Phra Chao Uthong (under the name of Phra Ramathibodi) became King at Ayuthia in A.D. 1350 and thereafter was fully occupied in bringing the outlying states and provinces into line, in organizing his government, and in setting up a system of law, parts of which continue in use to the present time. Before his death in 1369, he had brought together the whole of the components of the Sukhothai-Sawankhalok Kingdom and had welded them so closely together that, when Cambodia, annoyed by the independent attitude of what was theoretically its vassal, sent an army to reassert its rights of suzerainty, the united Siamese not only defeated the enemy but pursued him well within the confines of his own country.
Under Ramathibodi's successors the Kingdom continued to prosper. During the next two centuries, Buddhism definitely succeeded Brahmanism as the popular religion throughout the country and great treasure was expended in beautifying the cities by the erection of graceful temples and reliquaries in the adapted Cambodian style which persists in Siam to this day.
About A.D. 1527, the King of Pegu, enraged by the exploits of Siamese marauders in his frontier province of Tavoy, collected an army at Moulmein and sent it into Siam under the leadership of the heir apparent, Bureng Naung. Defeating the Siamese near Supanburi, the Peguan prince advanced to the walls of Ayuthia itself; so stout was the resistance, however, and so prolonged the siege that his supply system broke down and he was forced to return to his own country, fighting rear-guard actions and losing heavily all the way. After 3 years, Bureng Naung, now King, taking the assumption by the King of Siam of the title "Lord of the White Elephants" as a casus belli, again attacked Siam with a great army and once more besieged the capital. This time, to save the city, the "Lord of the White Elephants" was compelled to negotiate and to turn over several of the animals in question to the invader, who then retired. Only a few years later, however, the Siamese King repudiated Peguan suzerainty; Bureng Naung returned, by treachery gained admission to the city, sacked and partially destroyed it, and sent the King, with many of his followers, in chains to Pegu. Leaving the Siamese governor of Pitsanulok as his viceroy in Ayuthia, Bureng Naung pressed on to subdue other cities but was scarcely out of sight when a Cambodian army, burning to avenge recent defeats and to reestablish ancient rights, appeared to begin a new siege of Ayuthia; this enemy was repulsed but not before the unprotected districts around the capital had been thoroughly looted.