US advisers were sent to South Vietnam in 1955. Special Forces (Green Beret) in 1961. US Regulars in 1965.
North Vietnam and the United States. South Vietnam wasn't the main combatant, because they did NOT conduct the Air War over North Vietnam. The Vietnam War had three categories: US Navy-Riverine War; US Air Force-Air War; US Army/US Marines-Land War.
Vietnam's area is 126,000 square miles or so. It is a little larger than New Mexico.
The US Army has jurisdiction over all land wars.
land
The US never invaded Vietnam.
vietnam covers 127,246 sq mi; 329,566 sq km of land
Radios and land lines; US warships still used blinking lights.
1. Air War over North Vietnam (MIGS vs US JETS) 2. Land/Ground War in South Vietnam (Patton tanks and ACAVs) 3. Riverine War in South Vietnam. (Swift Boats & Monitors)
South Vietnam was considered an ally of the US.
Catholic versus Buddhist, Communist versus Democracy and the constants of poor versus rich, haves versus have nots, land owner versus peasant.
US Military personnel were in Vietnam from 1955 thru 1975.
See: Vietnam