Mongolia basically acted as a part of the Soviet Union. The Battle of Khalkhyn Gol in eastern Mongolia (modern day Dornod Province) was the decisive conflict in the Soviet-Japanese border wars, leading to Soviet victory, the rise of Georgy Zhukov, and a Soviet-Japanese neutrality pact that gave the USSR free reign to focus their manpower on the west, rather than also worry about the east as well. Mongolia also provided material support to the USSR, such as clothing, and funding for a tank brigade (one tank that survived until Berlin is preserved as a memorial in Ulaanbaatar) and air squadron. Lastly, Mongolia was a launching point for the invasion of Manchuria.

More importantly for Mongolia, though, WWII was the impetus for its recognition as an independent nation after Stalin forced the issue during the Yalta Conference.