After vietnam was divided at a peace conference in geneva,

The US supported the Bao Dai regime in South Vietnam. This, and later support of the Ngo Dinh Diem Regime, would be what  dragged us into the Vietnam Conflict. The Vietminh in the North, with the backing of China and the USSR, had hoped to regain control of the South and spread Communism. Our support of the South along with mission to contain, or eliminate, the Communism meant we could not walk away and had to help.