The Khmer Rouge were a Communist movement allied to the Viet Cong. When the United States military pulled out of Vietnam and Cambodia in 1975, they left a power vacuum that their opponents were quick to exploit. The Khmer Rouge, led by Pol Pot, championed a particularly oppressive form of dictatorship that called for a return to medieval technology and an abandonment of urbanisation.
With the fall of the Cambodian capital of Phnom Penh, the Khmer Rouge took control of the country. All the citizens of Cambodia were forced to leave the cities, to practice subsistence agriculture in the rural areas. The regime was infamous for its cruelty and brutality, to say nothing of its near genocidal policies. It is estimated that in the four years of their reign, as many as two million people were killed, either in concentration camps, summary executions or simple starvation. In fact, during the years of the Khmer Rouge, Cambodia became known as the Killing Fields – more than 20,000 separate mass graves were created in these years.