Vietnamese troops crossed the border in force on December 25, 1978, after months of Khmer Rouge cross-border massacres against Vietnamese villages. The offensive, which captured Phnom Penh two weeks later, ended the Khmer Rouge's four-year reign of terror that had killed roughly 1.7 million Cambodians, but triggered a decade-long Vietnamese occupation and a bruising border war with China.
By late 1978, Pol Pot's regime had turned Cambodia into a vast, blood-soaked experiment in agrarian communism, emptying cities, abolishing money, and executing anyone suspected of education, foreign ties, or dissent. Border raids by Khmer Rouge units into Vietnam, including massacres of civilians in villages like Ba Chuc, gave Hanoi its pretext, though ideological rivalry between the two communist states ran deeper.
On December 25, roughly 150,000 Vietnamese troops, backed by Cambodian defectors organized as the Kampuchean United Front for National Salvation, rolled across the frontier in a rapid, mechanized campaign. Phnom Penh fell on January 7, 1979, and Pol Pot's forces retreated to the Thai border to wage a guerrilla war that would drag on for another decade.
The invasion ended the Khmer Rouge's genocide but isolated Vietnam diplomatically — China, the United States, and much of the world condemned the occupation, and China launched a punitive invasion of Vietnam's northern border weeks later. Cambodia would not see a UN-brokered peace until 1991.