Beginning in the summer of 1965, 3/4 (Third Bn, 4th Marines) were faced with an expanding tactical area of operational responsibility (TAOR) and shrinking numbers due to insufficient replacements coming in to fill combat losses. As a result, the Marines began working directly with the Popular Forces, local militias made up of old men and boys too young to be drafted into the Army of the Republic of Vietnam. The plan was approved by General Lewis Walt, the commander of III Marine Amphibious Force and overall Marine Commander in Vietnam at the time, as well as General Nguyen Van Chuan, the local ARVN commander. The Marines were given operational control of the PF platoons near Phu Bai.
Working directly with the PF were General Walt and Lt. General Victor Krulak, both veterans of the Banana Wars in Central America in the ’30s. The Banana Wars were instrumental in the formation of Marine “Small Wars” doctrine, codified in the “Small Wars Manual,” which remained the Marine counter-insurgency manual until it was replaced by FM 8-2, “Counter-Guerrilla Operations.”
From its beginning, the combination of Marines and local Popular Forces were referred to as the Combined Action Program (CAP). Under the CAP-Platoon concept, a squad-sized element of Marines was combined with a platoon of PF in a local Vietnamese village. These Marines would stay in place for a long time; an entire year or more was not unheard of. They lived with the people of the village and worked with the Popular Forces to secure the village and the surrounding area.
The initial experiment was limited to picked volunteers from 3/4, and began in August, 1965. There were seven CAP Platoons across I Corps by January, 1966. While it may have begun as a stopgap measure to adjust for the difficulties that 3/4 was facing with its expanding TAOR, it also reflected General Walt’s closeness to the Marine Corps’ Small-Wars Doctrine. From the Small Wars Manual: