The Soviet–Japanese War was part of the Second World War fought in China. Japan had set up a puppet state of Munchukuo in northern China; threatening Soviet's interests in the area. The Soviet invaded and took over control of Manchukuo.
Japan surrendered but the Soviet stayed in northern China and supported the communist movement in that country. China eventually became a communist country and fell under Soviet's influence.
US and western allies were very concerned about the development. The Chinese government they supported failed to the communist army. They ended up starting up NATO as a counter to the Warsaw Pact led by the Soviets. That resulted in the Cold War.