A very short history

Cyrus (a Latinized form derived from a Greek form of the Old Persian Kūruš) the Great, Cyrus II, conquered most of Southwest Asia plus much of Central Asia and the Caucasus, making the largest empire the world had yet seen; but he respected the customs and religions of the lands he conquered and established a government that worked well, much to the advantage of its subjects. He was preceded as king of Persia by his father, grandfather and great-grandfather. His father and son were named Cambysus, from which name that of the rulers of Angkor, the Khom, derives.
Ancestors of Darius, Xerxes and Cyrus the Great came down from rugged mountains east of Persia and Babylonia, perhaps soon after 1000 BCE. Like others did so many times from Manchuria, Altai, Mongolia and Tibet, they used larger stature and greater physical strength to overcome an agricultural people, and make a new empire. Most likely Gautama Buddha was a Khom descendent, more likely from Pakistan than Nepal.
Cambysus II died just west of Egypt, in a sandstorm that destroyed his army, but Khom descendents (and other Aryans) had already gone to northwest India, influencing not only the caste system (especially the Bramin and Kasytria castes), but also Pali and Sanskrit languages. The magic of language and commerce became the backbone of social distinction and authority in the sub-continent and coastal Southeast Asia, and an extensive maritime system, the Srivijaya 'Empire', arose, demonstrating more of the Khom organizational and administrative expertise. It had centers at Ligor (now Nakorn SriThammarat) and Borabadur, and extended from Madagascar to Cochin China (Vietnam). The Srivijayan empire, a commercial empire of manner utilized later in the earlier days of the Dutch and British East India companies, mainly exercised its influence around coastal areas of Southeast Asia, for trade (in which it was highly successful). About 770 CE, perhaps a decade after, Srivijian Khom prince "Jayavarman II" abandoned Khom antecedent principals, first usurping a name and then an empire. He proclaimed independence for Kambuja from Javanese (Srivijian) dominion, and established rule over local people, something his Srivijarian compatriots had been reluctant to attempt. He was most likely able to accomplish this through utilization of a '5th column' of already in-place Khom traders working the Lower Mekhong.
Wikipedia reports, "The most valuable inscription concerning Jayavarman II is the one dated in 1052 AD, two centuries after his death, and found at the Sdok Kak Thom temple in present day Thailand. 'When His Majesty Paramesvara came from Java to reign in the royal city of Indrapura,…Sivakaivalya, the family’s learned patriarch, was serving as his guru and held the post of royal chaplain to His Majesty,' states the inscription, using the king’s posthumous name. In a later passage, the text says that a Brahman named Hiranyadama, 'proficient in the lore of magic power, came from Janapada in response to His Majesty’s having invited him to perform a sublime rite which would release Kambujadesa [the kingdom] from being any longer subject to Java.' The text also recounts the creation of the cult of the devaraja, the key religious ceremony in the court of Jayavarman and subsequent Khmer people."
Angkor thrived for centuries, bedazzling locals with mysterious rituals, symbols, incantations and other exotic imports, then fell due to too few masters (less than one) per hundered slaves, plus a lack of, or at least insufficient, middle class (not Black Plague or Tai invasions or water problems, as have been claimed, although those might have contributed). One of the last of the Khom rulers, Ramkamhaeng of Sukotai, had an extensive economic (mostly pottery) empire which allowed him and his peers to envision uniting various cultural legacies, legacies influenced by the Khom and Srivijara (Mon, Malay, Lao, Shan, Khmer) cultures. With combining new realities (more inland influence, increased trade with China, increasingly perceived need for well-trained soldiers) into a new set of 'traditions' based on magic and pyrmidal social structuring, the new Tai/Siamese polity was able to successfully comptete with the equally multi-ethnic Burmese and Vietnamese kingdoms.

When, about 1350, a trading port for a new empire to replace Angor was founded at Ayudhaya, it's law utilized the ancient Brahmin Laws of Manu, which state, "Do not let the producing classes, the lowest castes, accumulate wealth. Dispossess them of their wealth as soon as they may gain it. They are there to serve the higher-caste people.” Despite retaining the weaknesses of Angkor, by propping the system up through advances recieved from Chinese, Indian and Persian influences, Ayudhaya was able to become the largest metropolis in the world - for a while.