The March 1965 deployment of two battalions from the Third Marine Division on Okinawa to guard the Da Nang air base marked the first explicit commitment of U.S. combat forces to South Vietnam's defense. Composed ultimately of the 3d, 4th, and 9th Marine Regiments, plus the 5th Marine Division's 26th Marines, the division's headquarters moved north to the Hue area in October 1966, to Quang Tri eighteen months later, then ultimately to Dong Ha in June 1968.
Medals of Honor were awarded to twenty-nine marines and one corpsman who served with the division. Units from the Third Marine Division became primarily responsible for setting up a defensive barrier along the Demilitarized Zone (DMZ) and fought and operated out of Gio Linh and Con Thien, both within five kilometers of the DMZ and together with Dong Ha and Cam Lo forming "Leatherneck Square"; Camp Carroll, the Rockpile, and Ca Lu along Route 9; and successfully defended Khe Sanh in the spring of 1968.
The division, which had not seen action since landing on Bougainville, Guam, and Iwo Jima during World War II, received a Presidential Unit Citation and Vietnamese Cross of Gallantry with Palm, redeployed to Okinawa in late November 1969, and in 1975 provided troops involved in the Pnom Penh, Da Nang, and Saigon evacuations as well as the units which attacked Koh Tang island during the Mayaguez incident.