Why did the United States support Chiang's Chinese government - in - exile?

Due to the fact that the Communists were aligned with the Soviet Union, which was Americas nemesis at the time.

Explanation:

The Chinease Civil war, was one of the many wars that took place early in the Cold War. The Cold War was a battle between Capitalism and Communism. So naturally the United States supported the nationalists who opposed Communism, while the Soviets supported the Communists.