When the Korean War started in 1950, Stalin invited china to enter on the side of communist North Korea. At first, Mao hesitated, but then decided to join. this would prove to the Americans that China was capable of acting without their help. It would also show the world how capable China was militarily. It also gave the Americans more fear about the spread of communism, especially since geopolitically, Korea was almost surrounded by communist countries (including Soviet Union and China).