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COSVN, pronounced "CŎS-vĭn" and standing for Central Office, South Vietnam, was a People's Army of Vietnam (PAVN) military headquarters (HQ) during the Second Indochina War (Vietnam Conflict). It was in overall command of the communist effort in the southern half of the Republic of Vietnam (RVN or South Vietnam), which included the efforts of both PAVN and the National Front for the Liberation of South Vietnam (NLF or derogatively Viet Cong). Whether COSVN actually existed, and if so, where it was located at any one time, and how important it was, have been contentious subjects, but U.S. military intelligence generally believed that COSVN existed and that it was a valuable target (Westmoreland 1976:55,56,206,389).

During the early 1960s, COSVN was reputed to be in Tay Ninh Province northwest of Saigon, near the Cambodian border (Westmoreland 1976:55). During the period of 1965-1975, COSVN was not within the borders of South Vietnam, but was based in and around the Mimot rubber plantation, in what was called the “Fishhook” area of Cambodia, on the Vietnamese/Cambodian border north of Tay Ninh and west of Loc Ninh.

It was believed that COSVN had several subdivisions that dealt with the political, logistical, and military aspects of the war in South Vietnam. For tactical reasons U.S. Radio Research direct support units were primarily concerned with the military divisions, which were known as “MAS-COSVN” (Military Affairs Section) and “MIS-COSVN” (Military Intelligence Section), with the political and logistical sub-divisions left to the Radio Research Field Station at Phu Bai. These two sub-divisions usually occupied a location removed from, but generally near, the central office itself, as determined by ARDF or airborne radio direction finding.

One of the central frustrations of the U.S. military during the war was the Democratic Republic of Vietnam (DRV or North Vietnam)'s use of Laos and Cambodia for supply routes and Base Areas. During the Johnson administration, the U.S. military was generally not allowed by its civilian commanders to widen the war by attacking the supply routes and sanctuaries in both neutral countries. Later, President Richard Nixon allowed such attacks, at first in the form of secret aerial bombing (Operation Menu in 1969), wherein the suspected site of COSVN in Cambodia was repeatedly and heavily bombed, and later in the form of an overt ground invasion (the Cambodian Incursion of 1970), wherein South Vietnamese (ARVN) and U.S. ground forces attempted to "clean out the sanctuaries" (Westmoreland 1976:389).

With the announcement of a 20-mile limit to the depth of U.S. penetration, COSVN and its sub-divisions quickly retreated outside the operational area and successfully eluded capture. A marked reduction in radio traffic and transmitter power also made them difficult to place accurately at their new location, despite close 24-hour monitoring. The military benefits and tragic side effects of such bombing and invasion have been contentious subjects. Westmoreland thought that it was "unfortunate" that Nixon mentioned COSVN to the press in 1970 as one of the primary objectives of the incursion (Westmoreland 1976:389). This invited critics, who were already scornful of Nixon, to mock the notion of the president obsessing over COSVN as if it were a "holy grail" sort of objective. The idea that "it probably didn't even exist" was in line with this scornfulness, but Westmoreland in his memoir described COSVN as something whose existence U.S. military intelligence sources did not doubt.

• Westmoreland, William C. (1976). A Soldier Reports. Garden City, New York: Doubleday & Company, Inc., pp. 55, 56, 206, 389. ISBN 0-385-00434-6.  [General Westmoreland was COMUSMACV (Commander, U.S. Military Assistance Command, Vietnam) 1964-1968 and U.S. Army Chief of Staff, 1968-1972. A Soldier Reports is his memoir on the war.]