The public unveiling of the KNUFNS dashed any remaining expectations that Cambodian-Vietnamese disagreements could be solved without further armed conflict, because the Hanoi-backed front openly called for the ouster of the "reactionary Pol Pot-Ieng Sary clique." Because the KNUFNS was far too weak to topple the regime of Democratic Kampuchea, virtually the entire combat burden would fall on Vietnamese forces, which, for this purpose, had been steadily building up troop strength on the border during the preceding months.

Nervous Khmer Rouge leaders in Phnom Penh did not have long to wait after the KNUFNS announcement, for, on December 25, 1978, Hanoi launched its offensive with twelve to fourteen divisions and three Khmer regiments (that later would form the nucleus of the KPRAF), a total invasion force comprising some 100,000 people. Vietnamese units struck across the Cambodian frontier in five spearheads that thrust initially into northeastern Cambodia. One task force drove west from Buon Me Thuot (in Dac Lac Province, Vietnam) along Route 13 and Route 14 to capture Kracheh City (the capital of Kracheh Province). A second column attacked west from Pleiku (in Gia Lai-Cong Tum Province, Vietnam), and followed the circuitous Route 19 to capture Stoeng Treng City (the capital of Stoeng Treng Province). In thus concentrating its initial thrusts in the northeast, Hanoi may have had several objectives. One of these may have been to capture quickly substantial expanses of the Cambodian territory that had been an early spawning ground for the Khmer Rouge and its fledgling RAK in the late 1960s. The remoteness of this region would have rendered it difficult to dislodge Vietnamese forces, no matter what the outcome of the war. An early occupation also would have preempted Khmer Rouge units, if they were pressed harder elsewhere, from falling back to this area where they might have enjoyed a measure of public support. The attacks in the northeast also may have been intended to confuse the leadership of Democratic Kampuchea about where the full brunt of the Vietnamese offensive would fall.

Khmer Rouge commanders were not deceived by the Vietnamese thrusts toward Kracheh and Stoeng Treng, however, and made no attempt to reinforce the northeast. Instead, they erected their main defense line in an arc across the flat, rice-growing plains of southeastern Cambodia, astride the most probable Vietnamese axes of advance. Their calculation of Vietnamese intentions proved correct, as Hanoi's forces unleashed the full weight of their offensive in this area. From Vietnam's Tay Ninh Province, heavily armed Vietnamese units drove along the axis of Route 7 toward their objective, the river port of Kampong Cham. Farther south, Vietnamese units with air support attacked along Route 1, in the direction of Phumi Prek Khsay (also known as Neak Luong), the Mekong River gateway to Phnom Penh. The fifth and final Vietnamese spearhead drove west from Ha Tien, Vietnam, to capture the ports of Kampot and Kampong Saom, and thus to prevent the resupply by sea of retreating Khmer Rouge forces.

Resistance to the invading Vietnamese units by the RAK could have been suicidal, given the disregard for human life previously displayed by the forces of Democratic Kampuchea. Instead, heavy fighting was localized. Major engagements were fought before Kampong Cham and Phumi Prek Khsay and at Tani, inland from the coast of Kampot Province. RAK units, already deprived of experienced commanders by party purges, withered under sustained pounding by Vietnamese artillery and airstrikes, and many of them simply scattered before the Vietnamese offensive, some to regroup later in western Cambodia.

By January 5, 1979, the main Vietnamese spearheads had driven to the eastern banks of the Mekong River. Incomplete evidence hints that the Vietnamese offensive originally may have intended to go no farther. The way to Phnom Penh lay open, however, because the Khmer Rouge units were falling back. Vietnamese forces paused briefly, perhaps to wait for bridging and ferrying equipment and the latest orders from Hanoi, then proceeded to carry out the final assault on Phnom Penh. Khmer Rouge leaders elected not to defend the city, and it fell on January 7.

After the fall of the capital, Vietnamese units continued their advance in two columns into western Cambodia, capturing Batdambang and Siemreab. The columns met at Sisophon and drove on to the Thai border, where there was heavy fighting in March and in April. In the meantime, ÍÍÍÍÍÍÍÍÍsome remaining Khmer Rouge units offered scattered resistance before they melted away into less accessible areas. There the Khmer Rouge leaders soon rekindled an insurgency against the new government in power, just as they had in the late 1960s, and insecurity persisted in the countryside in spite of the continued Vietnamese presence.

On the diplomatic front, Vietnam, maintaining it had no troops in Cambodia and attributing the lightning-like victory to the KNUFNS, at first denied responsibility for the invasion. When called before the UN Security Council, however, Hanoi's representative, tacitly admitting the presence of Vietnam and citing numerous Western press reports of Pol Pot's genocidal actions, implied that his country had overthrown the Pol Pot regime in the name of humanitarian and human rights.

The Vietnamese sweep through Cambodia produced an unprecedented level of turmoil on the Thai border, as disorganized and bypassed Khmer Rouge units and civilian refugees fled before their advancing enemy. Amid this chaos, in 1979, two anti-Vietnamese insurgent movements, besides the Khmer Rouge, came into being. The first of these was the Khmer People's National Liberation Armed Forces (KPNLAF--see Appendix B), the armed wing of the Khmer People's National Liberation Front (KPNLF--see Appendix B), which gave allegiance to Son Sann, a noncommunist, perennial cabinet minister in successive Sihanouk administrations. The other was the Sihanouk National Army (Armée Nationale Sihanoukiste--ANS--see Appendix B), the armed wing of the National United Front for an Independent, Neutral, Peaceful, and Cooperative Cambodia (Front Uni National pour un Cambodge Indépendant, Neutre, Pacifique, et Coopératif-- FUNCINPEC--see Appendix B), which owed allegiance to Sihanouk. Fighting independently, these noncommunist guerrilla movements and the Khmer Rouge fomented continuous rebellion in the early 1980s that could not be quelled, despite a substantial Vietnamese military commitment to this purpose. Operating from refugee camps on the Thai frontier, the insurgents made forays into the Cambodian border provinces and kept the countryside in a permanent state of insecurity.

In the 1984 to 1985 dry season, the Vietnamese military command in Cambodia, frustrated because of depredations by the guerrillas, undertook a sustained offensive to dislodge them from their sanctuaries in the refugee camps. These installations were pounded by artillery and were overrun by Vietnamese tactical units. The operation, which was intended to cripple the Khmer guerrillas, had the opposite effect, however. It drove them away from the border, and they undertook prolonged forays deeper into the Cambodian interior.

To restrict guerrilla activity, the Vietnamese erected a physical barrier on the Thai-Cambodian border. Code-named Project K-5, the effort consisted of clearing jungle growth of erecting obstacles, such as ditches, barbed wire, and minefields and of building a road parallel to the border. Construction of the project, which began in 1985, was performed by corvée labor. All districts in Cambodia were tasked to provide able-bodied males for tours of duty on the project that ranged from three to six months. Living conditions were primitive in the construction camps, and the diet was inadequate the area was malarial, and unexploded ordnance from past conflicts was a constant threat. The barrier was completed in 1987 at an unrecorded cost in Cambodian lives. Preliminary indications shortly thereafter revealed that it was having little effect on guerrilla movements to and from the Cambodian interior.

Data as of December 1987