Siamese–Vietnamese War (1840–1845)

The Siamese–Vietnamese War of 1840–1845 (Thai: อานามสยามยุทธ (พ.ศ. 2383 – พ.ศ. 2388), Vietnamese: Chiến tranh Việt–Xiêm (1840–1845), Khmer: សង្គ្រាមសៀម-យួន (១៨៤០-១៨៤៥)) was a military conflict between the Đại Nam, ruled by Emperor Minh Mạng and then Emperor Thiệu Trị, and the Kingdom of Siam, under the rule of Chakri King Nangklao.

In effort to pull Cambodia away from Siamese influence, King Ang Chan II of Cambodia allied with Vietnam during his conflict with Siam in 1812–1813, putting Cambodia under Vietnamese influence. Siam attempted to reclaim domination over Cambodia by sending Chaophraya Bodindecha to lead Siamese forces to invade Cambodia in late 1833 and continued offensives into Southern Vietnam but the Siamese were defeated by the Vietnamese at Vàm Nao in early 1834. Concurrent Cambodian uprising against Siamese incursion pushed the Siamese back all the way back to the Siamese base at Battambang in the Siam-controlled Northwestern Cambodia. As the pro-Vietnamese Cambodian king Ang Chan died in 1835 without any male heirs, the Vietnamese enthroned Princess Ang Mey as the puppet Queen Regnant of Cambodia under Vietnamese rule without any powers, while the Siamese had male candidates including Prince Ang Em and Prince Ang Duong, half-brothers of Ang Chan. In order to utilize Cambodia as defense against Siam, the Vietnamese Emperor Minh Mạng established the Tây Thành province or Trấn Tây province (Western Commandery) over Cambodia with Trương Minh Giảng as the Governor-General, annexing Cambodia into Vietnamese direct rule. Vietnam sought to politically and culturally integrate Cambodia into Vietnam, imposing administrative personnel, reforms, cultural mandates and assimilation policies. These Vietnamese policies on Cambodia, combined with the Vietnamese decision to dethrone and exile Queen Ang Mey to Saigon in 1840, bred resentment among the Cambodian Oknha nobility and the Khmer populace, resulting in the general Cambodian uprising against Vietnamese rule in September 1840.

The Siamese–Vietnamese War of 1840–1845 can be divided into three phases;

• First phase: Siamese Invasion of Cambodia (November 1840–February 1841); Taking opportunity of the Cambodian revolt against the Vietnamese Trấn Tây regime, the Siamese king Rama III or King Nangklao sent Chaophraya Bodindecha as the supreme commander to lead the multiethnic Northern Khmer–Lao–Siamese forces of 21,753 men to invade and reclaim control over Cambodia, diving into two routes from Battambang and Siemreap to attack Pursat and Kampong Thom, respectively. Trương Minh Giảng defeated the Siamese at Stoung and Chikraeng but Bodindecha struck a peace deal with the Vietnamese commander at Pursat, confusing the Vietnamese.

Kingdom-wide restive sentiments of the Khmers pushed the Vietnamese out of most places in Cambodia. The Vietnamese were restricted to the Phnom Penh Quatre Bras area, while the Khmers welcomed Prince Ang Duong, who was brought from Bangkok to Cambodia in 1841 by Bodindecha to rally Khmer support for Siam. The Siamese under Bodindecha pushed forward Ang Duang as the Siam-endorsed candidate to take control over most parts of and to establish Siamese domination over Cambodia. Death of Emperor Minh Mạng and ascension of the new Vietnamese Emperor Thiệu Trị in January 1841 marked the shift of the Vietnamese policies towards Cambodia. In effort to salvage Vietnamese rule in Cambodia, Trương Minh Giảng brought back Princess Ang Mey and Prince Ang Em (who had earlier defected from Siam to Vietnam in 1839) to Cambodia as the Vietnam-backed candidates to rally Khmer support but to no avail. The Vietnamese eventually decided to retreat from Cambodia in November 1841, ending the Trấn Tây province and ending the six-year period of direct Vietnamese rule over Cambodia. Trương Minh Giảng the Governor-General of Trấn Tây reportedly committed suicide upon the Vietnamese loss of Cambodia.

• Second phase: Siamese attack on Hà Tiên and Vĩnh Tế canal (January–April 1842); As Bodindecha had taken control over most parts of Cambodia, the Siamese king relayed a new mission to Bodindecha in Cambodia to fill up and destroy the Vietnamese Vĩnh Tế canal, which connected Châu Đốc to the Gulf of Siam via Hà Tiên. Ever since its completion in 1824, the Vĩnh Tế canal had been perceived by the Siamese court as a strategic threat as it allowed Vietnamese naval access to Gulf of Siam. In January 1842, Siamese king Nangklao sent his younger half-brother Prince Itsaret Rangsan, accompanied by Chuang Bunnag, to lead the Siamese naval forces of 9,000 men, including some Western-style square-rigged warships, from Bangkok to the attack the Vietnamese port of Hà Tiên in order to divert Vietnamese attention for Bodindecha to attack Vĩnh Tế canal, reaching Hà Tiên in March. Chaophraya Bodindecha at Phnom Penh also sent Chaophraya Yommaraj Bunnak and Ang Duong to lead the Khmer–Lao–Siamese forces of 11,900 men to attack Vĩnh Tế canal at the northern bank, where the Vietnamese had fortified. The Vietnamese, who had been focusing on suppressing the Khmer Krom uprisings in Southern Vietnam, were again caught off guard but were able to prevail over the invading Siamese. In April 1842, Vietnamese commanders Đoàn Văn Sách successfully repelled the Siamese naval attack on Hà Tiên and Phạm Văn Điển crushed the Khmer–Siamese at Vĩnh Tế canal in a surprise attack, during which thousands of Cambodians and the Siamese died – the most humiliating Siamese defeat since the Battle of Vàm Nao in 1834.

Vietnamese victory over the invading Siamese in 1842 turned the tide of war in favor of Vietnam and forced the Siamese into defensive position. Chaophraya Bodindecha the Siamese commander expected Vietnamese retaliatory attack but both the Siamese and the Vietnamese had been exhausted by the continuing conflicts as thousands died from disease and starvation in Cambodia in this period, resulting in the three years (1842–1845) of an undeclared truce between Siam and Vietnam in Cambodia. As the Siamese continued to occupy Cambodia, Bodindecha and Ang Duong constructed several fortresses in Southeastern Cambodia along the Mekong in preparation of a prospective Vietnamese invasion, while Vietnam contemplated a Cambodian–Siamese fallout, during which they would take the opportunity to reclaim Cambodia. The Khmers, however, saw Siamese rule as nearly oppressive as the Vietnamese rule, leading to formation of an anti-Siamese faction within Ang Duong's court. In early 1845, Chaophraya Bodindecha and the bulk of Siamese forces had to leave Cambodia due to the persisting rice shortage, exposing Ang Duong to Vietnamese threats. In May 1845, Ang Duong discovered an anti-Siamese rebellion plot and conducted a purge on the conspirators. A rebellion against Ang Duong broke out in Srey Santhor, which was quickly quelled.

• Third phase: Vietnamese invasion of Cambodia (July–December 1845); Nguyễn Văn Chương (Nguyễn Trí Phương) the governor of An Giang and Hà Tiên provinces saw the anti-Siamese uprising in Cambodia as the long-awaited opportunity for Vietnam to reclaim Cambodia. In July 1845, Nguyễn Văn Chương and his deputy Doãn Uẩn, with the consent of the Vietnamese Emperor Thiệu Trị, led the Vietnamese riparian fleet to invade and conquer the Ba Phnum area of Southeastern Cambodia. The Khmers were caught off guard as Ang Duong himself led the defenses but the Cambodian Mekong fortifications built earlier did not manage to resist the Vietnamese attack. Chaophraya Bodindecha had to lead Siamese armies to aid Ang Duong against the Vietnamese attack, this time with 40,000 men. The Vietnamese then went to seize Phnom Penh in September 1845 as Ang Duong had to retreat to Oudong. The Vietnamese fleet under Nguyễn Văn Chương and Doãn Uẩn pursued the Siamese further to attack Oudong, where Bodindecha resisted and repelled the Vietnamese assault. The Siamese and the Vietnamese again reached an uneasy stalemate in Cambodia. The Vietnamese kept attacking Oudong but the Siamese did not yield, whereas the Siamese managed to defend Oudong but the Vietnamese attack was persistent. As both sides became exhausted due to several years of continuous warfare in Cambodia, Siam and Vietnam pursued peace in December 1845.

The Siamese and the Vietnamese in Cambodia agreed to a peace deal in December 1845, in which Vietnam would accept Ang Duong the Siam-endorsed candidate for the Cambodian throne on the condition that Ang Duong would profess himself to be a vassal of both Siam and Vietnam in joint suzerainty. Despite the peace agreement, tension remained high in the whole 1846 year as both Siamese and Vietnamese monarchs were reluctant to accept the peace terms. By December 1846, Chaophraya Bodindecha realized the Siamese presence in Cambodia had become a desperate stand as the Vietnamese were assembling forces for a new attack. Bodindecha then had Ang Duong send diplomatic envoys to the Vietnamese imperial capital of Huế in January 1847 to pledge Ang Duong himself as a vassal of Vietnam. The Vietnamese held a Sinitic Confucian investiture ceremony of Ang Duong as Cao Miên quốc vương or King of Cambodia in Oudong in May 1847 and then withdrew from Cambodia, while the Siamese held an Indic Brahmanistic enthronement ceremony for Ang Duong in March 1848 and also withdrew from Cambodia. With Ang Duong accepted by both Siam and Vietnam as the King of Cambodia, intermittent wars that had been plaguing Cambodia, Siam and Vietnam for fifteen years since 1833 came to the end, securing peace in Cambodia for about a decade until another Cambodian dynastic conflict erupted again in 1861.