Well, when the Japanese kicked out the Chinese, they pretty much ruined the Nationalist forces, allowing Mao's Communists an easy victory over the Nationalists in 1948. When that happened, the Soviets gave Mongolia back, basically saying, "Hey, you're a Communist country now and we are too! We should be friends! Here's an old territory of yours as an enticement!"
Well, when the Japanese kicked out the Chinese, they pretty much ruined the Nationalist forces, allowing Mao's Communists an easy victory over the Nationalists in 1948. When that happened, the Soviets gave Mongolia back, basically saying, "Hey, you're a Communist country now and we are too! We should be friends! Here's an old territory of yours as an enticement!"