When the US invaded Cambodia in 1970, Prince Sihanouk was ousted and Lon Nol took charge. Sihanouk then allied with the independent Cambodian communists, and after a war in which the US bombed Cambodia more than Europe was bombed in WWII, the coalition of Sihanouk and the communists took over in 1975.
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.