The –VietnameseWar–Siamese–Vietnamese War of 1841–1845 (, ) was a military conflict between the Đại Nam, ruled by Emperor Thiệu Trị, and the Kingdom of Siam, under the rule of Chakri King Nangklao. The rivalry between Vietnam and Siam over the control of the Cambodian heartlands in the Lower Mekong basin had intensified after Siam had attempted to conquer Cambodia during the previous Siamese–Vietnamese War (1831–1834). Vietnamese Emperor Minh Mạng installed Princess Ang Mey to rule Cambodia as a puppet queen regnant of his choice in 1834 and declared full suzerainty over Cambodia, which he demoted to Vietnam's 32nd province, the Western Commandery (Tây Thành Province). In 1841, Siam seized the opportunity of discontent to aid the Khmer revolt against Vietnamese rule. King Rama III sent an army to enforce Prince Ang Duong's installation as King of Cambodia. After four years of attrition warfare, both parties agreed to compromise and placed Cambodia under joint rule.
Background
The once-powerful Khmer Kingdom during the 18th century became increasingly influenced by its eastern and western neighbors: Vietnam and Siam. During the reign of the youthful Khmer king Ang Eng (1779–96) Siam conquered Cambodia's Battambang and Siem Reap Provinces in the west. The provincial administrators became vassals, under direct Siamese rule.
In the early 17th century, Siam first adopted the tradition to take members of the Cambodian royal family hostage and took them to the court at Ayutthaya, where they were left to be influenced and to compromise each other under Siamese scrutiny. The Vietnamese court in Huế also established those methods and skillfully orchestrated their protégés and interfered in marriage policies. Quarrels among the royal contenders greatly diminished any chances of restoring an effective Cambodian kingship for many decades.
After Siam's defeat in the Siamese–Vietnamese War (1831–1834), the Vietnamese reinstalled King Ang Chan to the Cambodian throne. Prince Ang Em, who had been a Siamese hostage, was made the governor of Battambang by Chao Phraya Bodindecha (Battambang and Siem Reap had been under direct rule of Siam since 1794). However, King Ang Chan died in January 1834 and left four daughters but no male heir. In 1834, Emperor Minh Mạng chose Princess Ang Mey to rule Cambodia as a queen regnant with the title Quận chúa (郡主). However, Queen Ang Mey was only a puppet queen without royal powers, as Minh Mạng incorporated Cambodia into the Vietnamese Empire as Tây Thành Province. The Vietnamese administration of Cambodia was bestowed to Trương Minh Giảng, who was appointed as viceroy. The province's government was based in Phnom Penh.
Emperor Minh Mạng had decreed elaborate plans and designs for cultural, economic, and ethnic development and the assimilation of Cambodia and forwarded them to Trương Minh Giảng. However, the economic and societal realities of Cambodia frustrated all efforts, and hardly any progress had been made in more than a decade.
Prince Ang Em, the governor of Battambang, resolved to take actions against the humiliating reign of Queen Ang Mey. In December 1838, Ang Em defected from Siamese tutelage to Vietnam and arrived in Phnom Penh in the hope that Trương Minh Giảng would make him king. Trương, however, arrested Ang Em and sent him to Huế. Siamese General Chaophraya Bodindecha marched from Bangkok to Battambang in 1839 to alleviate the situation. In 1840, Minh Mạng ordered the demotion of Queen Ang Mey and her sisters, depriving them of their status. High-ranking Cambodian ministers, including Chauvea Tolaha Long, were deported to Huế where they were later exiled to Northern Vietnam. Princess Ang Baen, elder sister of Ang Mey, whose mother was a pro-Siamese queen, was caught collaborating with the Siamese at Battambang and was executed by drowning at Long Hồ. Ang Mey and other princesses were subsequently exiled to Poulo Condore. These events, combined with Vietnamese suppressions, led the Cambodians to rise against Vietnamese rule in open rebellion. Oknha Surkealok the governor of Pursat met Bodindecha in November 1840, urging for Siamese armies to expel the Vietnamese, who held garrisons in all of the notable settlements in Cambodia and presenting a formal petition to King Rama III to release Ang Duong to be the new Cambodian king. Siam then endorsed Prince Ang Duong, Ang Em's younger brother, as the new candidate for the Cambodian throne.