“..the Khmer Rouge was overthrown by a Vietnamese invasion.”
Going from worse to bad.
The rouge were pushed deep into the frontier jungle where they died mostly of disease.
IMO the KR relationship with Vietnam proves that it was some kind insane death cult.
The second secret agenda item of the KR (after destroying Khmer society) was to retake Kampuchea Krom (Little Cambodia), which is that part of Vietnam from about Tay Ninh/Saigon all the way down to the the tip of the country at Ca Mau. The territory had been lost by Cambodia several hundred years before, and the KR believed it still belonged to Cambodia (hmmm.... sounds just like the Muslims of today).
Anyway after multiple midnight attacks on Vietnamese border villages - in one case wiping out an entire town of about 1,500 by hand and knife - the KR made the big mistake of launching a rocket attack on the border town of Chau Doc.
That was the final straw, and the Vietnamese turned on the rather hardened war machine one more time, taking Phnom Penh in less than one week.
Pol Pot and his cronies ran into the jungle and conducted a useless insurgent campaign for the next 10 years.