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In 1975 the coalition led, nominally or not, by Sihanouk gained control of Kampuchea (again, in the case of Sihanouk). The US and west said a genocide was happening between 1975 and 1979, but immediately somewhat covertly, and then openly, began arming the coalition with Pol Pot and what the west called the Khmer Rouge.
Read the section on Chomsky. I have no desire to engage with someone who does not acknowledge what Chomsky himself admitted. It is the equivalent of saying that Jews were lying about the Holocaust because the allies happened to be the ones who liberated them
Also, as I said, the US began arming the Khmer Rouge in 1979, and began openly arming them in the 1980s. So if the Khmer Rouge had committed a genocide, then that's who the US was arming to put back in power.