Under Pol Pot's leadership, and within days of overthrowing the government, the Khmer Rouge embarked on an organised mission: they ruthlessly imposed an extremist programme to reconstruct Cambodia (now under its Khmer name Kampuchea) on the communist model of Mao's China. The population must, they believed, be made to work as labourers in one huge federation of collective farms. Anyone in opposition - and all intellectuals and educated people were assumed to be - must be eliminated, together with all un-communist aspects of traditional Cambodian society.
So, at short notice and under threat of death, the inhabitants of towns and cities were forced to leave them. The ill, disabled, old and very young were driven out as well, regardless of their physical condition: no-one was spared the exodus. People who refused to leave were killed; so were those who didn't leave fast enough, and those who wouldn't obey orders.
All political and civil rights were abolished. Children were taken from their parents and placed in separate forced labour camps. Factories, schools and universities were shut down; so were hospitals. Lawyers, doctors, teachers, engineers, scientists and professional people in any field (including the army) were murdered, together with their extended families. Religion was banned, all leading Buddhist monks were killed and almost all temples destroyed. Music and radio sets were also banned. It was possible for people to be shot simply for knowing a foreign language, wearing glasses, laughing, or crying. One Khmer slogan ran 'To spare you is no profit, to destroy you is no loss.'
People who escaped murder became unpaid labourers, working on minimum rations and for impossibly long hours. They slept and ate in uncomfortable communes deliberately chosen to be as far as possible from their old homes. Personal relationships were discouraged; so were expressions of affection. People soon became weak from overwork and starvation, and after that fell ill, for which there was no treatment except death.
Also targeted were minority groups, victims of the Khmer Rouge's racism. These included ethnic Chinese, Vietnamese and Thai, and also Cambodians with Chinese, Vietnamese or Thai ancestry. Half the Cham Muslim population was murdered, and 8,000 Christians.
The imposition of a murderous regime always leaves its leaders afraid: afraid of losing power, failing to prevent vengeance, and facing betrayal by ambitious rivals. The Khmer Rouge repeatedly interrogated their own members, imprisoning and executing them on the slightest suspicion of treachery or sabotage.
Civilian deaths in this period, from executions, disease, exhaustion and starvation, have been estimated at well over 2m.
With macabre bureaucratic precision the Kmer Rouge photographed their victims before torturing and killing them. This picture is all that remains of a mother and child murdered in Tuol Sleng Prison sometime between 1975 and 1979.