Prior to the 13th century, Tai kingdoms had existed on the northern highlands including the Ngoenyang (centered on Chiang Saen; predecessor of Lanna) kingdom and the Heokam (centered on Chiang Hung, modern Jinghong in China) kingdom of Tai Lue people. Sukhothai had been a trade center and part of Lavo, which was under the domination of the Khmer Empire. The migration of Tai people into upper Chao Phraya valley was somewhat gradual.
Modern historians stated that the secession of Sukhothai from the Khmer empire began as early as 1180 during the reign of Po Khun Sri Naw Namthom who was the ruler of Sukhothai and the peripheral city of Sri Satchanalai (now a part of Sukhothai Province as Amphoe). Sukhothai had enjoyed a substantial autonomy until it was re-conquered around 1180 by the Mons of Lavo under Khomsabad Khlonlampong.
Two brothers, Po Khun Bangklanghao and Po Khun Phameung (Po Khun was a Siamese title of high nobility) took Sukhothai from Mon hands in 1239. Bangklanghao ruled Sukhothai as Sri Indraditya – and began the Phra Ruang Dynasty - he expanded his primordial kingdom to the bordering cities. At the end of his reign in 1257, the Sukhothai kingdom covered the entire Upper Chao Phraya valley.
Traditional Thai historians considered the foundation of the Sukhothai kingdom as the beginning of their nation because little was known about the kingdoms prior to Sukhothai. Modern historical studies demonstrate that Thai history began before Sukhothai. Yet the foundation of Sukhothai is still a celebrated event