Which political party in China did the United States support, and why?

On October 1, 1949, Chinese Communist leader Mao Zedong declared the creation of the People’s Republic of China (PRC). The announcement ended the costly full-scale civil war between the Chinese Communist Party (CCP) and the Nationalist Party, or Kuomintang (KMT), which broke out immediately following World War II and had been preceded by on and off conflict between the two sides since the 1920’s. The creation of the PRC also completed the long process of governmental upheaval in China begun by the Chinese Revolution of 1911. The “fall” of mainland China to communism in 1949 led the United States to suspend diplomatic ties with the PRC for decades.

Communists entering Beijing in 1949.
The Chinese Communist Party, founded in 1921 in Shanghai, originally existed as a study group working within the confines of the First United Front with the Nationalist Party. Chinese Communists joined with the Nationalist Army in the Northern Expedition of 1926–27 to rid the nation of the warlords that prevented the formation of a strong central government. This collaboration lasted until the “White Terror” of 1927, when the Nationalists turned on the Communists, killing them or purging them from the party.

After the Japanese invaded Manchuria in 1931, the Government of the Republic of China (ROC) faced the triple threat of Japanese invasion, Communist uprising, and warlord insurrections. Frustrated by the focus of the Nationalist leader Chiang Kai-shek on internal threats instead of the Japanese assault, a group of generals abducted Chiang in 1937 and forced him to reconsider cooperation with the Communist army. As with the first effort at cooperation between the Nationalist government and the CCP, this Second United Front was short-lived. The Nationalists expended needed resources on containing the Communists, rather than focusing entirely on Japan, while the Communists worked to strengthen their influence in rural society.

During World War II, popular support for the Communists increased. U.S. officials in China reported a dictatorial suppression of dissent in Nationalist-controlled areas. These undemocratic polices combined with wartime corruption made the Republic of China Government vulnerable to the Communist threat. The CCP, for its part, experienced success in its early efforts at land reform and was lauded by peasants for its unflagging efforts to fight against the Japanese invaders.

The answer is crop yield.