of Cambodia. The type of Buddhism practiced in medieval Cambodia has been widely studied by professor François Bizot and his colleagues at the École française d'Extrême-Orient. They have identified tantric and esoteric elements in this tradition and thus call it "Tantric Theravada". After 1431 when the Cambodian kings permanently abandoned Angkor due to a Siamese invasion, the royal court was located on Udon Mountain, a few miles north of Phnom Penh. Siamese incursions from the west and Vietnamese invasions from the east weakened the Khmer Empire. The Vietnamese invaders attempted to suppress Theravada Buddhism and force the Khmer people to practice Mahayana Buddhism. The Siamese, on the other hand, would periodically invade Cambodia and attempt to drive out the "unbelievers" in an attempt to protect the Theravada religion. This power-struggle between the two ascendant powers continued until the arrival of the Europeans in the 16th century. Colonial era Buddhism continued to flourish in Cambodia in the sixteenth century. King Ang Chan (1516–1566), a relative of King Dhammaraja, was a devout Buddhist. He built pagodas in his capital and many Buddhist shrines in different parts of Cambodia. In order to popularize Buddhism, King Satha (1576–1549), son and successor of King Barom Reachea, restored the great towers of the Angkor Wat, which had become a Buddhist shrine by the sixteenth century. Each successive wave of European influence was accompanied by Catholic missionaries, but Theravada Buddhism proved surprisingly resistant to foreign attempts to convert the Khmer people. During the colonial period, the peace was periodically breached by outbreaks of religiously motivated violence, including periodic millenarian revolts. During the seventeenth, eighteenth, and nineteenth centuries, Thailand's involvement in Cambodian politics extended Thai influence into religious matters as well. On King Norodom's invitation, monks from the Thai Dhammayuttika Nikaya established a Dhammayuttika presence in Cambodia. The newly formed Thommayut order benefited from royal patronage, but frequently came into conflict with the existing Mohanikay (Maha Nikaya) lineage. The Thommayut were sometimes accused of holding loyalty to the Thai court, rather than to the Khmer nation. During the era of French rule, convulsions of violence, led by Buddhist holy men, would periodically break out against the French. Significant advances were made in the education of Cambodian monks, both in specifically Buddhist topics and more general studies. Primary education of Cambodian children continued to take place at temple schools. Monks were also encouraged to become involved in community development