The escalating tensions between the Thais and Vietnamese ultimately prompted the Cambodians to seek protection from France. Following the death of King Ang Eng, his eldest son and heir, Ang Chan, was only six years old and did not ascend to the throne for another nine years. During this interim period, the Thais annexed the province of Battambang, extending their control as far as Siem Reap, which remained under Thai dominion until 1907. By the time Ang Chan (r. 1806–34) was crowned, he had developed a strong anti-Thai sentiment and appealed for assistance from the Vietnamese, who subsequently annexed the entire Mekong Delta and took effective control of significant portions of Cambodia, relegating the king to a puppet role.

In 1812, Ang Chan moved the royal court to Phnom Penh, from where he discreetly sent emissaries to Bangkok, assuring the Thais of his loyalty. In 1832, following the death of the Vietnamese viceroy, Thai King Rama III seized the moment and dispatched an army to expel the Vietnamese forces. However, by the time Thai troops arrived, the Vietnamese had already departed, taking Ang Chan with them. The Thais initially attempted to install one of Ang Chan’s two exiled brothers as king but ultimately abandoned this plan due to a lack of local support.

A couple of years later, the Vietnamese returned to Phnom Penh, keeping Ang Chan under close supervision. He died shortly thereafter without a male heir, prompting the Vietnamese to install his second daughter, Mei, as queen (r. 1835–41), believing she would be more compliant. They set about imposing Vietnamese customs and culture on the Cambodians, disregarding Theravada Buddhism and enforcing the Vietnamese language, which led to significant resentment. Anti-Vietnamese riots erupted regularly beginning in 1836.

Frustrated by their inability to establish a disciplined administration, the Vietnamese blamed Queen Mei for their failures and arrested her in 1840. While the Cambodians had initially resented being forced to accept a Vietnamese-appointed queen, they now rallied against her detention, leading to further riots. Thai troops stationed on the border intervened, forcing the Vietnamese out of Cambodia. Despite some sporadic conflicts, the Vietnamese never regained control and withdrew entirely from Cambodia in 1847. The following year, Chan’s brother, Duang, was crowned king at Oudong with a full Buddhist ceremony (r. 1848–59).

Meanwhile, the French had begun to make inroads into Southeast Asia but faced resistance in their attempts to establish trade with Vietnam. Using the pretext of French missionaries facing persecution, they invaded the Mekong Delta and annexed southern Vietnam. Fearing another Vietnamese invasion, King Duang sought assistance from the French. Although a diplomatic mission was dispatched, it was turned back before it could reach him in Oudong, and Duang died before any negotiations could take place. This left the task of formalizing relations with the French to King Norodom (r. 1859–1904), who agreed to a treaty in 1863, establishing the foundation for the French protectorate in Cambodia.