During the Vietnam War, Cambodia was used as a base by the People's Army of Vietnam and the Viet Cong, and thousands of refugees from across the country flooded the city to escape the fighting between their own government troops, the People's Army of Vietnam, the Viet Cong, the South Vietnamese and their allies, the Khmer Rouge, and American air strikes. By 1975, the population was 2–3 million, the bulk of whom were refugees from the fighting. The Khmer Rouge cut off supplies to the city for more than a year before it fell on April 17, 1975. Reports from journalists stated that the Khmer Rouge shelling "tortured the capital almost continuously", inflicting "random death and mutilation" on millions of trapped civilians. The Khmer Rouge forcibly evacuated the entire city after taking it, in what has been described as a death march: François Ponchaud wrote that "I shall never forget one cripple who had neither hands nor feet, writhing along the ground like a severed worm, or a weeping father carrying his ten-year old daughter wrapped in a sheet tied around his neck like a sling, or the man with his foot dangling at the end of a leg to which it was attached by nothing but skin"; Jon Swain recalled that the Khmer Rouge were "tipping out patients from the hospitals like garbage into the streets....In five years of war, this is the greatest caravan of human misery I have seen". All of its residents, including the wealthy and educated, were evacuated from the city and forced to do difficult labour on rural farms as "new people". Tuol Sleng High School was taken over by Pol Pot's forces and was turned into the S-21 prison camp, where people were detained and tortured. Pol Pot sought a return to an agrarian economy and therefore killed many people perceived as educated, "lazy" or political enemies. Many others starved to death as a result of failure of the agrarian society and the sale of Cambodia's rice to China in exchange for bullets and weaponry. The former high school is now the Tuol Sleng Genocide Museum, where Khmer Rouge torture devices and photos of their victims are displayed. Choeung Ek (The Killing Fields),  away, where the Khmer Rouge marched prisoners from Tuol Sleng to be murdered and buried in shallow pits, is also now a memorial to those who were killed by the regime.