After years of deadly Khmer Rouge raids across its border, Vietnam invaded Cambodia on December 25, 1978. The Khmer Rouge army collapsed within two weeks, and on January 7, 1979, Vietnamese forces entered a nearly deserted Phnom Penh. Pol Pot's leadership escaped west toward Thailand. Vietnam put in a new government, the People's Republic of Kampuchea, but the Khmer Rouge regrouped along the border with backing from China and Thailand, and the civil war went on for years.
When Vietnamese troops arrived in Phnom Penh, the city was almost empty. The Khmer Rouge had driven its people into the countryside in April 1975, the start of what they called Year Zero. Under their rule money, schools and religion were abolished, and people who were educated, belonged to minorities or were suspected of disloyalty were killed. Starvation, forced labour and executions killed an estimated 1.5 to 2 million people, close to a quarter of the population.
In the days after the city fell, the invaders found Tuol Sleng, a former high school that had been turned into Security Prison S-21. Its guards had photographed and recorded thousands of prisoners before sending them to their deaths. Those files became some of the most damning evidence of the regime's crimes.
The liberation had a complicated aftermath. Most of the world refused to recognise the government Vietnam installed, and the Khmer Rouge kept Cambodia's seat at the United Nations for years. Vietnamese forces did not fully withdraw until 1989. In Cambodia, January 7 is still a public holiday marking the end of the regime, though it remains politically disputed.
Key people: Pol Pot, Heng Samrin