[ozborn]

It was communist controlled Vietnam which got rid of the Khmer Rouge about 45 years ago ending the genocide in Cambodia, it's entirely unclear what your hypothetical "winning" South Vietnamese government would have done. The US government was still mad enough with Vietnam that it applied more sanctions on Vietnam after Vietnam invaded Cambodia and the other major player (China) supported the Khmer Rouge and invaded Vietnam in retaliation.

[philwelch]

The United States and South Vietnam both opposed the Khmer Rouge during the Cambodian Civil War while the North Vietnamese supported them. The split between the Vietnamese communists and the Khmer Rouge only happened after the US allowed both South Vietnam and the previous Cambodian government to fall to communists.

[qwytw]

The genocide happened in the mid to late 70s, a couple of years after the US had already withdrawn from Vietnam.

Why did the US decide to support the Khmer Rouge only after they have significantly intensified their campaign of mass murder?