Since there never was a declaration of war and American military advisers were present very early in Vietnam, the commencement date of the Vietnam War is a matter for debate. Some view December 1956 when Hanoi authorized a low-level insurgency in South Vietnam as the start date, while some consider 26 September 1959 when the first battle between the North communist and South Vietnamese forces occurred. In 1998, after an extensive review by the DoD, November 1, 1955 is now officially considered by the America as the start date of the Vietnam War (or Vietnam conflict). That was the day when the Vietnamese Military Assistance Advisory Group (MAAG) was created from the Indochina MAAG, which had been established since 1950 under President Harry Truman.
Refer to when was the Vietnam war? as well.