Vietnam, as a part of Indochina, had been under French Control since 1887. After the withdrawal of French in 1954, the US intervened in the Vietnam War due to the lessons from China, Korea and domino theory, weaknesses of South Vietnam and Vietcong.
The US saw China and Korea fell as dominoes in 1949 and 1953. They were not prepared to see another falling domino. Not only this, if Vietnam fell, the countries around will be influenced in the mean time, including Cambodia, Laos, Burma, Thailand, Malaysia and even India and Indonesia. The Geneva Agreements in 1954 has divided Vietnam between 17th parallel, the North was under communist system. This was already a failure of Containment and a failure in the battle against communism. Thus the US saw more threats of Communist taking over in Asia and they were willing to do actions on it.
The unpopular and corrupted government in South Vietnam, Republic of Vietnam, was also another issue to the US.
This meant that the north Communists, Vietcong, could be welcomed by South Vietnamese and easier to take over. This was because of the crucial dictatorship under Ngo Dinh Diem and his family members. First of all, he was a Catholic in a country where Buddhists were in majority. He banned on the flying of the Buddhist flags on the Buddha's birthday and led to times of demonstrations. Some monks burn them up to show the anger and revolted. Secondly, the North has distributed lands to every peasant when Diem gave lands and wealth to Catholics and Governors. Thirdly, the National Liberation Front had been formed compose of 12 different national groups to oppose Diem's government.