Siamese and Vietnamese dominance intensified during the 17th and 18th century, resulting in frequent displacements of the seat of power as the Khmer royal authority decreased to the state of a vassal. Siam, which might otherwise have been courted as an ally against Vietnamese incursions in the 18th century, was itself involved in prolonged conflicts with Burma and in 1767 the Siamese capital of Ayutthaya was completely destroyed. However, Siam recovered and soon reasserted its dominion over Cambodia. The youthful Khmer king Ang Eng (1779–96) was installed as monarch at Oudong while Siam annexed Cambodia's Battambang and Siem Reap provinces. The local rulers became vassals under direct Siamese rule.^[72]

Siam and Vietnam had fundamentally different attitudes concerning their relationship with Cambodia. The Siamese shared a common religion, mythology, literature, and culture with the Khmer, having adopted many religious and cultural practices.^[73] The Thai Chakri kings followed the Chakravatin system of an ideal universal ruler, ethically and benevolently ruling over all his subjects. The Vietnamese enacted a civilising mission, as they viewed the Khmer people as culturally inferior and regarded the Khmer lands as legitimate site for colonisation by settlers from Vietnam.^[74]

A renewed struggle between Siam and Vietnam for control of Cambodia and the Mekong basin in the early 19th century resulted in Vietnamese dominance over a Cambodian vassal king. Attempts to force Cambodians to adopt Vietnamese customs caused several rebellions against Vietnamese rule. The most notable took place from 1840 to 1841, spreading through much of the country. The territory of the Mekong Delta became a territorial dispute between Cambodians and Vietnamese. Cambodia gradually lost control of the Mekong Delta.