Look at photos of Japanese, Korean &/or Thai people 50 or 100 years ago and it is easy to note dramatic changes. Dietary and lifestyle changes account for much of this, but also do attitude, expectations and self-image. Images engraved in stone at Angkor Wat show even more startling difference from modern Khmer.
Khmer is a language, not an ethnic group; same as with Vietnamese, Thai, Russian and Italian. As to who pulls who's strings, wires have become crossed, any more, and all understandings pretentious.
About 780 CE Jayavarman II came inland into mainland SE Asia, most likely from the Indonesian islands, and most likely with troops from there. At the time, the area was a patchwork of ethnic tribes, not unlike the Americas before "colonization".
Over a century earlier the Srivijaya Maritime Empire dominated the SE Asian seas as traders of forest product, luxury goods and whatever else they found source and market for. Mostly it eschewed administering locals, sticking to warehouses (for a while later called “factories” due to European managers called Factors) - but Ligor (Nakorn SriThammarat) was more than that, as also Borabadur, on Sumatra.
There's evidence that 1500 years ago coastal SE Asia was largely populated by dark skinned, wide nosed, kinky-haired people. As Angkor declined, Islam arrived in areas now Islamic, and Zhouang, Shan, Mon, Lue, Khoen and other lighter skinned, straight-haired people came down from the north. Migrations are a human norm, but even today many pockets of ethnically and linguistically unique remain. That history has been overwritten for nationalistic purposes is also a norm.

Massive ancient stoneworks aren't rare; a signigicant part of their utility may well have been population control. That many became hidden under jungle foliage speaks volumes as to how locals came to view them.
As far as border conflicts now, claims of historic justification are bogus. The conflicts are for gain, quite simply. Supposed hegemonic players like China, the CIA, International Jewry, the Thai elite &/or government (&/or military), secret societies – any and all appearance of hegemony is illusary. And factional alliance-shifting common (as per usual).
History may or may not repeat or rhyme, but perhaps legend and fairy tales do. Perhaps amusements for children and role-playing for teens (and DJT appointees) can tell us as much as all written 'history' combined.