In 1975 about 10% of the population in Cambodia, about 600,000 had already died as a result of US actions during the Vietnam War (1). The US invaded Cambodia several times looking for Vietcong bases of operation. This “searching” in practise was a steady bombing campaign of the Cambodian countryside.
It was US President Richard Nixon who ordered the bombings and then lied about it, turning the tide of public opinion of the war around against it. Hypocritically, most of the people who complained about Pol Pot supported the bombings of Cambodia.
The US bombing not only killed 2 or 3 million people, but it also created abut 2 million refugees who fled the country side and went into the cities. As the US was destroying the Cambodian countryside the country became dependent on U.S. food aid. Even this was horribly mismanaged because of the inefficiency of the right-wing Lon Nol regime.
What this means that when the Khmer Rouge came into power during the worst possible condition: Mass starvation. Cambodia was poorest country in the world and one-third of the population were refugees.
In the mass starvation that was taking place, it was absolutely necessary to get food production going again as fast as possible. If it didn’t then possibly the entire country could have gone through an even greater famine killing millions. In this emergency situation Pol Pot order people Phnom Penh and move to the countryside in order to begin farming again. The country’s economy was absolutely crushed and unable to function (2).
On a side note, this in ability of anti-communists to fail to understand that people need to eat is a recurring theme. After the shortage of grain due to weather conditions, pro-market farmers in the Soviet Union had deliberately withheld grain in order to inflate the price. As people were going hungry these farmers sought to make even more money by making more people hungry. Stalin responded by collectivizing these farms to get food production going again. Anti-communists think Stalin is bad guy for trying to minimize this suffering. Farms were collectivized because people need to eat.
After the atrocities of Pol Pot, actually communist Vietnam invaded to bring this nightmare to an end.
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This massive 2 or 3 million figure is a direct result of all deaths from 1975 to 1979, based on estimates of the population. In actuality a Finnish inquiry determined that 1 million or fewer people actually died in the Pol Pot regime. Several thousand of those were a the result of the clashes with the Vietnamese military.
Another famine started after the invasion by Vietnam, however international aid was already too late. By then 2 million or 30% of the population had died as a result of US bombings, Pol Pot regime and the Vietnamese invasion combined.