The Thais entered their present territory from south-western China at a comparatively late date. The earliest reports of their presence come from Chinese annals, which, however, are very vague until about 650 A.D., when we hear more definite accounts with the foundation of the first important Thai state of Nan Chao (though this is still a matter of controversy). From small beginnings, this Kingdom became a considerable power, embracing practically the whole of Yunnan and parts of Szechwan. It retained its independence till 1253, when it was destroyed by the Mongols under Kublai Khan.
Nan Chao maintained close, and mainly friendly, relations with China, under whose cultural influence it stood: the educated classes spoke Chinese, Nan Chao princes studied in China and Chinese artisans brought their skills into the country. But centuries before the collapse of the Nan Chao state various tribes began their southward trek in the ancient tradition of the Asiatic peoples.
The entry of the Thais into South-East Asia was generally peaceful. They infiltrated into the new territories in small groups in the course of centuries, and became assimilated to the local population. In the north of the area the first Thai principalities arose probably in the 11 th century. The figures of Thai warriors at Angkor Vat (12th century) show that at this time there must have been a number of such states.
The territory of modem Thailand was ruled from the 6th to the 13th century by the Mons and Khmers. In Central Thailand, from the 6th century onwards, the state of Dvaravati developed. This was conquered in the 11th century by the Khmers advancing over the Menam Plain. Only around Lamphun was the small Mon Kingdom of Haripunjaya able to maintain its independence.
The Khmer Kingdom reached its greatest extent from about 1000 to 1250, taking in practically the whole of Cambodia, Cochin-China, Laos and Thailand. In the north-east it bordered on Annam, which had freed itself from Chinese control in the 10th century, and farther south on Champa, a power strongly hostile to the Khmers. In Burma the kingdom of Pagan had developed, reaching the peak of its power in the 11th century. The Malay peninsula was under the control of the sea-power of Srivijaya. All of these South-East Asian states, Pagan, Haripunjaya, Champa and Cambodia, with the exception of the Chinese state of Annam, had for a thousand years been under the influence of Indian culture. Among the Khmers it was Brahmanism which determined religious life, and with it art and culture. Among the Mons, on the other hand, Buddhism, especially in its early Theravada form, prevailed. But even among the Khmers, who introduced the Brahman court ritual and the Brahman idea of the god-king, it is clear that the bulk of the population in Thailand were attached to Theravada Buddhism.
Accordingly the invading Thais did not find a cultural or political vacuum in their new homes. Probably with their greater vitality they simply assimilated the indigenous Mons and Khmers, absorbed their superior culture and gradually eliminated them from leading positions. By the end of the 13th century the Thais were the leading ethnic group in the entire peninsula.
In the north of Thailand, Thai princes conquered the old Mon kingdom of Haripunjaya and founded Lan Na, which retained its independence till the 16th century. A series of Thai principalities flourished on the upper Mekong. In the mid 14th century these were united to form the important Kingdom of Lanchang, which controlled virtually the whole of Laos. But the most important Thai state to arise in this period was the Kingdom of Sukhothai.