Siamese–Vietnamese War of 1831-1834

The Siamese–Vietnamese War of 1833–1834 (Thai: อานามสยามยุทธ (พ.ศ. 2376 – พ.ศ. 2377), Vietnamese: Chiến tranh Việt–Xiêm (1833–1834)), was a part of centuries-long conflict between Siam and Vietnam for suzerainty over Post-Angkor Cambodia. After the Siamese–Vietnamese conflict over Cambodia during 1811–1813, Cambodia shifted from being under Siamese domination to the period of Vietnamese influence. Though losing Cambodia, Siam had always entertained the idea of retaking Cambodia. In the early nineteenth century, both Siam and Vietnam were politically and militarily powerful in the region, with Laos and Cambodia sandwiched in the middle, shifting alliances and submissions between these two hegemons.

Instability in Southern Vietnam, namely the Lê Văn Khôi Rebellion at Saigon in 1833, allowed Siam to reassert its power over Cambodia. In November 1833, King Rama III of Siamese Rattanakosin Kingdom sent Siamese armies of 40,000 men under Chaophraya Bodindecha to attack Cambodia by land and 10,000 men under Chaophraya Phrakhlang Dit Bunnag to attack Hà Tiên by sea. Siam's goal was to install its sponsored candidates the Cambodian princes Ang Em and Ang Duong to the Cambodian throne against the pro-Vietnamese incumbent king Ang Chan and also to conquer Saigon, perhaps also to assist the Lê Văn Khôi rebels at Saigon. This Siamese invasion came as a surprise for Vietnam, who was occupied with the rebellion at Saigon. Emperor Minh Mạng of Vietnam's Nguyen dynasty had to rely on the forces, generals and commanders he had earlier mobilized against the rebels to defend Southern Vietnam from the Siamese in hurry.

After sweeping through Cambodia and Hà Tiên, the two Siamese commanders Bodindecha and Phrakhlang Dit Bunnag converged at Châu Đốc and proceeded downstream the Bassac River. In order to reach Saigon, the Siamese fleet had to cross from Bassac River to Mekong River at Vàm Nao canal or Thuận Cảng River, which was a choke point. Nguyễn Xuân and Trương Minh Giảng, who were fighting with the rebels at Saigon, were sent from Saigon to deal with the invading Siamese, leading to the climactic Battle of Vàm Nao in January 1834, during which the Vietnamese prevailed over the Siamese, who were utterly defeated and had to retreat back to Cambodia. After the victory at Vàm Nao, Nguyễn Xuân and Trương Minh Giảng pursued the retreating Siamese deep into Cambodia. During the Siamese retreat, the Cambodians, in anti-Siamese sentiments, arose, further inflicting damages and casualties onto the Siamese. By April 1834, the Siamese had all retreated, with Bodindecha retreating by land to Battambang and Phrakhlang by sea to Chanthaburi.

Vietnamese victory over the Siamese allowed Vietnam to put down the Lê Văn Khôi Rebellion and to annex Cambodia into its direct rule, establishing Trấn Tây Province over Cambodia, with Trương Minh Giảng as the Governor-General, both in 1835. Dissatisfactions of Vietnamese rule eventually drove the Cambodians to arise against Vietnam in 1840, during which Siam took advantage of by invading Cambodia in late 1840, leading to the Siamese–Vietnamese War of 1840–1842.

Even though the main Siam's interest was on the Cambodian front, Siam also sent armies through Laos to attack the Vietnamese-held Muang Phuan in the Northern Front, resulting in forced relocation of the Phuan people from Muang Phuan and Phuthai people from Laos to settle in Siam.