the king was able to buy peace. Around the 13th century, Theravavada missionaries from Sri Lanka reintroduced Theravada Buddhism to Southeast Asia; having sent missionaries previously in 1190s. The religion spread and eventually displaced Hinduism and Mahayana Buddhism as the popular religion of Angkor; however it was not the official state religion until 1295 when Indravarman III took power. The Khmer Empire was Southeast Asia's largest empire during the 12th century. The empire's centre of power was Angkor, where a series of capitals were constructed during the empire's zenith. In 2007 an international team of researchers using satellite photographs and other modern techniques concluded that Angkor had been the largest pre-industrial city in the world with an urban sprawl of . The city, which could have supported a population of up to one million people and Angkor Wat, the best known and best-preserved religious temple at the site, still serves as a reminder of Cambodia's past as a major regional power. The empire, though in decline, remained a significant force in the region until its fall in the 15th century. Post-Angkor Period After a long series of wars with neighbouring kingdoms, Angkor was sacked by the Ayutthaya Kingdom and abandoned in 1432 because of ecological failure and infrastructure breakdown. This led to a period of economic, social, and cultural stagnation when the kingdom's internal affairs came increasingly under the control of its neighbours. By this time, the Khmer penchant for monument building had ceased. Older faiths such as Mahayana Buddhism and the Hindu cult of the god-king had been supplanted by Theravada Buddhism. The court moved the capital to Longvek where the kingdom sought to regain its glory through maritime trade. The first mention of Cambodia in European documents was in 1511 by the Portuguese. Portuguese travellers described the city as a place of flourishing wealth and foreign trade. Continued wars with Ayutthaya and the Vietnamese resulted in the loss of more territory and Longvek being conquered and destroyed by King Naresuan the Great of Ayutthaya in 1594. A new Khmer capital was established at Oudong south of Longvek in 1618, but its monarchs could survive only by entering into what amounted to alternating vassal relationships with the Siamese and Vietnamese for the next three centuries with only a few short-lived periods of relative independence. The hill tribe people in Cambodia were "hunted incessantly and carried off as slaves by