King Si lntharathit and King Ramkhamhaeng were both warrior kings who extended their territories far and wide. Their successors, however, could not maintain such a far-flung empire. Some of these later kings were more remarkable for their religious piety and extensive building activities than for their warlike exploits. An example of this type of Buddhist ruler was King Mahathammaracha Lithai, believed to have been the compiler of the Tribhumikatha, an early Thai dook on the Buddhist universe or cosmis. The political decline of Sukhothai was, howeved , not wholly due to deficiencies in leadership. It resulted rather from the emergence of strong Thai states further south, whose political and economic power began to challenge Sukhorhai during the latter half of the 〖14〗^th century. These southern states, especially Ayutthaya, were able to deny Sukhothai access to the southern region.
The Sukhothai Kingdom did not vanish at once. Its decline occurred from the mid-〖14〗^th until the 〖15〗^th century. In 1378, the Ayutthaya King Borommaracha I subdued Sukhothai’s frontier city of Chakangrao (present-day Kamphaeng Phet), whereupon Sukhothai became a tributary state of Ayutthaya. Sukhothai attempted to break loose from Ayutthaya but with no real success. In the 〖15〗^th century it was incorporated into the territory of the Ayutthaya Kingdom. By then, the focus of Thai history and politics had shifted to the Ayutthaya Kingdom. By then, the focus of Thai history and politics had shifted to the central plains of present-day Thailand, where Ayutthaya was establishing itself as a centralized state, its power outstripping not only Sukhothai but also other neighbouring states such as Suphannaphum and Lawo(present-day Lop Buri)