Two Words. Vietnam Quagmire. I was fortunate to study the Vietnam War in the 70s during my time at Harvard. The United States entered into Vietnam with a simplistic and mostly ignorant view of Vietnam and Communist China. The argument for doubling down on fighting on behalf of South Vietnam was that if South Vietnam fell, Communism would triumph. Americans and their leaders are ethnocentric. They are ignorant of most other lands and cultures. That was true in the 70s. That is still true about Americans and their leaders vis a vis the Mideast in general and Syria, in particular, today.
In the 70s there was no monolithic Communism. Chinese Communism was different from Russian Communism. Vietnam was not Communist China. The Vietnam war was a civil war between the Vietnamese.