There are a number of events that led to the conflict in Vietnam (I assume you are talking about the Vietnam War in which the US was involved.). Among the most important are:
- French colonization of Vietnam. This makes Vietnam want freedom.
- World War II. France is driven out of Vietnam by Japan.
- Declaration of Vietnam's independence. This was done by Ho Chi Minh at the end of the war.
- French war against the Vietminh. The Vietminh fought to remain independent while the French fought to keep control of their colony.
- Geneva Accords of 1954. This split Vietnam into two countries, one controlled by the Vietminh, one controlled by a regime friendly to the US.