From 1600 until the arrival of the French in 1863, Cambodia was ruled by a series of weak kings who, because of continual challenges by dissident members of the royal family, were forced to seek the protection - granted, of course, at a price - of either Thailand or Vietnam. In the 17th century, assistance from the Nguyen lords of southern Vietnam was given on the proviso that Vietnamese be allowed to settle in what is now the Mekong Delta region of Vietnam, at that time part of Cambodia and today still referred to by the Khmers as Kampuchea Krom (Lower Cambodia).

In the west, the Thais controlled the provinces of Battambang and Siem Reap from 1794; by the late 18th century they had firm control of the Cambodian royal family. Indeed, one king was crowned in Bangkok and placed on the throne at Udong with the help of the Thai army. That Cambodia survived through the 18th century as a distinct entity is due to the preoccupations of its neighbours: while the Thais were expending their energy and resources in fighting the Burmese, the Vietnamese were wholly absorbed by internal strife.