Twenty years later, a royal coup in Cambodian court ousted the ruling Satha II prompted a Vietnamese intervention led by lord Nguyễn Phúc Khoát (r. 1738–1765) to back the usurper king Prea Srey Thomea.
Nguyen remnants fled to the Mekong Delta to seek refuges, where they found support from Cambodian monarchs Ang Eng (r. 1779–1796) and Siamese Rama I.
In early 1785, Siamese-Cambodian-Nguyen fleets marched forward Saigon, but they were routed by Tay Son leader Nguyễn Huệ at the Battle of Rạch Gầm-Xoài Mút.
[1] During the Vietnamese Civil War (1789–1802), Cambodia as a Siamese vassal sent at least 20,000 men and fought alongside Nguyen Anh's army against the Tay Son.
Requested by Chan, Gia Long sent an army numbered 13,000 men commanded by Trương Tấn Bửu (1752–1827), invaded Cambodia, seized Oudong.
[6] In 1817, tens of thousand Cambodian workers were employed by Vietnamese officials to repair and extend the 40-miles Vĩnh Tế Canal that connects the Gulf of Thailand with the Mekong.
That time Minh Mạng wanted to replace the Viceroy of Cambodia and Saigon, Lê Văn Duyệt, with officials loyal to him in order to consolidate his centralization efforts.
Hearing the news, Rama III of Siam, believed that he could scold Minh Mạng with assistance from pro-Siamese Chinese mercantile class in Saigon, sending a large army led by Bodin to attack the Vietnamese positions in Cambodia in early 1833 at the outbreak of the First Siamese–Vietnamese War.
Entire the Cambodian administration, tax resisters, military, foreign affairs, economy departments,... were transferred to Vietnamese control.
Minh Mạng appointed Queen Ang Mey as the puppet ruler, and the enthronement ceremony was celebrated following Vietnamese traditions.
Wealthy Vietnamese landowners who have contributed more than 12 tons of rice and supplies for the troops in the province received government appointments as trophies.
The governor Trương Minh Giảng enacted a notorious repression campaign against Khmer dissents to put down these revolts by using forces and terrors.
[11] Prince Ang Duong (Chan's brother exiled in Bangkok) led the opposition faction to resist the Vietnamese occupation.
[17] Feeling anxiety about the Vietnamization progress of Cambodia, in June 1840 Minh Mang demoted Queen Mey and her sisters appointed them as low-rank officials of the province.
In August, Ang Mey and her sisters had been poisoned by Vietnamese secret police in an opera house, then deported to Saigon along with the royal regalia.
Vietnamese offices and soldiers in Tây Thành Province were torn down by Cambodian guerillas while the governor Trương Minh Giảng had been calling back by the newly crowned emperor Thieu Tri to deal with internal rebellions.
[18] After a failed attempt to bring Prince Im to Cambodian throne in late 1841, Trương Minh Giảng committed suicide.
[19] Took advantage, Thai forces numbering 35,000 troops invaded Cambodia, occupying Phnom Penh, and asserted their rule over the abandoned Vietnamese province.