The Vietnam War was the first combat opportunity for "mechanized" infantry, a technically new type of infantry with its roots in the armored infantry of World War II, now using the M113 Armored Personnel Carrier. In addition, Armored Cavalry squadrons in Vietnam consisted largely of M113s, after replacing the intended M114 in a variety of roles, and Armor battalions contained M113s within their headquarters companies, such as the maintenance section, medical section, vehicle recovery section, mortar section, and the scout (reconnaissance) section. U.S. Army mechanized infantry units in Vietnam were fully equipped with the M113 APC/ACAV, which consisted of one headquarters company and three line companies, normally with an authorized strength of approximately 900 men. Ten U.S. mechanized infantry battalions and one mechanized brigade were deployed to Vietnam from 1965 until their departure in 1972: 2/2nd Mechanized Infantry, 1/5th Mechanized Infantry, 2/8th Mechanized Infantry, 1/16th Mechanized Infantry, 2/22nd Mechanized Infantry, 4/23rd Mechanized Infantry, 2/47th Mechanized Infantry, 1/50th Mechanized Infantry, 5/60th Mechanized Infantry, 1/61st Mechanized Infantry, and the 1st Brigade, 5th Infantry Division (Mechanized).
The U.S. Army 1st Brigade, 5th (Mech) Infantry Division in Vietnam was not composed of strictly mechanized infantry battalions. The 5th (M) ID (1st Bde), consisted of: the 5/4th Field Artillery, 1/11th Light Infantry (straight leg-no armored vehicles), 1/77th Armor (M48 Patton tanks), 1/61st Mechanized Infantry, "A" Troop" 4/12th Armored Cavalry (only one Troop of Cavalry), and the 3/5th Armored Cavalry OPCON (Operationally Controlled) /Attached from the 9th Infantry Division. The one troop of the 12th Armored Cavalry and the full squadron of the 5th Armored Cavalry were M551 Sheridan and M113 ACAV equipped.