The Russians had a previous stake and interest in Manchuria before the world wars centered around their naval and military base named Port Arthur that was in Manchuria. They lost Port Arthur in the Russo-Japanese War after it was besieged for 5 months and subsequently the Russians had to embarrassingly abandon their hold over Manchuria to the Japanese as well as their designs for further expansion in the East. In 1938 Japanese and Russian forces clashed in a border dispute that would prove costly for both sides, leading them to sign a neutrality pact in 1941. The Russians were initially delighted to not have to fight a two front war against Germany and Japan and generally respected the neutrality pact they signed with Japan. I am not certain, but Its safe to guess that as the tide turned against Germany in the West, Stalin and his generals began to draw up plans for expansion in the east and to get revenge for Russia's humiliation in the Russo-Japanese War, also a warm water port in the East would be a nice addition to Stalin's territory.