The Khmer Serei had its origin in the Issarak (pro-independence) armed bands active within Cambodian borders during the later French colonial period. Thanh, who had briefly been Prime Minister during the Japanese occupation in World War II before being exiled by the returning French, attempted to gain overall leadership of the Issarak movement, but had little success.
After Cambodia gained its independence in 1954 under King Norodom Sihanouk, Thanh organized the Khmer Serei irregulars to maintain an armed struggle against Sihanouk. They operated mainly from bases on the Thai and Vietnamese borders, recruiting largely amongst the Khmer Krom minority of the Mekong Delta in Vietnam, and made clandestine anti-Sihanouk radio broadcasts. Although observers regarded the Khmer Serei's activities as more of a "nuisance" than a genuine threat, the Cambodian regime viewed the insurgency seriously: allegations later surfaced that Sihanouk had executed as many as 1000 Khmer Serei suspects during his rule.
The 1963 arrest, public caging and subsequent execution of Preap In, a Khmer Serei activist who had offered to go to the National Assembly to negotiate directly with Sihanouk, was a prominent example of the Sangkum's repression of the Khmer Serei. The execution of In, a former member of the Democratic Party, was filmed and shown in all cinemas for a month, an event which remained in the memories of Cambodians for many years.
The Khmer Serei and the Khmer Kampuchea Krom militia
The Khmer Serei also had loose links with the US-backed Front de Lutte du Kampuchea Krom militia, or "White Scarves" (Khmer: Kangsaing Sar; Vietnamese: Can Sen So), of southern Vietnam. This group, originally founded by a Khmer Krom monk named Samouk Sen, sought Khmer Krom independence and regularly clashed with the Viet Cong: it expanded in the 1960s and later became affiliated to FULRO, a paramilitary organisation for Vietnamese minority groups. Members of both the Khmer Kampuchea Krom and the Khmer Serei were trained by the US military for clandestine operations during the Second Indochina War as part of MIKE Force, and were partly financed and armed by the Central Intelligence Agency. At their peak in 1968, the Khmer Serei and related forces were thought to number up to 8000 men.
After the 1970 Lon Nol coup, Son Ngoc Thanh became a government minister, and members of the Khmer Serei and Khmer Kampuchea Krom militias - some of whom had been infiltrated into the Cambodian army prior to the coup - appeared on the streets of Phnom Penh. However, due in part to the desperate military situation, and in part to Lon Nol's suspicion that this comparatively well-trained force might be used against him by his rivals, they were deliberately thrown into the most grueling battles and largely shattered.
Son Ngoc Thanh was dismissed from the government in 1972, and exiled himself in South Vietnam, where he was arrested following the Fall of Saigon; the remaining Khmer Serei in South Vietnam were eliminated by the North Vietnamese victory, while those in Cambodia itself were hunted down by the Khmer Rouge after the overthrow of Lon Nol's regime.
Subsequent to the 1975 Khmer Rouge victory and the proclamation of Democratic Kampuchea, the few remaining "Free Khmer" operating in the north-west, in the area of the Dangrek Mountains, found themselves mostly in refugee camps along the Thai-Cambodian border, such as Nong Chan and Nong Samet. Son Sann was to use some of these former Khmer Serei supporters as the basis of the Khmer People's National Liberation Front movement, formed in 1979.
Thanh himself was to die in Vietnamese custody in 1977.
- Library of Congress Country Studies: Cambodia - The Second Indochina War, 1954-75, 1987
- Short, P. Pol Pot: Anatomy of a Nightmare, Henry Holt, 2005, p.156
- Vietnam Studies, U.S. Army Special Forces 1961-1971, CMH Publication 90-23, Department of the Army, Washington, D.C. 1989 (First Printed, 1973)
- Radu, M. The New Insurgencies, Transaction Publishers, 1990, p.202
- Shawcross, W. Sideshow: Kissinger, Nixon and the Destruction of Cambodia, 1979, p.132
- Kershaw, R. Monarchy in South-East Asia: the faces of tradition in transition, Routledge, 2001, p.221