On the seventeenth of April, 1975, the Khmer Rouge marched into Phnom Penh, and declared at one o'clock that the city of two million inhabitants be abandoned. The sick and the infirm were forced from hospital beds at gunpoint; surgeons were forced to abandon patients mid-operation; orphaned babies were left abandoned in the Phnom Penh paediatric centre. This was the first day of Year Zero in what was renamed Democratic Kampuchea.
Over the course of the following four years, approximately a fifth of the population of Cambodia died at the hands of the Khmer Rouge, led by Brother Number One, Pol Pot. People who lived in cities were executed as "economic saboteurs"; people with educations, people with more than one language were executed as "bourgeoisie"; people who wore glasses were assumed to be literate and therefore deserving of execution. What happened in Cambodia in those four years is almost without parallel in recorded history; survivors recall people being executed for smiling at each other. In Tuol Sleng prison, where seventeen thousand people were incarcerated over the duration of the Khmer Rouge's regime, twelve people are known to have survived. It wasn't a prison in any meaningful sense; it was a death factory. The entire population was forced into agricultural labour camps, where hundreds of thousands of people starved; anyone caught picking wild fruit was guilty of private enterprise and executed.
By 1978, as a result of waves of refugees fleeing into Vietnam, relations between the two countries collapsed, and Pol Pot ordered a preemptive invasion. Unsurprisingly, given that the Vietnamese had spent the previous decade or so fighting the United States military, they were battle-hardened and well equipped; it took them about a month to force the Khmer Rouge into the mountains near the Thai border.
This is where the United States comes in.
The new Vietnamese-installed government never took its seat at the United Nations; instead, the Khmer Rouge were officially classed as the legitimate government of Kampuchea by most Western governments, led by the USA and the UK. This continued until 1993; incredibly, when Vietnam proposed a full withdrawal in return for the exclusion of the Khmer Rouge from any government, the offer was rejected. Thanks to this decision, aid from the World Food Programme was handed over to the Khmer Rouge, to sustain their troops while their victims starved to death. Meanwhile, American government agencies disseminated stories to the effect that the famine killing people in Cambodia was the fault of the Vietnamese occupiers. All this was because the Cambodians had the misfortune to be rescued from the Khmer Rouge by the wrong people.
Fifteen years. Fifteen fucking years of feeding an army that committed one of the worst genocides in history while starving their surviving victims. It would have been a moral failure of historic proportions to side with the Khmer Rouge for a single day, and the American government did it for a decade and a half. Go and stand in the killing fields outside Phnom Penh, see the bone fragments coming up through the soil of unmarked mass graves, see the tree where the Khmer Rouge smashed in the heads of newborns, and then remember that the people who did this were fed at the orders of American government officials for fifteen years. Then see how comfortable you feel claiming that US foreign policy is driven by altruism.