virtually godlike.
They were concerned that the USSR would abolish the Emperor if they invaded whereas the yanks would allow it to remain.
Sure the bomb helped but stalin was the last straw. They rushed to surrender before the USSR got more than a few islands.
