The first US Military operation was President Reagan's "Urgent Fury" the invasion of Grenada in 1983. From then until our current ones; Iraqi Freedom & Enduring Freedom; they were quick, surgical, and efficient.
Lyndon Johnson authorized this and most of the military operations in Vietnam.
Nearly ALL operations involved US personnel.
US Citizens facing the military draft.
The NVA conducted conventional military offensive operations.
A North Vietnamese military patrol supposedly attacked a US destroyer Maddox in the Gulf of Tonkin which led to the Gulf of Tonkin Resolution.Rebecca
Other than Military Operations; since WW2, Korea & Vietnam.
Vietnam escalated like a cancer. From 1955 thru 1964 Vietnam was a guerrilla war fought in South Vietnam by US military advisers and Special Operations men (Green Beret and SEALs). From 1964 thru 1975 the war was fought, in addition to the above, by conventional forces (Tanks, Artillery, Infantry, Jets, Bombers, Aircraft Carriers, Cruisers, Destroyers, and one battleship) in both North & South Vietnam.
October 26, 1955 US promises aid to the newly established Republic of South Vietnam; US military men are in country and commencing operations April 1956. Last US military men depart 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.