This is a difficult question to answer, as you say there is no shortage of opinions about United States’ foreign policy and what things can be ascribed to them, what can’t… It is a game that people like to play, the blame game, and it is rarely a game that you win with nuance, even-handedness, or looking closely at a country’s history that people generally don’t have a thorough knowledge of.

That being said…

It is not an especially nuanced take on the rise and fall of the Khmer Rouge that does not include US misadventures in Indochina.

I feel that people often fall into at least one of a few pitfalls around talking about this part of history, you’ve outlined a couple in your question itself. People will often say ‘the US supported Pol Pot!’, without saying they did so after they fell from power and part of a strategy of rapprochement based on Chinese foreign interests, less so out of a simple ‘punishment’ of the Vietnamese backed PRK regime. So timing is key, and you’ve specifically asked about the rise of the Khmer Rouge, as opposed to those other people who also blame the US for what the Khmer Rouge did while they were in power.

So if we are not talking about during, or after, and the question is to what extent is the US responsible for the rise of the Khmer Rouge… well the answer on that particular front is not yunno, ‘none’. It almost goes without saying that without the particular circumstances of the Second Indochina War, the Khmer Rouge would not have came to power in the way they did. Historians generally do acknowledge that the extensive US bombing campaign of the countryside, the backing of the corrupt and ineffectual Lon Nol regime, the general destabilising of the region due to their attempts at containment of the USSR or PRC… these were all factors that have to be considered, and in a game of counterfactuals, you might be able to succinctly say that without the Vietnam War, there would be no Khmer Rouge.

That being said…

The Khmer Rouge (the name they are generally referred to in the west, they didn’t call themselves that, they are the Communist Party of Kampuchea) have a slightly more complicated and certainly longer history than most people talk about. Generally, particularly in the kinds of circles that you mention in your question, people just assume they popped into existence the day that the Cambodian Civil War began in 1970. The movement can be traced to the Indochinese Communist Party and efforts led by the Vietnamese (via the COMINTERN) to create a federation of socialist states to replace the French colonial control of Indochina.

Here is a quote from David Chandler’s Tragedy of Cambodian History, where he speaks about this early Vietnamese guidance for the Cambodian communists;

This trajectory was invisible in 1950, but prospects for a successful revolution were also difficult to discern. Cambodia’s revolution began, at Vietnamese insistence, without an indigenous proletariat or any communally organised grouping, aside from the Buddhist sangha. There was also a shortage of trained and dedicated cadre. That shortage combined in a volatile manner with the perceived necessity of ‘awakening’ Cambodian peasants to revolutionary violence. In the 1970’s, when the inflammable mixture became a national policy, one in four Cambodians died as a result.

We could get right into the weeds here about the circumstances of this meeting of the Indochinese Communist Party in Hatien and the subsequent split into the Vietnamese Workers Party, the Pathet Lao and the Kampuchean Revolutionary Peoples Party. The point I am making is that the Vietnamese were instrumental in beginning the Khmer Rouge, back in 1950 – before the US was even significantly supporting the French in the First Indochina War, let alone taking their own steps into the region.

In the late 1950’s and 60’s, another regime needs to be spoke about in this discussion of ‘the rise of the Khmer Rouge’, China. From their involvement in the Geneva Accords in 1954, their example of socialist revolution based on the peasantry, their involvement in Cambodian politics… not to mention their eventual funding of both the NLF, DRV and CPK, both in money and materiel. Perhaps not directly related to your question, but worth mentioning, that the most influential ideological source for the CPK while in power was Mao and Maoism. Their stated aim (and all of the suffering that came from it) was a ‘Super Great Leap Forward’, it isn’t too hard to imagine where they got that idea.

The French are another responsible party in the rise of the Khmer Rouge, not only during the colonial conflicts they clung to in the region, but also their influence in creating the conditions of modern Cambodia. The lack of education they introduced, the lack of modernising, the lack of effort in creating a political system that was functional – and not simply reliant on a figure like Prince Sihanouk. We could take that slightly further, the influence they brought to those future leaders of the CPK that studied in Paris in a period of French history with a highly Stalinist and widespread communist party.

Now, lastly, the group that is most responsible for the rise of the Khmer Rouge. The Cambodians themselves.

Here is a quote from Haing Ngor’s auto-biography;

But sad to say, the country that is most at fault for destroying Cambodia is Cambodia itself. Pol Pot was Cambodian. Lon Nol was Cambodian and so was Sihanouk. Together the leaders of the three regimes caused a political chain reaction resulting in the downfall and maybe the extinction of our country.

There is a lot in that that I feel people don’t often contemplate… Was S21 an invention of… the United States? China? Capitalism? Communism? Well, there are lots of different ingredients… but S21 was a creation of the Cambodian Communist Party – it was Cambodian. Cambodians aren’t just some mindless group that were told what to do by others or had no agency in what they did, whether their rise or what they did in power that was them.

Was the US ‘to some extent’ responsible for the circumstances that saw them come to power? Sure. US involvement in the region was, awful, but they had their reasons. Just like the Chinese and the Soviets had their reasons in supporting the other half of that equation. But Sihanouk’s decision to side with the CPK, lending his figurehead to their movement, that was the decisive moment in the expansion of the Khmer Rouge… not those that were forced into their arms by the destructive US bombing.

History is complex, and those using the Khmer Rouge to score ‘points’ for some ideological team are playing a game that is different to understanding history. While they aren’t completely wrong, they aren’t really trying to understand what happened and the circumstances involved.