On Jan. 7, 1979, the communist Vietnamese army captured the Cambodian city of Phnom Penh, signaling the downfall of the bloodiest per-capita regime in world history: the Khmer Rouge.

Like Vietnam, Cambodia had been part of the French colonial empire in Indochina since the late 19th century. In 1953, Cambodia gained its independence and was ruled a constitutional monarchy until a coup in 1970. What followed were several years of civil war between a nationalist, pro-Western government and a radical new communist movement, the Khmer Rouge.

During the Vietnam War, the communist North Vietnamese funneled weapons and logistical support to the Viet Cong guerrillas in South Vietnam through the Ho Chi Minh trail, which ran partially through Cambodia. In 1973, U.S. President Richard Nixon authorized the bombing of the trail in Cambodia. Many historians suggest that this move marginalized the nationalist government and swelled the ranks of the Khmer Rouge.

The Khmer Rouge ultimately triumphed in the civil war and came to power in 1975. Changing the name of the country to Democratic Kampuchea, the regime and its leader, Pol Pot, sought to completely expunge the past and create a more radical form of communism. What followed was one of the most uncompromising experiments in social engineering in history.

In his book “The Red Flag: A History of Communism,” historian David Priestland wrote: “Money was abolished, and everybody ... became laborers on collective farms. Urban life was destroyed, the cities emptied, schools closed. The country became one large agricultural labor camp, and the lives of all were devoted to labor and political education.

"The regime sought to destroy old hierarchies of all sorts. Children were expected to call their parents 'comrade father' and 'comrade mother' and the use of the term 'sir' was banned. Only marriages approved by the party were allowed. Pol Pot even declared that 'mothers should not get too entangled with their offspring' and communal dining halls were introduced to stop family bonding.”

Sexual relations outside of marriage were outlawed and punishable by death, as was the consumption of alcohol. Anyone with any kind of formal education was suspect, and anyone with a college degree was executed. Anyone who had been involved in any “free-market activities” was also executed. Those who appeared to shirk their duty to cultivate the fields could be murdered as saboteurs. Anyone participating in any kind of religious practice or ritual was put to death.

One terrifying policy that echoed the famous “Twilight Zone” episode “It's a Good Life” saw children put in command of revolutionary cadres. With this power the regime hoped to indoctrinate the next generation in brutality, as they were allowed to oversee torture and executions.

From 1975 to 1979, during the period of the Khmer Rouge's rule, Cambodia became known as “The Killing Fields.” In addition to those who were executed as declared political enemies, many Cambodians were worked to death or died from starvation as the collective farms failed to keep up with demand.

It's estimated that somewhere between 1.5 million and 2 million Cambodians were killed during this period. If the higher number is correct, that represented more than 20 percent of the population of Cambodia in 1975.

For all of its xenophobic policies, the Khmer Rouge did not operate in a vacuum, however, and Cambodia soon found itself drawn into the larger communist division known as the Sino-Soviet split. Far from being socialist allies, by the late 1960s communist China and the Soviet Union had become bitter rivals and both sought to lead the wider communist world.

While the Vietnamese communist party had looked to the USSR for guidance, the Khmer Rouge looked to China. This ideological difference sparked conflict along the Vietnamese-Cambodian border, and soon ethnic Vietnamese living in Cambodia were also targeted by the regime.

This conflict came to a head in late 1978 when the Vietnamese government, no stranger to atrocities itself, decided to intervene and stop the rampant killing in Cambodia. On Jan. 7, 1979, the capital city fell to the Vietnamese.

In his book “The Rise and Fall of Communism,” historian Archie Brown wrote: “After almost two years of border clashes, Vietnamese forces invaded Cambodia/Kampuchea at the end of December 1978. Their intention was to dislodge the Khmer Rouge — not, of course, to put an end to communist rule. Most of the Cambodian population welcomed them as liberators, and in the course of that year a more 'normal' communist government than Pol Pot's was established, under Vietnamese supervision.”

The ousting of the Khmer Rouge from power was not the end of the movement, however. China still supported the party as a resistance group, which operated out of western Cambodia and Thailand, and even invaded Vietnam itself to restore the Khmer Rouge in Cambodia. The Chinese invasion of Vietnam, however, proved short-lived and unsuccessful.

Despite U.S. President Jimmy Carter's emphasis on international human rights, the United States opposed the actions of the Soviet-backed Vietnamese and gave support to the defunct Khmer Rouge. This was also done because the U.S. was seeking better relations with China (Carter extended formal diplomatic recognition to China on Jan. 1, 1979).

The Khmer Rouge continued to formally represent Cambodia in the United Nations until the early 1990s. Pol Pot died in 1998 and the Khmer Rouge officially dissolved the next year.

In his book “The Cold War: A New History,” historian John Lewis Gaddis wrote of the Khmer Rouge's legacy: “No tyrant anywhere had ever executed a fifth of his own people, and yet the Khmer Rouge leader Pol Pot did precisely this in the aftermath of the Vietnam War. The future will surely remember that atrocity when it has forgotten much else about the Cold War, and yet hardly anyone outside of Cambodia noticed at the time.

"There was no trial for crimes against humanity: Pol Pot died in a simple shack along the Thai border in 1998 and was unceremoniously cremated on a heap of junk and old tires. At least there was no mausoleum.”

Cody K. Carlson holds a master's degree in history from the University of Utah and currently teaches at Salt Lake Community College. He is also the co-developer of the popular History Challenge iPhone/iPad apps. Email: ckcarlson76@gmail.com