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, 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.