If you mean who the enemy was, it were the:
North Vietnamese Army
Vietcong
Khmer Rouge
and the Pathet Lao
While the "Main Enemies" Were the vietcong and the north vietnamese army.
Vietnam war
The full might of the US military minus nuclear weapons.
Stop communist aggression.
Military advisors first arrived in Vietnam in 1950. America started sending troops into Vietnam as early as 1960, but they took no military action until 1965. The United States was actively (militarily) involved in the Vietnam War from 1965 to 1975.
US Military personnel were in Vietnam from 1955 thru 1975.
US military advisors were training the ARVNs in April 1956 in Vietnam. There was no "US invasion."
SEE: Vietnam Veterans Memorial Wall, and Statistics About the Vietnam War (Recommended by the History Channel).
If your referring to the Vietnam War, the first U.S military advisors arrived in Vietnam in 1950.
Westmoreland
Lyndon Johnson authorized this and most of the military operations in Vietnam.
US Military records were not kept prior to 1968, in Vietnam. 3,747 war dogs (officially designated military working dogs during the war) are documented as having served in South Vietnam. Estimates run over 5,000 canines. 10,000 US servicemen operated as dog handlers in South Vietnam during the war: 65% were used by the USA; 26% by the USAF; 7% by the USMC; and 2% were used by the USN. 73 dog handlers and 43 K9's were killed in Vietnam.
Nearly 7,000 US military officers were killed in Vietnam.