1) Introduction of Theravada Buddhism
The pillar of the survival of the Angkor Civilization was supported by the religious belief of Hinduism and Mahayana Buddhism. The monarchs being regarded as the god-king were able to motivate the dedication of their people to serve the throne. The empire's irrigation system and military troops required massive laborers and manpower to maintain. The introduction of Theravada Buddhism in 13th century to the Khmers had turned out to hurt the basic foundation of the Angkor Empire. Theravada Buddhism taught the people to seek self-enlightenment, abandon worldly things and discourage any superstition belief which directly or indirectly means all deities and all evils. The sovereignty of the Angkorian monarch as a "god-king" or deva-raja was basically challenged.
2) Loss of Water Control
Less devotion of the people to the "god-king" led to negative impact on the empire. The Angkor people seemed unwilling to work with dedication for the king as a holy service as they had previously did. The formerly efficient irrigation and drainage system became silted up with less water supply and the rice crops, used to be cultivated two or three times a year, were dramatically dropped, thus weakend the productivity and the strength of the Angkor Empire.
3) External Threats
As neighboring states of the Angkor grew, they became a major threat to the empire, especially the Thai State of Ayuthaya in the Chaophaya River Basin to the West.In order to protect the empire, the Angkor had to direct some of its manpower to secure strong armed forces, which resulted in less workers and manpower to give maintenance to its irrigation system.
4) Double-edged sword of Roads Network
The road network built by Jayavarman VII had aided the transports of products and trades throughout the empire and also facilitated the Khmer troops to out an end to its neighbors. It had become problem when the Angkor became weak as the invaders could easily invade in through this road network, instead of previously sailing up from the Mekong River. This turned out to be true when the newly emerged Ayuthaya, a Thai kingdom in the West became stronger. They use this road to march from the Chaophaya River basin through Phnomrung (in Burirum of modern Thailand) and then through Aranyapathet to attack right at the heart of Angkor and finally destroyed the empire in 1431. The glory of the Angkor Civilization was eliminated since that time.
By Blake Preusker