By the end of the fourteenth century, Ayutthaya was regarded as the strongest power in southeast Asia, but it lacked the manpower to dominate the region. In the last year of his reign, Ramathibodi had seized Angkor during what was to be the first of many successful Thai assaults on the Khmer capital. The policy was aimed at securing Ayutthaya's eastern frontier by preempting Vietnamese designs on Khmer territory. The weakened Khmer periodically submitted to Ayutthaya's suzerainty, but efforts to maintain control over Angkor were repeatedly frustrated. Thai troops were frequently diverted to suppress rebellions in Sukhothai or to campaign against Chiang Mai, where Ayutthaya's expansion was tenaciously resisted. Eventually Ayutthaya subdued the territory that had belonged to Sukhothai, and the year after Ramathibodi died, his kingdom was recognized by the emperor of China's newly established Ming Dynasty as Sukhothai's rightful successor.
The Thai kingdom was not a single, unified state but rather a patchwork of self-governing principalities and tributary provinces owing allegiance to the king of Ayutthaya under the mandala system. These states were ruled by members of the royal family of Ayutthaya who had their own armies and warred among themselves. The king had to be vigilant to prevent royal princes from combining against him or allying with Ayutthaya's enemies. Whenever the succession was in dispute, princely governors gathered their forces and moved on the capital to press their claims.