Khmer Rouge
The Cambodian communist movement that emptied the cities in 1975, declared Year Zero, and killed roughly a quarter of the population in under four years.
The Khmer Rouge ruled Cambodia for three years, eight months, and twenty days, and in that time killed somewhere between 1.7 and 2 million people, close to a quarter of the country. It is the purest experiment in what happens when a movement decides that a society is a blank page and that the people on it are obstacles. There were no death camps in the industrial sense and very little machinery. Most victims were worked to death, starved, or killed with farm tools.
How they got power
The movement was small, rural, and going nowhere until the wider war arrived. From 1969 the United States bombed eastern Cambodia to hit North Vietnamese supply lines, dropping a tonnage on a neutral country comparable to what fell on Japan in the Second World War. In 1970 General Lon Nol overthrew Prince Norodom Sihanouk in a coup, and the deposed prince, from exile in Beijing, told his rural subjects to support the communists. Bombed peasants and a royal endorsement turned a fringe guerrilla band into an army. On 17 April 1975 it walked into Phnom Penh.
Year Zero
Within 72 hours the Khmer Rouge emptied the capital, driving roughly two million people, including hospital patients on their beds, into the countryside on foot. Money was abolished, and the central bank was blown up. Markets, private property, schools, courts, newspapers, religion, and eventually the family meal were abolished with it. The calendar restarted at Year Zero. The population was sorted into "base people," peasants who had lived under Khmer Rouge control, and "new people" from the cities, who were presumed contaminated and expendable. Wearing glasses, speaking French, or having soft hands could be evidence.
The leadership, known only as Angkar, the Organization, was largely a group of Paris-educated intellectuals: Saloth Sar, who took the name Pol Pot, along with Ieng Sary, Khieu Samphan, and Nuon Chea. Their goal was an agrarian communist utopia reached in a single leap, funded by tripling rice yields through sheer labor. The quotas were fantasy, so the rice was requisitioned anyway and the workers starved. When the plan failed, the leadership concluded it was being sabotaged, and turned inward. At S-21, a Phnom Penh high school converted into an interrogation center, around 15,000 people were tortured into naming accomplices and then driven to the killing fields at Choeung Ek. A handful survived.
The Cold War protects them
Vietnam ended it. After border raids and massacres of Vietnamese civilians, Hanoi invaded in December 1978 and took Phnom Penh on 7 January 1979. What followed is one of the coldest facts of the Cold War: because Vietnam was aligned with Moscow, China backed the ousted Khmer Rouge, and the United States and its ASEAN partners went along. The genocidal regime kept Cambodia's seat at the United Nations into the 1990s, its guerrillas were resupplied on the Thai border, and the government that stopped the killing was treated as the occupier.
Pol Pot died in the jungle in 1998, never tried. The UN-backed tribunal that finally sat in Phnom Penh took sixteen years and hundreds of millions of dollars to convict three men.
Cambodia today has a median age in the twenties, an economy built on garments and tourism, and a political class descended from Khmer Rouge defectors, including Hun Sen, who ruled for 38 years. The country never got a reckoning so much as an agreement to move on. The lesson it left is narrower and harsher than the usual one about ideology: utopian projects kill in proportion to how little friction the state meets, and great powers will shelter almost anyone who is inconvenient to their rival.