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.

Sukhothai
The region of Sukhothai was at first a province of the Khmer Kingdom, but the power of the Cambodian governor did not extend very far. Thus, when the Kingdom of Angkor was in decline, two Thai governors were able to stage a successful revolt and declare the independence of Sukhothai (1238). One of these leaders was proclaimed King of the new state.

The Thais today regard the Kingdom of Sukhothai as the first organised state in their history. This view is amply justified, for it is from this time that a Thai national consciousness finds expression in language and writing: the reception of Indian and Chinese cultural influences continued, but with a distinctively Thai character. In contrast to Nan Chao, which was strongly marked by Chinese influence, Thai art and culture developed independent features. There are many legends and tales about the first king of Sukhothai, Sri Intharathitya or Phra Ruang.

Supernatural powers are ascribed to him, and also the deeds of other rulers. It is said that at his command a fishskeleton which had been thrown into the water came back to life and swam away: hence the saying, "he has Phra Ruang's tongue", meaning that a person can cause things to happen by mere words.

The earliest document of the new state dates from the reign of Rama Khamheng, a son of Phra Ruang and the third king. About 1270 the story of his accession and an account of the life of the times was engraved on a stone stele which is now in the National Museum in Bangkok. We learn how the young prince saved his father's life in battle by a bold charge on his elephant, snatching victory from defeat. His father gave him the title of Rama Khamheng (Rama the Brave), and after the deaths of his father and elder brother he became King. The inscription further extols the wealth of the country in fish and rice, and the freedom of the individual (there were no taxes of any description), and declares the king's concern for the welfare of his subjects.