The Khmer Rouge executed between half a million and a million Cambodians between 1975 to 1979[0]. These were the intentional killings, estimates range to as many as 2 million Cambodians or 25% of the population died as a result of Khmer Rouge polices.

The end of the regime was brought about by an incursion into the Vietnamese border town of Ba Chúc, resulting in the massacre of more than 3000 civilians. Vietnam invaded, toppled the Khmer Rouge and brought an end to the executions although civil war would continue for much of the next decade.

For these actions Vietnam was extensively sanctioned[1]. The parallels with ongoing conflicts today are hard to ignore.

[0] https://en.wikipedia.org/wiki/Khmer_Rouge#Crimes_against_hum...

[1] https://en.wikipedia.org/wiki/Cambodian%E2%80%93Vietnamese_W...