In 1979 the Vietnamese invaded Cambodia and took over much of the country with the help of Vietnamese aligned communists.
The US then began arming the coalition of Sihanouk and the communists not aligned with Vietnam. It also fought for their seat at the UN.
So the US began arming and supporting Pol Pot in 1979, as did China. This was covered to one extent or another in the New York Times, Nightline etc.
Don't really get people tut tutting Chomsky being a Vietnam and Cambodia dove when the US was arming Pol Pot starting in 1979.