Immediately following the Khmer Rouge victory in 1975, there were skirmishes between their troops and Vietnamese forces. A number of incidents occurred in May 1975. The Cambodians launched attacks on the Vietnamese islands of Phu Quoc and Tho Chu and intruded into Vietnamese border provinces. In late May, at about the same time that the United States launched an air strike against the oil refinery at Kampong Saom, following the Mayaguez incident, Vietnamese forces seized the Cambodian island of Poulo Wai. The following month, Pol Pot and Ieng Sary visited Hanoi. They proposed a friendship treaty between the two countries, an idea that met with a cool reception from Vietnam's leaders. Although the Vietnamese evacuated Poulo Wai in August, incidents continued along Cambodian's northeastern border. At the instigation of the Phnom Penh regime, thousands of Vietnamese also were driven out of Cambodia.

Relations between Cambodia and Vietnam improved in 1976, in part because of Pol Pot's preoccupation with intraparty challenges. In May Cambodian and Vietnamese representatives met in Phnom Penh in order to establish a commission to resolve border disagreements. The Vietnamese, however, refused to recognize the Brévié Line--the colonial-era demarcation of maritime borders between the two countries--and the negotiations broke down. In late September, however, a few days before Pol Pot was forced to resign as prime minister, air links were established between Phnom Penh and Hanoi.

With Pol Pot back in the forefront of the regime in 1977, the situation rapidly deteriorated. Incidents escalated along all of Cambodia's borders. Khmer Rouge forces attacked villages in the border areas of Thailand near Aranyaprathet. Brutal murders of Thai villagers, including women and children, were the first widely reported concrete evidence of Khmer Rouge atrocities. There were also incidents along the Laotian border. At approximately the same time, villages in Vietnam's border areas underwent renewed attacks. In turn, Vietnam launched air strikes against Cambodia. In September, border fighting resulted in as many as 1,000 Vietnamese civilian casualties. The following month, the Vietnamese counterattacked in a campaign involving a force of 20,000 personnel. Vietnamese defense minister General Vo Nguyen Giap underestimated the tenacity of the Khmer Rouge, however, and was obliged to commit an additional 58,000 reinforcements in December. On January 6, 1978, Giap's forces began an orderly withdrawal from Cambodian territory. The Vietnamese apparently believed they had "taught a lesson" to the Cambodians, but Pol Pot proclaimed this a "victory" even greater than that of April 17, 1975.

Faced with growing Khmer Rouge belligerence, the Vietnamese leadership decided in early 1978 to support internal resistance to the Pol Pot regime, with the result that the Eastern Zone became a focus of insurrection. War hysteria reached bizarre levels within Democratic Kampuchea. In May 1978, on the eve of So Phim's Eastern Zone uprising, Radio Phnom Penh declared that if each Cambodian soldier killed thirty Vietnamese, only 2 million troops would be needed to eliminate the entire Vietnamese population of 50 million. It appears that the leadership in Phnom Penh was seized with immense territorial ambitions, i.e., to recover the Mekong Delta region, which they regarded as Khmer territory.

Massacres of ethnic Vietnamese and of their sympathizers by the Khmer Rouge intensified in the Eastern Zone after the May revolt. In November, Vorn Vet led an unsuccessful coup d'état. There were now tens of thousands of Cambodian and Vietnamese exiles on Vietnamese territory. On December 3, 1978, Radio Hanoi announced the formation of the Kampuchean (or Khmer) National United Front for National Salvation (KNUFNS--see Appendix B). This was a heterogeneous group of communist and noncommunist exiles who shared an antipathy to the Pol Pot regime and a virtually total dependence on Vietnamese backing and protection. The KNUFNS provided the semblance, if not the reality, of legitimacy for Vietnam's invasion of Democratic Kampuchea and for its subsequent establishment of a satellite regime in Phnom Penh (see Vietnamese Invasion of Cambodia , ch. 5).

In the meantime, as 1978 wore on, Cambodian bellicosity in the border areas surpassed Hanoi's threshold of tolerance. Vietnamese policy makers opted for a military solution and, on December 22, Vietnam launched its offensive with the intent of overthrowing Democratic Kampuchea. An invasion force of 120,000, consisting of combined armor and infantry units with strong artillery support, drove west into the level countryside of Cambodia's southeastern provinces. After a seventeen-day blitzkrieg, Phnom Penh fell to the advancing Vietnamese on January 7, 1979. From new redoubts in the mountain and jungle fastness of Cambodia's periphery, Pol Pot and other Khmer Rouge leaders regrouped their units, issued a new call to arms, and reignited a stubborn insurgency against the regime in power as they had done in the late 1960s. For the moment, however, the Vietnamese invasion had accomplished its purpose of deposing an unlamented and particularly loathsome dictatorship. A new administration under the mentorship of Hanoi was quickly established, and it set about competing, both domestically and internationally, with the Khmer Rouge as the legitimate government of Cambodia. Peace still eluded the war-ravaged nation, however, and although the insurgency set in motion by the Khmer Rouge proved unable to topple the new Vietnamese-backed regime in Phnom Penh, it did nonetheless keep the country in a permanent state of insecurity. The fledgling Khmer administration, weak and lacking in manpower and in resources, was propped up by a substantial Vietnamese military force and civilian advisory effort. As events in the 1980s progressed, the main preoccupations of the new regime were survival, restoring the economy, and combating the Khmer Rouge insurgency by military and by political means. The fostering of activity to meet these imperatives and the building of institutions are described in subsequent chapters (see The People's Republic of Kampuchea , ch. 4; Vietnamese Invasion of Cambodia , ch. 5).

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Probably the most definitive account of Cambodian history in English is David P. Chandler's A History of Cambodia, which covers this subject from the earliest centuries to the attainment of independence. An earlier work, Martin Herz's A Short History of Cambodia from the Days of Angkor to the Present, is somewhat dated but contains highly specific material on the early Sihanouk years. The sections on Cambodia in D.G.E. Hall's classic, A History of South-East Asia, are also useful. Hall's parallel treatments of Vietnamese and Thai history provide interesting perspective. Georges Coedès' Angkor: An Introduction and The Making of Southeast Asia are informative on the ancient kingdoms. The former provides good descriptions of the monuments of Angkor, their architectural motifs, and their religious significance. For an understanding of the rise of communism in Cambodia an essential source is Ben Kiernan's How Pol Pot Came to Power.

William Shawcross's Sideshow: Kissinger, Nixon, and the Destruction of Cambodia is a critical account of United States involvement in the country before and during the years, 1970 to 1975, when Lon Nol was in power. Another perspective is provided by Henry Kissinger in White House Years and in Years of Upheaval.

A large number of books have been written on the horrors of the Democratic Kampuchea period. Molyda Szymusiak's The Stones Cry Out is one of the most gripping. Others include François Ponchaud's Cambodia: Year Zero and Sydney Schanberg's The Death and Life of Dith Pran. The film "The Killing Fields" is also based on the experiences of Dith Pran-- journalist Schanberg's cameraman in wartime Phnom Penh--in Cambodia during and after the war. Norodom Sihanouk's War and Hope provides another closeup . Useful scholarly treatments of the period include Craig Etcheson's perspective on the Khmer Rouge, The Rise and Demise of Democratic Kampuchea; two collections of essays, Peasants and Politics in Kampuchea, 1942-81, edited by Ben Kiernan and Chanthou Boua; and Revolution and Its Aftermath in Kampuchea, edited by David P. Chandler and Ben Kiernan. One well-reviewed account of life in Democratic Kampuchea and the transition to its successor regime is Elizabeth Becker's When the War Was Over; another account, equally well-regarded by critics, addressing the abiding enmity between Cambodia and Vietnam is Nayan Chanda's Brother Enemy. Familiarity with both sources is essential for an understanding of what occurred in Cambodia after 1975. (For further information and complete citations, see Bibliography.)

Data as of December 1987