Truman and Churchill had realized that Stalin and the Soviet Union wanted to spread communism to every corner of the world. In 1949, it seemed like they were on their way.
Remember, Japan had invaded China before the start of WWII. At the time, there was a civil war within China between democratic Chiang Kai-shek and communist Mao Zedong, but once Japan invaded China, Chiang and Mao worked together to defeat the Japanese (the enemy of my enemy is my friend). Once WWII ended though, so did the partnership between these two political rivals and the civil war started up again.
The USSR supported Mao and the US sent several billions of dollars in aid to Chiang. America was worried that a victory by Mao and the communists would lead to a communist superpower spanning most of Asia.
Chiang’s people were not up to the task of defeating Mao. They didn’t want to fight and, even though the masses in China were faced with starvation, corrupt officials diverted US aid dollars into their own pockets. Now remember, communism thrives in places with there is economic chaos and where people in desperate situations might turn away from capitalism and demand communism. Mao promised to feed the people and was able to win their support.
In 1948, Mao’s forces dominated the war so Chiang appealed to America for military help. America refused to help the corrupt leader, forcing Chiang to flee to Taiwan. Mao and the communists were then able to control the world’s most populous country and they renamed it the People’s Republic of China.