The communists were able to gain control in China in 1949 because America did not provide the promised support to Chiang Kai-shek. (The Soviets were helping Mao.)
In 1950 North Korea invaded South Korea and America went to South Korea's aid. In October of 1950 China joined North Korea in the war.
McCarthy believed that Chiang Kai-shek did not receive the promised support because the U.S. State department was heavily influenced by communist spies and sympathisers (which resulted in some very bad decisions being made and millions of lives being lost).
We now know that McCarthy was right. The following puts the whole sorry story into perspective.