[haittiweerved]

Before the Thai people arrived in what is now called Thailand, there was a old civilisation called Dvaravati, consisting of the Mon people. Mon people spoke a language similar to khmer, but they had been theravada buddhists, not mahayana and hindu like the khmer!
The civilisation dates back to the late 6th century. The first Thai arriving in the today Thailand, came in contact with this buddhist civilisation and it is likely that they adopeted buddhism from them. King Mangrai of the northern kingdom Lanna even married the daughter of the Mon king from Pegu. Also the majority of the south part of his northern empire was Mon population. Wyatts write: "Mangrai was particularly respectful of Mon culture and Buddhism and early demonstrated his royal patronage of the religion. By encouraging the propagation of a new, more scholarly, and ostensibly authentic Sinhalese Buddhism toward the end of the century, however, Mangrai could begin to build a new common social and political idenity on a religious ground that was not so readily identifiable with either Mon or Thai." The Tai ruling families intermarried with the older indigenous ruling family of the Mon kingdom Haripunjaya and through the propagation of a common religion, Theravada buddhism they developed a new "thai" identity, which was acutally a mixture of Tai and Mon. I guess this happend not only in the north but everywhere where the Tai meet the Mon people. Mon people have been less organised then the khmer of angkor, and they treated their kings not like gods. In uniting with the tai immigrants, the built up a common idenity against the khmer civilisation. The leader in building this opposed idenity have been the frist tai kings, mangrai, ramkhamhaeng and Nam Müang (Payao). Under a common religion the different people united under the Tai kings against the Khmer people, who try to keep them under control and slavery.

Source: David K. Wyatt