From 1976 to 1978, the xenophobic government in Phnom Penh instigated a series of border clashes with Vietnam, and claimed the Mekong Delta, once part of the Khmer empire. Khmer Rouge incursions into Vietnamese border provinces left hundreds of Vietnamese civilians dead. On 25 Decem­ber 1978 Vietnam launched a full-scale invasion of Cambodia, toppling the Pol Pot government two weeks later. As Vietnamese tanks neared Phnom Penh, the Khmer Rouge fled westward with as many civilians as it could seize, taking refuge in the jungles and mountains on both sides of the Thai border. The Vietnamese installed a new government led by several former Khmer Rouge officers, including Hun Sen, who had defected to Vietnam in 1977. The Khmer Rouge's patrons, the Chinese communists, launched a massive reprisal raid across Vietnam's northernmost border in early 1979 in an attempt to buy their allies time. It failed, and after 17 days the Chinese withdrew, their fingers badly burnt by their Vietnam­ese enemies. The Vietnamese then staged a show trial in which Pol Pot and leng Sary were condemned to death for their genocidal acts.

The social and economic dislocation that accompanied the Vietnamese invasion - along with the destruction of rice stocks and unharvested fields by both sides-(to prevent their use by the enemy) - resulted in a vastly reduced rice harvest in early 1979. The chaotic situation led to very little rice being planted in the summer of 1979. By the middle of that year the country was suffering from a widespread famine.

As hundreds of thousands of Cambodians fled to Thailand, a massive international famine relief effort, sponsored by the UN, was launched. The international community wanted to inject aid across a land bridge at Poipet, while the new Phnom Penh government wanted all supplies to come through the capital via Kompong Som (Sihanoukville) or the Mekong River. Both sides had their reasons - the new government did not want aid to fall into the hands of its Khmer Rouge enemies, while the international community didn't believe the new government had the infrastructure to distribute the aid - and both were right.

Some agencies distributed aid the slow way through Phnom Penh, and others set up camps in Thailand. The camps became a magnet for half of Cambodia, as many Khmers still feared the return of the Khmer Rouge or were seeking a new life overseas. The Thai military bullied and blackmailed the international community into distributing all aid through their channels and used this as a cloak to rebuild the shattered Khmer Rouge forces as an effective resistance against the Vietnamese. Thailand demanded that, as a condition for allowing international food aid for Cambodia to pass through its territory, food had to be supplied to the Khmer Rouge forces encamped in the Thai border region as well. Along with weaponry supplied by China, this international assistance was essential in enabling the Khmer Rouge to rebuild its military strength. The Khmer Rouge regrouped with food and shelter from willing donors and managed to fight on for another 20 years.

In June 1982 Sihanouk agreed, under pressure from China, to head a military and political front opposed to the Phnom Penh government The Sihanouk-led resistance coalition brought together - on paper, at least - Funcinpec (the French acronym for the National United Front for an Independent, Neutral, Peaceful and Cooperative Cambodia), which comprised a royalist group loyal to Sihanouk; the Khmer People's National Liberation Front, a noncommunist grouping formed by former prime minister Son Sann; and the Khmer Rouge, officially known as the Party of Democratic Kampuchea and by far the most powerful of the three. The undisputed crimes of the Khmer Rouge were conveniently overlooked to ensure a compromise to suit the great powers.

During the mid-1980s the British government dispatched the Special Air Service (SAS) to a Malaysian jungle camp to train guerrilla fighters in land mine-laying techniques. Although officially assisting the smaller factions, it is certain the Khmer Rouge benefited from this experience. It then used these new-found skills to intimidate and terrorise the Cam­bodian people. As part of its campaign to harass and isolate Hanoi, the USA gave more than US$15 million a year in aid to the noncommunist factions of the Khmer Rouge-dominated coalition and helped the group retain its seat at the UN assembly in New York. Those responsible for the genocide were representing their victims on the international stage.

For much of the 1980s Cambodia remained closed to the Western world, save for the presence of some aid groups. Government policy was effectively under the control of the Vietnamese so Cambodia found itself very much in the Eastern-bloc camp. The economy was in tatters for much of this period, as Cambodia, like Vietnam, suffered from the effects of a US-sponsored embargo.

In 1985 the Vietnamese overran all the major rebel camps inside Cambodia, forcing the Khmer Rouge and its allies to retreat into Thai­land. From that time the Khmer Rouge - and, to a limited extent, the other two factions - engaged in guerrilla warfare aimed at demoralis­ing its opponents. Tactics used by the Khmer Rouge included shelling government-controlled garrison towns, planting thousands of mines along roads and in rice-fields, attacking road transport, blowing up bridges, kidnapping village chiefs, and killing local administrators and school teachers. The Khmer Rouge also forced thousands of men, women and children living in the refugee camps it controlled to work as porters, ferrying ammunition and other supplies into Cambodia across heavily mined sections of the border. The Vietnamese for their part laid the world's longest minefield, known as K-5, stretching from the Gulf of Thailand to the Lao border, in an attempt to seal out the guerrillas. They also sent Cambodians into the forests to cut down trees on remote sections of road to prevent ambushes. Hundreds, surely thousands, died of disease and from injuries sustained from land mines.

By the late 1980s the military wing of Funcinpec, the Armée Nationale Sihanoukiste, had 12,000 troops; Son Sann's faction, plagued by inter­nal divisions, could field some 8000 soldiers; and the Khmer Rouge's National Army of Democratic Kampuchea was believed to have 40,000 troops. The army of the Phnom Penh government, the Kampuchean People's Revolutionary Armed Forces, had 50,000 regular soldiers and another 100,000 men and women serving in local militia forces.