The main reason the Chinese Communists gained control of mainland China in 1949 was due to a combination of factors, including their military strength, popular support, and weaknesses in the Nationalist government. The Nationalist government, led by Chiang Kai-shek, was weakened by corruption, economic instability, and internal divisions. Meanwhile, the Communists, led by Mao Zedong, had built a strong military and were able to mobilize popular support through their land reform policies and promises of social and economic equality. Additionally, the Communists were able to exploit the weaknesses of the Nationalist government, which was divided and lacked a clear strategy for dealing with the Communist threat. Ultimately, these factors allowed the Communists to gain control of mainland China and establish the People's Republic of China in 1949.