From the 9th to the 13th centuries, the Mahāyāna and Hindu Khmer Empire dominated much of the Southeast Asian peninsula.
Under the Khmer Empire, more than 900 temples were built in Cambodia and in neighboring Thailand.
While Buddhism in Thailand remained under state centralization in the modern era, Buddhism experienced periods of tight state control and periods of liberalization depending on the government at the time.
Three major forces have influenced the development of Buddhism in Thailand.
By tradition, Pāli is the language of religion in Thailand.
Scriptures are recorded in Pāli, using either the modern Thai script or the older Khom and Tham scripts.
By the 19th century, and especially with the coming to power in 1851 of King Mongkut, who had been a monk himself for twenty-seven years, the sangha, like the kingdom, became steadily more centralized and hierarchical in nature and its links to the state more institutionalized.
The details of the history of Buddhism in Thailand from the 13th to the 19th century are obscure, in part because few historical records or religious texts survived the Burmese destruction of Ayutthaya, the capital city of the kingdom, in 1767. In Thailand, as in other Theravada Buddhist kingdoms, the king was in principle thought of as patron and protector of the religion (sasana) and the sangha, while sasana and the sangha were considered in turn the treasures of the polity and the signs of its legitimacy.When a king was weak, however, protection and supervision of the sangha also weakened, and the sangha declined.This fluctuating pattern appears to have continued until the emergence of the Chakri Dynasty in the last quarter of the 18th century.The administrative and sangha reforms that Mongkut started were continued by his successor.In 1902 King Chulalongkorn (Rama V, 1868–1910) made the new sangha hierarchy formal and permanent through the Sangha Law of 1902, which remained the foundation of sangha administration in modern Thailand.Religion and polity, however, remained separate domains, and in ordinary times the organizational links between the sangha and the king were not close.