You also have to factor in how much the US bombing of Cambodia led to the conditions ripe for the Khmer Rouge.

IIRC Cambodia is the most bombed country in history.

You are jokig, right? I mean, the Vietnam and China-backed communist regime of neighboring Cambodja, who murdered millions implementing the same political reforms that killed millions in communist China, is only guilty of crimes against humanity because the US bombed them? Is that your argument?

And yet it was Vietnam that overthrew the Khmer Rouge, which was supported by China and the US. The ideology of the Khmer Rouge was very different from that of the Viet Minh. It was an ideology that demonized industry, cities and workers - all very much the opposite of the ideology of the Viet Minh (and of most of the Communist Party of China). It's not for nothing that the Khmer Rouge ideology has been called "Stone-Age Communism."

> And yet it was Vietnam that overthrew the Khmer Rouge

...in response to the Khmer Rouge's attacks on Vietnam.

https://en.m.wikipedia.org/wiki/Cambodian%E2%80%93Vietnamese...

> which was supported by China and the US.

Vietnam was concerned Cambodja's regime was too China-friendly, but the millions being killed under the banner of advancing socialism did not featured as a pressing concern by neither China nor Vietnam.

The Khmer Rouge's ideology had almost nothing in common with the ideology of the Vietnamese Communists, or really any Marxist movement. It's difficult to see how an ideology that is intensely hostile towards all forms of industrial society and even towards the very existence of cities has anything to do with an ideology that has rapid industrial development as one of its central pillars. After the Vietnamese overthrew the Khmer Rouge, they put a very different type of government in place.

There were, of course, geopolitical calculations on all sides, which is why the United States and China supported the Khmer Rouge. I don't think moral revulsion alone often leads any government to declare war on another.