Prior to World War II, China participated in the Civil War. Nationalists (led by Chain Kai-shek) fought to keep control of the country from the Communists (led by Mao Zedong). When Japan invaded China in July 1937, both sides in the civil war agreed to a ceasefire.
However, after the end of World War II, fighting resumed. From 1945-1949, Mao's forces were supported by common Chinese citizens. Communism was gaining momentum in one of the world's largest nations