So the same old question why did America go to war with Vietnam... Allow me to explain. In the 1950s and the 1960s, Communism was an extremely popular way to govern people in a developing region such as Vietnam since it seems. Then in America, 1949 is the year of the fear of Communism in America. The 1950s were under the scare of the Communism taking over America, aptly named the Red Scare, led by an anti-communist Senator Joseph McCarthy, who ironically shared a name with Stalin. Anyway, McCarthy went on a witch hunt against the Communist that was rising in America, creating a sense of distrust and hysteria. After WWII, Eastern Europe and China had fallen under Communism and this trend reached over to Latin America, Africa and part of Asia.
There is a need to contain Communism and it's up to the US to do so after feeling that they are losing in the Cold War. So the US sent some people to help fight the French fight the Communist Northern Vietnam. The French were in a battle to maintain colonial power and not as concerned about the Communism part of the war. France pulled out soon after in 1954 since they don’t deem the blood sacrifice worth the colony. The US continues to send in their army and war support until 1975, all in order to maintain that Democracy. The capital of Saigon fell to the North Vietnamese soon after they left. So pride and scare mongering in the US brought them to the war and somehow accomplished nothing.