Soviets and Japanese forces fought twice during World War Two.

Once at Nomohan/Khalkhin Gol in Mongolia in 1939 and once in Manchuria, Korea and the Kuril Islands in August of 1945. Both engagements were handily won by the Soviet forces.

Nomohan was an escalation of a border dispute between Japanese occupied Manchuria and Soviet dominated Mongolia. Both sides imagined that a strong display of force on the border would prevent the other side from attempting a military incursion. This policy backfired and escalated into a multi-division clash (approximately 30,000 Japanese and 60,000 Soviets) in which the Japanese initial invasion was blunted in July 1939 and then a Soviet mechanized counterattack in August routed the Japanese forces with a double envelopment. Soviet forces won because they outnumbered the Japanese, and their military doctrine emphasizing motorized logistics, coordination with the air, and the combination of firepower and mobility proved superior. Soviet leadership in Georgy Zhukov, who later became famous fighting the Germans in 1941-45, was much superior to the Japanese Army's confused leadership, in which the IJA high command and the Kwantung army command in Manchuria were often at cross-purposes.

After Khalkhin Gol, both sides negotiated a peace which became the Soviet-Japanese Neutrality pact and settled down to a watchful period on the border while they dealt with conflicts elsewhere. The Soviets prepared for the clash they knew would come with the Germans eventually, The Japanese tried to bring their invasion of China to a successful conclusion.

The watchful peace lasted until 1945. The Soviets no longer needed a peace in the Far East after defeating Germany and they were not interested in mediating for the Japanese attempts to end the war without an unconditional surrender. Accordingly, the Soviets begin to build up their Eastern forces with battle-hardened troops from the war against Germany. Japan's Kwantung army in Manchuria was hollowed out by the war in China and the Pacific, and they were late in their decision to redeploy against the Soviets and were confused by Soviet deception efforts. When the Soviet declaration of war and attack came on August 8th, 1945, the Kwantung army was overwhelmed. To the end, control between Japanese high command and the Kwantung army was poor, with many Japanese units fighting well after the surrender of the home Islands, but by August 24th the Soviets were in Pyongyang after acquiring control over Manchuria, Northern Korea, the Kuril islands and South Sakhalin island.