Historians are not certain why Angkor gradually declined during the 13th and 14th centuries, but several factors may have contributed. Jayavarman VII's massive building program eventually exhausted the kingdom's resources. At the same time, Angkor's vassal states began to assert their independence, no longer paying tribute into Angkor's treasuries. Defending Angkor from frequent invasions by the neighboring Thais also resulted in a huge loss of manpower, so that the irrigation system that sustained Angkor's highly productive agriculture could not be properly maintained. At the end of the 13th century a new branch of Buddhism—Theravada Buddhism—was introduced from Sri Lanka. Its more egalitarian spiritual teachings undermined the hierarchical structure of Khmer society and the political power of prominent adherents of Hinduism. Thai raids on Angkor increased in the 14th century, and after a prolonged siege in 1431, the Khmers gradually shifted their capital south to Phnom Penh. Angkor never regained its former glory. Its monuments—except for Angkor Wat, which was maintained by Buddhist monks—became shrouded in jungle.
