According to the Truman doctrine: "it must be the policy of the United States to support free people who are resisting attempted subjugation by armed minorities or by outside pressures". The doctrine allows America to intervene wherever communism is spreading. The US was already fearing the Soviet union during the end of WWII, and thus created this doctrine in order to legitimize their intervention in Vietnam.

So the basic point of the Vietnam war was to stop the spread of communism in southeast Asia.