Eisenhower doctrine. If a country was under the threat of communism and wanted help, we’d help. NOT ALL VIETNAMESE WANTED COMMUNISM!! That’s a very important part. Many, many Vietnamese were against communism. USA teamed up with south (or maybe north) Vietnam to fight out a bunch of communists who were taking over Vietnamese government, pushed by China and the USSR. In the end it wasn’t about Vietnam, it was more of the Cold War principle of capitalism v communism. When it came to countries, it wasn’t much of USA v Vietnam, nor Vietnam and USA v Vietnam. It was yet another show of strength between USA and Soviets, with the Chinese playing a small role assisting the communists in Vietnam, by letting them hide out in China to avoid American attacks, and by funding there army. In the end, it was really a way of America attacking USSR, or maybe the other way around, without actually attacking the USSR. I believe Korea was also very similar to this.