On April 17, 1975, after five years of civil war, the totalitarian communist party known as the Khmer Rouge (French: Red Khmers) invaded the Cambodian capital Phnom Penh and took over government of the country. Three days after taking the city, the Khmer Rouge ordered the evacuation of the city on the pretext that American bombers would attack the city once it had fallen to the Communists. Within hours, the city was largely emptied of its nearly 2 million inhabitants.
In the ensuing months, the majority of Cambodian civilians were herded onto collective farms and forced to work under brutal conditions, under which between 1.5 and 3 million Cambodians died due to starvation, illness, forced labor and execution.
The Khmer Rouge regime worked fervently to root out its perceived enemies, targeting politicians and soldiers of the former government. Believing that peasant class were "untainted" by capitalist sentiment, the government worked to eliminate all others to ensure a pure Maoist ideology. In pursuit of this goal, the Khmer Rouge sought out anyone fitting its broad definition of "intellectuals," including those who spoke a foreign languages or wore glasses or a wristwatch.
Torture, interrogation and execution
Those accused of disloyalty to Angkar (the supreme ruling body of the country, a mysterious organization to which all Cambodians were forced to pledge allegiance) were subjected to a brutal process of "re-education," which typically meant vicious interrogations where they were often beaten and forced to confess to their crimes against the state. Re-education was typically done at Khmer Rouge detention centres, such as s-21 (also known as Tuol Sleng) prison in Phnom Penh. Few survived the re-education process: between 1975-1979, some 20,000 prisoners were brought to s-21 and six are known to have survived.
While many prisoners died during the brutal interrogation sessions at the Khmer Rouge prisons, the majority of condemned civilians were taken to "killing fields" in rural areas, where they were forced to kneel at the edge of a mass grave before executioners killed them with blows to the head or neck. To save on ammunition, the executioners used improvised weapons like bamboo poles, axe handles and other blunt objects. At the Choeung Ek killing field outside of Phnom Penh, executioners often killed small children by holding them by their feet and swinging their heads against a large Chankiri tree.
Vietnam invaded Cambodia in 1979, ending the short but deadly rule of the Khmer Rouge. Vietnamese troops discovered several killing fields as they poured into the country, climaxing in the discovery of s-21, where they found they found many prisoners freshly beaten to death, chained to iron bedframes.
Since 1979, there have efforts and preservation and commemoration of the atrocities and victims of the Khmer Rouge. S-21 has since become the Tuol Sleng Genocide Museum. Many of the mass graves at were dug up and the site is now a tourist attraction graced by a large Buddhist stupa containing 5,000 skulls of those executed there.
The precise death toll of the Khmer Rouge has never been recorded. The remains of 8,895 victims were found at Choeung Ek, although some of the mass graves there were left undisturbed. 
Many of those complicit in the executions were never brought to justice, although efforts have continued well into the 21st century. Kang Kek Iew (Comrade Duch), the overseer of s-21, was charged with crimes against humanity in 2007.
The term "killing fields" was coined by Dith Pran, a Cambodian journalist who lived under the Khmer Rouge regime for over four years before escaping to the United States.