The US was an ally of China—the Republic of China (ROC)—during World War II. After WWII, the Kuomintang (KMT) that ruled the ROC and the Chinese Communist Party (CCP) engaged in a bloody civil war. With the Communist victory imminent, the KMT government retreated to Taiwan. In October 1949, the People’s Republic of China (PRC) was established on the mainland.
During and after WWII, the US became disenchanted with the corrupt and inefficient KMT. As the Communist forces swept across the Chinese mainland, it appeared to be just a matter of time before Taiwan would be absorbed, and the US was prepared to abandon Taiwan. The US rediscovered Taiwan’s strategic value when the Korean War broke out in June 1950. Soon afterward, the US Seventh Fleet was dispatched to the Taiwan Strait, essentially blocking any PRC attempt to take over Taiwan by force. The US and the ROC also signed a mutual defense treaty in 1954.