The entire unit of American Marines and Popular Forces militia members together was designated as a Combined Action Platoon (CAP).

The program was said to have originated as a solution to one Marine infantry battalion's problem of an expanding Tactical Area of Responsibility (TAOR).

The concept of combining a squad of Marines with local (PFs) and assigning them a village to protect proved to be a force multiplier.

"Counterinsurgency operations and, in particular, the establishment of a foreign internal defense lends itself for the greatest utility of employing a CAP-style organization.

[2] The CAP concept seems to have been at least partially based on Marine pacification programs in Haiti, Nicaragua, the Dominican Republic, and elsewhere, during the Banana Wars in the late 19th and early 20th centuries.

In these programs, Marine units would pacify and administer regions, while providing training and security for local forces and villages.

[5] Opinions differ about exactly how and where Combined Action originated, but it seems to have started in August 1965 as a unit drawn from 3rd Battalion, 4th Marines, under LtCol William W. Taylor in the Phu Bai(3/4) area.

GEN Nguyễn Văn Chuân, the local Army of the Republic of Vietnam (ARVN) CO, gave Walt control of the Vietnamese platoons near Phu Bai.

This was in Zimmerman's mind when he developed the plan that called for combining a U.S. Marine rifle squad with a PF platoon to form an integrated self-defense force that was able to protect the village from low level Viet Cong threats.

"[7][8] MG Walt formalized the program in February 1967, appointing LtCol William R. Corson as the III MAF deputy director for Combined Action.

[9] In spite of this rocky start, CAP became an official "hearts and minds" civic action program, and a school of sorts was eventually established near Da Nang.

Training was brief (ten days) and covered a few bare essentials – some Vietnamese phrases, customs, and culture, some civic action precepts, and some military topics – far too short to be of much real good, though a step in the right direction.

Initially, CAP Marines were issued a special cloth and leather insignia to be worn from the button on the breast pocket of the uniform jacket.

The CAP concept in Vietnam was opposed by some who considered "hearts and minds" programs a waste of money, men, and materiel.

However, the concept eventually gained backing from Marine generals Wallace Greene, Victor Krulak and Lew Walt, and with their support, the program expanded.

By 1969, despite losses during the 1968 Tet Offensive, the program had expanded to 102 platoons comprising 19 companies and 4 groups, and was even mentioned by President Lyndon B. Johnson in a speech.

In addition, "cac" is a Vietnamese word for the male generative organ, and the motto included the phrase "suc manh", which means strength.

To work with the PFs, III MAF instituted the combined action platoon (CAP), consisting of a 13-man Marine rifle squad (if you were fortunate enough to have 13) augmented by a U.S. Navy Corpsman and paired with a 15- to 30-man PF platoon to defend one particular village (The PF was roughly equivalent to the US National Guard, but with less training and poorer equipment).

In southern I Corps, the CAC units (including CAC "India", out of the 1st Bn., 5th Marine Reg., aka; Cottage Tiger Company, from Dec. 1966 until late 1967, west of Tam Ky, along the Tam Ky river), became part of Task Force X-ray (CAC "India", was later in 1967, known as Sub Unit # 2 of Task Force X-ray), a brigade command of the 1st Marine Division, were placed under the administrative control of the 7th Communications Battalion when the Chu Lai TAOR was turned over to the U.S. Army in late April/May, 1967.

The report mentions building CAP sites, patrols, and other events, but generally doesn't break them down by company or platoon, with references to Alpha, Hotel, and Papa as separate entities.

The remaining 1 officer and 16 enlisted in SU #4 continued to function as CAG members until normal attrition reduced them to zero effective 30 November 1967.

According to the III MAF staff letter, the justification for this conversion included the facts that; the links with the PFs were still intact; it avoided the "mole" mentality of a static position; it denied the enemy information as to the exact location of the unit, thus, reducing casualties; it allowed the Marines to make better use of supporting arms by being outside the populated areas; and allowed the Marines to concentrate their strength by not requiring the unit to guard a base.

However, most Marine units worked in the lower lying areas with Vietnamese RF / PF units, while Special Forces tended to work in more remote areas using a variety of troops, including indigenous minorities such as the Sino-Vietnamese Nung and Dega (aka "Montagnard") tribesmen.

The main difference between the Marine CAP and the Army programs was that the Marine program was a "hearts and minds" civic action program seeking to gain the trust and friendship of the Vietnamese they lived and worked with through a combination of military training and civic action projects, while the Special Forces Civilian Irregular Defense Group (CIDG) combined village defense units with mobile strike forces of mercenary light infantry.

An additional combined operation involved MACV-SOG Military Assistance Command Vietnam Studies and Observations Group.

The HQ was in a nominally secure area, and they ventured out to arranged meeting places to provide instructional support in weapons maintenance, etc.

[citation needed] "Of all our innovations in Vietnam none was as successful, as lasting in effect, or as useful for the future as the Combined Action Program [CAP].

[citation needed] Elsewhere in his article he states: "From 1966 until 30 June 1969 they {CAP NCOs} lead small units which killed over 4400 VC/ NVA."

[citation needed] Harold P. Ford, who held senior positions in both the National Intelligence Council and the Directorate of Operations, offers some insights on Secretary of Defense Robert McNamara's evaluation of the situation in Vietnam; "The large-unit operations war, which we know best how to fight and where we have had our successes, is largely irrelevant to pacification as long as we do not have it.

Success in pacification depends on the interrelated functions of providing physical security, destroying the VC apparatus, motivating the people to cooperate and establishing responsive local government.