Border tensions between Cambodia and Vietnam (aside from traditional Khmer fear and hatred of the Vietnamese) goes back to the controversy over the Brévié Line, drawn in 1939 by French colonial administrators and considered by Vietnam to be the official international boundary between the two countries. For years after the French departure, various Cambodian governments attempted to negotiate the return of Cochinchina--known in Cambodia as Kampuchea Krom, which they maintained was a French colony, not a protectorate, that had been promised to Cambodia by early French colonial authorities. Negotiations to solve the border dispute were held between 1975 and 1977, but they made no progress and were suspended. The Khmer Rouge also felt an abiding distrust of the Vietnamese, who, they believed, had never renounced their determination to incorporate Cambodia into a larger, Hanoidominated Indochina federation.

Clashes between the RAK and Vietnamese communist forces began in Cambodia as early as 1970, when there were reported incidents of Khmer Rouge units firing on North Vietnamese. Reports continued of engagements of growing intensity, particularly after 1973. The North Vietnamese, because they urgently needed sanctuaries in Cambodia in order to pursue their war in South Vietnam, chose to ignore the incidents and were still prepared, at the end of Cambodia's long civil war, to send sapper and artillery groups to help the CPNLAF take Phnom Penh. After the communist victories of April and May 1975, clashes between Vietnamese and Khmer Rouge units centered on the border. Skirmishing began about a month after the fall of Phnom Penh, when Hanoi accused the Khmer Rouge of trying to seize Phu Quoc Island and of making forays into several Vietnamese border provinces.

Ironically, some analysts believe that the Khmer Rouge would have made more noise about their offshore claims had it not been for the destruction by the United States of their air force and much of their navy during the Mayaguez incident. On May 12, 1975, a Khmer Rouge sector commander, zealously asserting Cambodia's territorial rights in the Gulf of Thailand, boarded and captured the American container ship S.S. Mayaguez, which carried a crew of forty, near the island of Wai (which later fell under Vietnamese jurisdiction). Failing to receive a timely response to demands for return of the ship, Washington notified the UN and invoked the right to self-defense under Article 51 of the UN Charter. The ensuing four-day engagement involved U.S. bombing raids on the airfield at Ream and on the port of Kompong Saom, as well as naval barrages and a Marine assault on the nearby island of Kaoh Tang. On orders from the Khmer Rouge leadership, the Mayaguez crew was released unharmed and was returned to United States custody.

Deteriorating relations between Cambodia and Vietnam reached a crescendo of recrimination when, on December 31, 1977, Radio Phnom Penh, citing "ferocious and barbarous aggression launched by the Vietnamese aggressor forces against Democratic Kampuchea," denounced the "993 "so-called Socialist Republic of Vietnam" and announced the "temporary severance" of diplomatic relations. Rhetorical exchanges between the two sides became more acrimonious, and border skirmishes involving Cambodian and Vietnamese units erupted into pitched battles in the summer and the fall of 1978. Major engagements were reported in the Parrot's Beak (part of Svay Rieng Province), in the Fishhook (part of Kampong Cham Province), and in Rotanokiri Province. In an effort to court world public opinion, in September 1978 the Ministry of Foreign Affairs of Democratic Kampuchea published its so-called "Black Book," the Black Paper: Facts and Evidence of Aggression and Annexation Against Kampuchea. The tract denounced Vietnam's "true nature" as that of "aggressor, annexationist and swallower of other countries' territories."

In November 1978, rhetoric was succeeded by full-scale action: Vietnamese forces launched a sustained operation on Cambodian soil in the area of Snuol and Memot (both in Kracheh Province). This action cleared a liberated zone where anti-Khmer Rouge Cambodians could launch a broad-based political movement that would offer an alternative to the odious Pol Pot regime. Proclamation of this movement, the Kampuchean (or Khmer) National United Front for National Salvation (KNUFNS--see Appendix B), took place in a rubber-plantation clearing on December 2, 1978, amid rigid security provided by heavily armed Vietnamese units reinforced with airdefense weapons.

Data as of December 1987