Khmer (Civ6)/Civilopedia

Between the Ninth and Fifteenth Centuries, the god-warrior-kings of the Khmer Empire dominated Southeast Asia. Theirs was a formidable agricultural-martial kingdom whose wealth impressed even the mighty Chinese to the North. Unfortunately, all of that gold and rice would prove too attractive for their regional rivals, ultimately making the conquerors of the Suvarnabhumi the conquered.
Archaeology and Chinese historical records tell us the Khmer Empire can be traced back to the Mekong region in Southeast Asia during the First Century CE. The Chinese called this region and its disparate collection of peoples the Funan Kingdom (offhandedly putting a diverse, often warring group of principalities under one umbrella).
According to Khmer legend, these people were the product of the union of Cambodia's first king, Indian prince Preah Thong (Huntian, in the Chinese record) and Neang Neak, a divine serpent [naga] princess from a magical seas kingdom. The region is said to have been a wedding gift from the princess' father, who drained the waters around Nokor Kauk Thlork Island, making it habitable for the happy couple and their descendants.
The legend speaks to the tremendous influence of Hindu culture in the region. The Mekong-based people of Funan was the perfect waypoint for Indian travelers and traders headed west. The Indians would end up bringing Hinduism, their laws, commerce, and Sanskrit with them, ultimately blending with local animist traditions.
The people of Funan's Indian-influenced mini-principalities battled amongst themselves for centuries, and although they would briefly sustain centralized governments, it would take the firm hand of Jayavarman II in the Ninth Century to usher in the Angkor era of centralized rule.
The first conqueror king of the Khmer Empire got his start in the Ninth Century. Until that point, Jayavarman II was either a guest of the Javanese or their prisoner. Whichever the case, once he returned to his homeland, he seemed eager to get about the messy process of crushing the competition in the Mekong.
That messy business out of the way, the only thing left to do was to declare himself 'Cakravartin' or 'universal ruler' during a ceremony atop Mount Mahendra in the Kulen Mountains. And in 802 CE, that's precisely what Jayavarman II did, granting himself the backing of the gods in establishing his empire.
There's something to be said for his approach: at the height of their six century empire, the Khmer would dominate what is now modern most of Thailand and half of Vietnam, with over a million people living in the capital. By the Tenth Century, the empire would stretch from the South China Sea, with the Mongolian and Tang empires penning it in to the North. Not a bad spread if you wanted to maintain control of trade in the Mekong.
Between 1296 and 1297, Chinese official Zhou Daguan would visit the Khmer Empire. In his chronicle of that visit, 'A Record of Cambodia: A Land and Its People,' he would say: 'It has long been a trading country.' Zhou would describe a land of gold and stone towers, with cloth flowing from Siam and Champa and silk for locals' parasols from China.
The vast wealth of the Khmer would come from the constant flow of raw materials in and out of the Empire. The empire would feed Southeast Asia's need for rice, with some 80% of the Khmer population participating in either the production or trade of this staple food.
This would be the golden age of the Khmer Empire, when Suryavarman II would start construction on the temple complex at Angkor Wat (it would be completed 27 years after his death). Angkor Wat would mirror the shape of the mythical Mount Meru which was said to be the convergence of the physical, metaphysical, and spiritual worlds. In this way, the Khmer kings would try to mirror heaven on Earth.
This wasn't simple piety; these Hindu (and latest Buddhist under Jayavarman VII) royals were eager keeping the gods on their side. A king would do his best to maintain a bearing which would best reflect the gods, and build temples which would emulate the shape of the heavens. Under the Khmer's rule, the more powerful and attractive the king, the more followers and land he would have.
Of course, that would mean a weak king whose bearing wasn't sufficiently divine would no longer be worthy of his kingdom.
Ironically, the wealth and majesty of the Khmer Empire would prove its undoing. Between the Twelfth and Fourteenth Centuries, the Tai people in the North (now modern Thai, Laotian, and Shan) were contending with the ever-expanding Mongol Empire. So they pulled up stakes from their rugged, mountain lives and moved south, creating the smaller kingdoms of Sukhothai, Lanna, and Ayutthaya, which gnawed at the periphery of the Khmer Empire.
The Khmer were unable to contend with the Northern invaders as well as longstanding rivalries with Champa in the East. So by 1431, the Thai kingdom of Ayutthaya would take Angkor and the Khmer kings retreated to Phnom Penh, the current capital of Cambodia.
The empire may be gone, but their temples still stand. And to this day, the people of Cambodia still speak of coming from the sea, of their ancestors, a Brahmin and a Naga princess.