The Soviet Union had fought with Japan over the Manchurian border. An uneasy truce had existed since September 1939, when, in 1941, both nations realized that the coming war in Europe would mean that they would have to protect their interests elsewhere. Still, both nations had significant forces in Northern China in case the other attacked.
When the Germans were at the suburbs of Moscow in the Winter of 1941, Soviet Premier Josef Stalin’s spy in Tokyo was pressured to report if any attacks on the Russian-Manchurian border were forthcoming before he moved crack Siberian winter troops to the West for a major offensive.
The Soviets interned both Americans and Japanese when they landed in Soviet-controlled territory. While Stalin professed neutrality, he was intent on entering the Pacific War as soon as Hitler was subdued. Massive losses in the West prevented him for doing that until 1945.
Two days after the Hiroshima atomic bomb dropped, Stalin invaded Manchuria. The once-powerful Kwantung Army was brushed aside as major areas lost in the Russo-Japanese War were retaken. Soviet Forces also occupied Sakhalin Island, north of Japan.
Korea was split in two when Allied and Soviet Forces agreed in September 1945 to use the 38th Parallel as their demarcation line. This would lead to war in 1950 between the Korean governments, sponsored by their respective opponents in a new Cold War.
The Soviet Union suffered grievous casualties in the Second World War, which they called the Great Patriotic War. While the final tally may never be known, it is estimated that 20 million soldiers and civilians died on all fronts. This was a major contribution to their Cold War policies; they desperately wanted to ensure that such losses would not happen again.