Siamese–Vietnamese War (1841–45)

The 1841–1845 Siamese-Vietnamese War in Cambodia was a war between Vietnam (then under the rule of the Nguyen Dynasty) and Siam (Thailand), triggered by Siam's attempt to expand its influence in Cambodia and to prevent its rival, Vietnam, from territorial gains the region at the expense of the declining Khmer Empire.

After Vietnam defeated Siam in the 1831–1834 war, the Vietnamese-installed Queen Ang Mey held on to the Cambodian throne during a period of increased Vietnamese influence over the country. Vietnamese occupation and oppression triggered a rebellion in 1841 with Cambodians overthrowing the pro-Vietnamese faction. The rebels appealed to Siam who supported another claimant to the Cambodian throne, prince Ang Duong. Rama III responded and sent Ang Duong back from exile in Bangkok with Siamese troops to install him on the throne.

Facing increasing instability, the Vietnamese abandoned Cambodia and retreated to An Giang. Seizing the opportunity, the Siamese launched and offensive in February 1842, crossing into Vietnamese territory, capturing Cô Tô mountain. However Vietnamese reinforcements from Huế and central Vietnam quickly moved into the Mekong delta, halting the Siamese advance. By May the Vietnamese counter-offensive repelled the Siamese, recapturing Cô Tô and taking a large number of prisoners of war.

Meanwhile in Cambodia, the Siamese occupying forces repeated the same heavy-handed mistakes of the Vietnamese, squandering Khmer's trust. The Cambodians then sought Vietnamese assistance to fend off the Siamese overlords. In July 1845, the Vietnamese cross into Cambodia, advancing toward Phnom Penh. Within two months, on September 13, the capital fell to Vietnamese troops. The Siamese commander Bodindecha and Ang Duong retreated to Oudong. The Vietnamese continued the pursuit and in October, Oudong was besieged.

Encircled, Bodindecha sought for peace, and in November 1845, representatives of Bangkok and Hue were signatories the peace treaty, bringing the war to an end. Cambodia was placed under joint Siamese-Vietnamese protection. In January 1847, Ang Duong sent emissaries to pay tributes to both Siam and Vietnam, recognizing its joint vassalage to both the Court of Bangkok and the Court of Hue. This lasted until the French established a protectorate in 1863.