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.g