The rise of the Khmer Rouge to power was preceded by a CIA coup against the Cambodian government and a masssive US assault on Cambodia. US bombing of Cambodia killed at least several hundred thousand people. Apologists for right-wing genocide say nothing about this US-sponsored genocide prior to Pol Pot. It was this US assault which lead to the rise of the Khmer Rouge as it fought against the US-backed dictatorship. Prior to the CIA coup the Khmer Rouge were a minor party but they gained much more popularity after it.
Eventually the Khmer Rouge took power and committed the genocide rightists like to harp on. At the same time Indonesia invaded and conquered East Timor, murdering between one-forth and one-third of the population. This was done with US support & funding. As atrocities increased against the East Timorese US funding also increased to insure that Indonesia wouldn't run out of weapons to exterminate them with. Coverage in US media (which largely consisted of regurgitation of state department & indonesian lies) declined as atrocities increased. When they reached their highest point coverage dropped to zero. This happened at the same time as Pol Pot's genocide in Cambodia, which the media & capitalists were giving lots of attention to while ignoring the US-backed genocide in East Timor.
The Khmer Rouge were kicked out of power by a Vietnamese invasion, which ended the genocide. That's right, those other totalitarian lefties are the ones who put a stop to Pol Pot's genocide. The US's response was to support Pol Pot's guerilla war against Vietnam. The US assisted Pol Pot's attempt to get back in power, which failed, fortunetly.
Of course, capitalists only talk about the period from 1975-1979, when Cambodia's genocide wasn't supported by the US. They prefer to ignore the preceding US-sponsored genocide in Cambodia, the US-sponsored genocide in East Timor at the same time (which as a percentage of population was actually worse than Pol Pot) and the US's subsequent support for Pol Pot after the Vietnamese threw them out of power.