Khun Bang Klang Thao and Khun Pha Muang who held the governorship of Ban Yang and Rad respectively decided to join their fores together and to overthrow the Khmer rule. Through the support of the Thais, Khun Bang Klang Thao was proclaimed King Si Inthrathit of Sukhothai. Thus was born the Thai kingdom of Sukhothai in 1238.
Si Inthrathit was succeeded by his second son, Ban Muang, as King of Sukhothai. At the death of Ban Muang in 1279, Ram Khamhaeng, who was Si Inthrathit’s third son, ascended the throne.
Ram Khamhaeng proved himself to be a great king. During his reign, Sukhothai was an extensive kingkom, bordering in the north on Lanna and in the east on Vientiane, covering in the south the upper part of Malay Peninsula, and including in the west Tenesserim, Tavoy, Martaban and Pegu.
As an absolute monarch, he governed his people with justice and magnanimity. Whenever they wished to submit a complaint to him, they rang a bell at the palace gate. He would then grant them a audience so as to afford an oportunity to find out by himself the causes of the complaint, and he then decided it according to its nature. He evinced considerable interest in the moral education of the people. He persuaded them to observe the simple Buddhist precepts, to make merit and give alms, and to attend a sermon regularly. He was in fact, responsible for the introduction of the Thevarada Buddhism of the Ceylonese school, or Lanka Buddhism, from Nakhon Si Thammarat to Sukhothai. He had a stone seat of Managcasila throne erected in a palmyra palm grove, where, at his request, a priest preached a sermon on every pre-sabbath day and sabbath day, and where he conducted the affairs of the State on other days. In short, Ram Khamhaeng’s rule of Sukhothai was endowed with the characteristics of a paternal government.
Realizing the importance of a national language as a unifying force of his people as well as a symbol of their independence, he created in 1283 the first Thai alphabet, using as a basis the Mon and Khmer scripts which had in turn been derived from a south Indian script. He employed for the first time the new script in his stone inscription of 1292. His Thai alphabet has since undergone some changes and is still in use to-day.
In the field of diplomacy, he cultivated cordial relations with King Mengrai of Lanna and Prince Ngam Muang of Payao, so that he could concentrate his attention on the Khmers who might at any moment open hostilities against Sukhothai. At the same time he safeguarded Sukhothai in the west through the conferment of the title of Chao Fa Rua on Makato, the ruler of Pegu, who swore fealty to him. He opened direct relations with China and exchanged missions with her, resulting in the introduction of Chinese artisans who then helped to improve the production of pottery wares at Sukhothai.
Ram khamhaeng died in 1299 at the age of more than sixty. Had his successors been warriors like him, the Kingdom of Sukhothai might have lasted longer than it actually did. Loe Thai was nowhere near his calibre, while Li Thai (1347-1368) won fame as a great scholar and patron of Lanka Buddhism. Throughout his reign, he devoted himself to the advancement of this religion, thereby neglecting the defence of the country. Consequently Sukhothai became an easy target for the expansionist policy of King Borom Rachathirat of Ayuthaya, and King Sai Lu Thai made submission to him 1378. Thus Sukhothai lost its 140-years old independence and became a vallal state of the Kingdom of Ayutthaya until it was annexed by Ayutthaya in 1438.