Just after we hit Japan with A-bombs, Russia declared war on Japan and moved in quickly, grabbing Manchuria and part of Korea in one month. From this news account it sounds like Stalin was being opportunistic as usual. It seems a bit out of character because Russia had been purely defensive up till then, purely focused on defeating the Krauts and driving them out of Russia.
A Wiki article gives a more complete picture. This move was part of the 1943 Yalta agreement. Stalin was supposed to join us against Japan as soon as Germany was safely down, and the date of August 1945 was determined in advance by the agreement and the events. Russia was, of course, well aware of our A-bomb program since the physicists at Los Alamos were Soviet spies. Because Stalin understood the potential, he didn't think the A-bomb would be enough to stop Japan.
And in fact it wasn't the prime determiner. Japan's government surrendered after the A-bomb and the equally deadly firebombing of Tokyo, but the Jap military was still in fighting mood. It was the surprise Soviet second front that finally dispirited the Jap army.
This series of events doesn't fit the crazed wacked-out modern narrative that we suddenly and illegally attacked the poor innocent peaceful long-suffering Zen Buddhist Japanese Peoples in 1945 without any provocation, so it has been in the memory hole.
Russia kept Sakhalin and the Kuril Islands but wasn't interested in occupying Manchuria, and officially handed it back to China in 1955. Korea was set up as a divided occupation zone like Germany, but the North quickly became a Chinese tool even while nominally Soviet. And of course it remains a Chinese tool.
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