For a little more context: The rise of the Khmer Rouge and Pol Pot was basically triggered by the US carpet bombing Cambodia. After the US military got thrown the fuck out of Viet Nam, the direct invasion may have ended, but the CIA decided to double down and keep the war going through regional proxy forces, namely the Khmer Rouge, which the CIA was arming, funding and providing military intelligence to. The Khmer Rouge butchered a lot of civilians, but failed pathetically at invading Viet Nam and repelling counterforces. Border regions of Cambodia spent a decade under NVA occupation as a result, while the CIA and KR ran operations out of N. Cambodia and Thailand.

The CIA also kept stirring up fear and conflict with Viet Nam's other neighbors, which is where that narrative of "The NVA and USSR won't stop at the border" came from. My understanding is that the CIA was directly in contact with and trying to convince the PRC that Pol Pot was the real communist revolutionary and needed help, and that the Soviets were using a puppet regime to encircle China. Ofc it was just projection and domino theory bullshit. Deng and the entire PRC should have fucking known better.