Although a factor, most dedicated historians agree that it was the support of the Khmer Rouge by then-deposed King Sihanouk that caused their ascendancy. Historically and culturally, the people in the countryside have always unquestioningly heeded the word of their King out of respect for his sacred role and apparent divinity. Sihanouk thought he was leveraging this factor to his benefit to regain power, as he had lost face at home and abroad, and he also possessed a well documented passion for revenge – which he leveled against those who dissed him via the coup* (i.e., Lon Nol and Sirik Matak).
As a result of Sihanouk’s support, the KR went from a minor faction to a major and well armed fighting force very rapidly. In the end, it was the KR who were using Sihanouk towards their ends, and the King didn’t realize this until it was too late. Even so, even if he did not intend the results that eventually occurred – facts are facts, even if they do not neatly coincide with the bluechip notion of the US as the cause of all the regions’ problems during the 2nd Indohcina conflict.
This does not excuse the US from their awful mistake in taking over the from the French in Vietnam – the US is solely accountable for their decision to intervene there. But to blame the rise of the KR solely on the US bombing of Cambodia, which was undertaken mainly per the request of the collapsing Lon Nol government against the verified combined forces of the KR, the NVA, VC (*and various foreign national advisors) is neither historically accurate or reasonable.
But, it’s an attribution so frequently made, without much critical thinking or research, that’s this notion has by now become a ‘pop culture historical nugget’: bite sized, easy to chew, and easy to digest and share in semi-researched articles and documentaries about the subject. As I recall, even Noam Chomsky has unwittingly regurgitated and shared this particular quasi-historical nugget in his movie, ‘Manufacturing Consent’.
Of course, this suited his aims and thesis, so a little shoe-horning of facts can fit the bill for an eager audience – especially when one is singing to the choir!
(*the ‘US supported coup’ in support of Lon Nol is also one of those constantly regurgitated and bite sized historical nuggets, repeated so frequently that it’s taken on an aura of fact. In actuality, there has never been any definitive link between the US and the deposition of Sihanouk….any doubters can go and do the research. Again, though it’s easier and more fashionable at times just to go with the flow and repeat and ingest what is ‘common knowledge’ )