After a long series of wars with neighboring kingdoms, Angkor is sacked by the Ayutthaya Kingdom and abandoned in 1432 because of ecological failure and infrastructure breakdown.

This leads to a period of economic, social, and cultural stagnation when the kingdom's internal affairs come increasingly under the control of its neighbors.

By this time, the Khmer penchant for monument building has ceased.

Older faiths such as Mahayana Buddhism and the Hindu cult of the god-king have been supplanted by Theravada Buddhism.The court moves the capital to Longvek, where the kingdom seeks to regain its glory through maritime trade.

The first mention of Cambodia in European documents is in 1511 by the Portuguese.

Portuguese travelers describe the city as a place of flourishing wealth and foreign trade.

The attempt is short-lived however, as continued wars with Ayutthaya and the Vietnamese result in the loss of more territory and Longvek being conquered and destroyed by King Naresuan the Great of Ayutthaya in 1594.

A new Khmer capital is established at Udong south of Longvek in 1618, but its monarchs can survive only by entering into what amountsto alternating vassal relationships with the Siamese and Vietnamese for the next three centuries with only a few short-lived periods of relative independence.The hill tribe people in Cambodia are "hunted incessantly and carried off as slaves by the Siamese (Thai), the Anamites (Vietnamese), and the Cambodians."

In the nineteenth century a renewed struggle between Siam and Vietnam for control of Cambodia results in a period when Vietnamese officials attempt to force the Khmers to adopt Vietnamese customs.

This leads to several rebellions against the Vietnamese and appeals to Thailand for assistance.

The Siamese–Vietnamese War (1841–1845) ends with an agreement to place the country under joint suzerainty.

This later leads to the signing of a treaty for French Protection of Cambodia by King Norodom I.