Some scholars maintain that decline was hovering in the wings at the time Angkor Wat was built, when the Angkorian empire was at the height of its remarkable productivity. There are indications that the irrigation network was overworked and slowly starting to silt up due to the massive deforestation that had taken place in the heavily populated areas to the north and east of Angkor. Massive construction projects such as Angkor Wat and Angkor Thom no doubt put an enormous strain on the royal coffers and on thousands of slaves and common people who subsidised them in hard work and taxes. Following the reign of Jayavarrnan VII, temple construction effectively ground to a halt, in large part because Jayavarrnan VII's public works quarried local sandstone into oblivion and the population was exhausted.

Another important aspect of this period was the decline of Cambodian political influence on the peripheries of its empire. At the same time, the Thais were ascendant, having migrated south from Yunnan to escape Kublai Khan and his Mongol hordes. The Thais, first from Sukothai, later Ayuthaya, grew in strength and made repeated incursions into Angkor, finally sacking the city in 1431 and making off with thousands of intel­lectuals, artisans and dancers from the royal court. During this period, perhaps drawn by the opportunities for sea trade with China and fearful of the increasingly bellicose Thais, the Khmer elite began to migrate to the Phnom Penh area. The capital shifted several times in the 16th cen­tury but eventually settled in present day Phnom Penh.