Two catastrophes hit Cambodia in the 1970s. On March 18, 1970, Cambodia was finally dragged into the Indochina conflict. Prince Norodom Sihanouk was overthrown in a pro-American coup. At the end of April, United States and South Vietnamese troops crossed into Cambodia.
The ground war was accompanied by some of the most intense aerial bombardment in history. US aircraft had started bombing parts of Cambodia near the Vietnamese border in 1969. After the coup, this was extended into the Cambodian interior. Much of it was carried out without the knowledge of Congress. By 1975, more than 2 million people had been made refugees and hundreds of thousands had been killed.
When Sihanouk was overthrown, the Khmer Rouge were a handful of guerrillas, ignored and unaided even by the Vietnamese communists. After the 1970 coup, the Khmer Rouge formed a Royal Government of National Unity (known by its French acronym, GRUNK) with Sihanouk supporters, and started receiving aid from China and Vietnam. In April 1975, they defeated the corrupt Lon Nol republic. and in January 1976, renamed the country Democratic Kampuchea. The Khmer Rouge immediately emptied the cities. Over the next 31/2 years, hundreds of thousands of Khmers died. Most died of hunger, exhaustion, and neglect, the result of the brutally utopian dreams of Pol Pot. But hundreds thousands more were executed -- mostly officials of the old regime, or members of the ruling Kampuchean Communist Party who who fell victim to the successive waves of purges. Until late 1976, the Khmer Rouge leadership was a shaky alliance of at least three factions: Vietnamese-trained communists; another group, basically pro-Chinese, but opposed to the wilder extremes of Pol Pot's policies; and the Pol Pot group. By early 1977 Pol Pot had consolidated his power. The worst phase of Democratic Kampuchea began. Purges picked up momentum: Workers in the cooperatives were made to eat communally; Khmer Rouge troops attacked villages on both the Thai and Vietnamese borders, massacring the inhabitants. The villages, they claimed, were on Khmer soil. On Christmas Day 1978 the Vietnamese invaded. On Jan. 7, 1979, they entered the capital city of Phnom Penh. For a regime that claimed its natural allies were the poorer peasantry -- 85 percent of the population -- Democratic Kampuchea had collapsed with surprising ease.