Siamese–Vietnamese conflicts over Cambodia

The once-powerful Khmer Kingdom during the 18th century became increasingly influenced by its eastern and western neighbors: Vietnam and Siam. Cambodian dynastic rivalries over the throne aggravated the conflicts^[6] as each side sought support from and manipulated by either Siam or Vietnam. In 1794, King Phuttha Yotfa Chulalok or King Rama I of Siam installed the youthful Ang Eng as the king of Cambodia under Siamese domination.^[7][8][9] King Rama also carved Northwestern Cambodia, including Battambang and Siemreap, for Chaophraya Aphaiphubet, a pro-Siamese Cambodian minister, to govern under direct Siamese control. Thus, Northwestern region of Cambodia was annexed into Siam. King Ang Eng died prematurely in 1796, leaving his four sons Ang Chan, Ang Snguon, Ang Em and Ang Duong. Ang Chan was installed as the new king of Cambodia by Siam in 1802 but he became resentful of Siamese influence in Cambodia and approached Vietnam. Ang Chan refused to attend the funeral of the Siamese King Phuttha Yotfa in 1809, executing his pro-Siamese ministers and taking anti-Siamese stance.^[9] In 1811, Prince Ang Snguon, Ang Chan's pro-Siamese younger brother, rebelled against Ang Chan.^[1] Siamese forces invaded Cambodia in 1812 in support of Ang Snguon and the panicked King Ang Chan fled to Cochinchina to take refuge at Saigon under Vietnamese protection.^[10] The Siamese burnt down the Cambodian royal capital of Oudong and returned. Princes Ang Em and Ang Duong, Ang Chan's other brothers, decided to join with Ang Snguon and went to Bangkok to be under Siamese custody.^[7] The Vietnamese Emperor Gia Long assigned Lê Văn Duyệt to restore Ang Chan to the Cambodian throne in 1813.^[6] This brought Cambodia under Vietnamese domination as Ang Chan moved his seat to Phnom Penh for closer Vietnamese protection.^[7] In 1819, Gia Long ordered the construction of Vĩnh Tế canal that connected Châu Đốc and Hà Tiên on the Cambodian–Vietnamese border,^[5] putting Cambodian people into the labor works. Siamese court at Bangkok was greatly alarmed by construction of this canal, suspecting that the canal was to facilitate mobilization of Vietnamese navy fleet into the Gulf of Siam, threatening Bangkok.

In 1826–1827, King Anouvong of the Lao Kingdom of Vientiane arose to free his kingdom and his people from Siamese rule. However, Anouvong was soon overcome by the Siamese forces and took refuge in the Vietnamese Nghệ An Province. Minh Mạng the Vietnamese emperor, posing to act as the mediator, sent Anouvong back to negotiate with the Siamese in 1828 but Anouvong instead arose and killed Siamese officials in Vientiane. Chaophraya Ratchasuphawadi (Chaophraya Bodindecha) was convinced that Vietnam supported Anouvong in his rebellion against Siam. Minh Mạng sent another Vietnamese mission to Nakhon Phanom to broker peace for Anouvong but the Vietnamese envoys were massacred by the order of Bodindecha. This was the breaking point of Siamese–Vietnamese relations.

Siamese Invasion of Cambodia and Southern Vietnam (1833–1834)

Lê Văn Duyệt, who had been the viceroy of Cochinchina or Southern Vietnam and also held influences over Cambodia, died in 1832.^[11] Minh Mạng installed his officials to replace Lê Văn Duyệt in Cochinchina who soon found out that Lê Văn Duyệt had been exceptionally powerful and came up with accusations against the deceased minister,^[1][11] leading to political purge, punishments and executions. The body of Lê Văn Duyệt was exhumed from his grave and humiliated. Lê Văn Khôi, adopted son of Lê Văn Duyệt, arose in rebellion at Saigon against the Nguyen in 1833 in Lê Văn Khôi revolt. Minh Mạng sent his generals to subjugate the rebels in mid-1833 and the rebels then sought assistance from Siam.^[12]

Siamese invasion of Cambodia and Southern Vietnam by land and sea from November 1833 to January 1834. The Siamese forces were defeated by the Vietnamese in the Battle of Vàm Nao in January-February 1834.

King Nangklao or King Rama III of Siamese Rattanakosin Kingdom, upon knowing of the rebellion at Saigon, took this opportunity to dismantle Vietnamese influence in Cambodia and to aid the Lê Văn Khôi rebellion at Saigon.^[8] In November 1833, the Siamese king sent his generals Chaophraya Bodindecha to lead the land armies of 40,000 men, bringing the Cambodian Princes Ang Em and Ang Duong with him, to invade Cambodia and Chaophraya Phrakhlang to lead the navy fleet of 10,000 men to attack the Vietnamese port of Hà Tiên. Siam endorsed Ang Em and Ang Duong as the candidates for Cambodian throne. With initial Cambodian defeats, the Cambodian king Ang Chan fled from his court at Phnom Penh to take shelter at Long Hồ in Cochinchina. The Siamese quickly took Hà Tiên, Châu Đốc (An Giang province) and proceeded along the Bassac river invading Southern Vietnam, crossing the Vàm Nao canal into the Mekong heading towards Saigon. The Vietnamese diverted their forces against the Lê Văn Khôi rebels to face the invading Siamese. Vietnamese riparian fleet under Nguyễn Xuân and Trương Minh Giảng intercepted the Siamese fleet at Vàm Nao canal, leading to the Battle of Vàm Nao in January 1834. Even though the Siamese under Bodindecha initially prevailed, the Vietnamese navy was superior to the Siamese. Unable to break through Vietnamese blockade, Chaophraya Bodindecha decided to retreat along with his Siamese forces to Châu Đốc and Phnom Penh.

With the events turned in Vietnam's favor, the Cambodians arose against the Siamese invaders. Pro-Vietnamese Cambodian commanders, Oknha Chakrey Long and Oknha Yumreach Hu, organized Cambodian resistance in Prey Veng against Siam, ambushing and inflicting damages onto the retreating Siamese. Nguyễn Xuân and Trương Minh Giảng capitalized the victory and pursued the retreating Siamese into Cambodia. By early 1834, Bodindecha and the Siamese had retreated to Siam-controlled Battambang,^[10] where the Siamese forces rested and recuperated. Minh Mạng ordered his minister Lê Đại Cương to bring the Cambodian king Ang Chan to return to Phnom Penh to resume the rule. Ang Chan rewarded his meritorious ministers Oknha Chakrey Long and Yumreach Hu by appointing them to the positions of Chauvea Tolaha (Prime Minister) and Samdech Chauponhea, respectively.

Vietnamese annexation of Cambodia

Map of Cambodia as Vietnam-occupied Trấn Tây Province

King Ang Chan of Cambodia died in January 1835,^[10] leaving no male heirs but four daughters; Princesses Ang Pen (Ang Pen's mother was Neak Neang Tep, daughter of Chaophraya Aphaiphubet the pro-Siamese Cambodian governor of Siam-controlled Battambang who had died in 1809.),^[10] Ang Mey, Ang Peou and Ang Snguon, born to different consorts of Ang Chan.^[13] Ang Chan's brothers Princes Ang Em and Ang Duong, who were Siam-endorsed candidates to the Cambodian throne, were under Siamese custody at Battambang. This left Vietnam with no male candidates to Cambodian kingship. In 1835, at the suggestion of Trương Minh Giảng, Emperor Minh Mạng created the Trấn Tây Province (鎮西) or Western Commandery over Cambodia, bringing Cambodia under direct Vietnamese rule and dividing Cambodia into thirty-three phủ or districts. Minh Mạng appointed the Cambodian princess Ang Mey as puppet Queen regnant of Cambodia with the title of Quận chúa^[13] (郡主) as the first and only historical female ruler in Cambodian history^[14] without any real powers. Minh Mạng deliberately passed over Princess Ang Pen or Ang Baen the eldest daughter of Ang Chan due to her connections with Siam.^[13] Other three princesses were given the title Huyện quân (縣君) or governors. Minh Mạng also appointed Trương Minh Giảng as Trấn Tây tướng quân (鎮西將軍, called Ong Tien Kun) as supreme commander of Cambodia with Lê Đại Cương as Tham tán đại thần or counselor. Phnom Penh, Cambodian royal capital, was renamed as Nam Vang^[15] as the center of Vietnamese administration in Cambodia. Vietnamese bureaucracy was introduced into Cambodia, superseding indigenous Cambodian noble-aristocracy, which still existed but was deprived of actual administrative roles.

Trương Minh Giảng brought 5,000 Vietnamese military men to Phnom Penh to occupy Cambodia and to be trained.^[10] Trương Minh Giảng the supreme Vietnamese minister in Cambodia posed himself as de facto ruler of Cambodia. Native Cambodian mandarins paid daily obeisance to Trương Minh Giảng. In 1837, two Cambodian noble brothers Oknha Chey and Oknha Chu rebelled at Kampong Som against Vietnam and fled to Bangkok.^[16] In the same year, Oknha Dechu Ream the governor of Kampong Svay rebelled but was caught and executed. Trương Minh Giảng appointed Oknha Chap as the new governor of Kampong Svay with Snang Ey as deputy. However, Snang Ey soon revolted next year in 1838.^[16] Trương Minh Giảng ordered Chauvea Tolaha Long the pro-Vietnamese Cambodian Prime Minister to march against Snang Ey at Kampong Svay. Snang Ey then fled to Siam, taking with him a number of local Cambodian population. Trương Minh Giảng was enraged and killed Oknha Chap the governor of Kampong Svay for retaliation.

Siamese preparations

After Siamese defeat in early 1834, King Nangklao of Siam appointed the Cambodian Princes Ang Em as the governor of Siam-occupied Battambang and Ang Duong as governor of Mongkolborey^[10] to give the Cambodian princes the impression that they ruled at least some parts of Cambodian and to rally pro-Siamese supporters. Siam expected offensive retaliation from Vietnam so Siam reinforced its defense in preparation for future military confrontations with Vietnam. In December 1834, King Rama of Siam sent Chaophraya Phrakhlang to fortify Chanthaburi. Phrakhlang moved the city of Chanthaburi five kilometers to the upland position for better defensive site and constructed the Noenwong Fort (Thai: ป้อมเนินวง).^[17] King Rama also ordered the renovation of Chachoengsao city walls and construction of a new fort called Khong Kraphan Fort (Thai: ป้อมคงกระพัน)^[18] at Phra Samut Chedi, Samut Prakarn. With the death of Ang Chan in early 1835 and the advent of Vietnamese rule, a group of Cambodian nobles sent a secret letter to Bangkok, urging the return of Cambodian princes Ang Em and Ang Duong.^[19] Siam, however, had not yet recovered from its losses to conduct any new expeditions into Cambodia. Lack of provisions and food resources had been issues since the Siamese forces operated in Cambodia far from home base. King Rama then had Phraya Ratchasuphawadi Ng To establish supply line at Prachinburi and Krabinburi on the way to Cambodia.^[18]

Siamese conquest of Lao kingdom of Vientiane allowed access to vast manpower resource in the Isan-Laos region. In 1836, King Rama III ordered Chaophraya Bodindecha to conduct a manpower census in Siam-controlled parts of Cambodia and the Lao region of Khorat Plateau in order to utilize the available manpower forces in prospective campaigns against Cambodia and Vietnam. Siam also sought to strengthen Battambang as its own political base.^[19] The Siamese king ordered Bodindecha to fortify Battambang. Chaophraya Bodindecha went to Battambang in February 1837 and moved the Battambang city from Baset to the present-day site on the Sangkae River, constructing a new city-fort there.^[19] In February 1839, Phraya Ratchasuphawadi was sent to fortify the Siam-controlled Siemreap town.^[18]

Cambodian uprising against Vietnam

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.^[1][10]

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,^[10] 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.^[13] Ang Mey and other princesses were subsequently exiled to Poulo Condore.^[13] These events, combined with Vietnamese suppressions,^[10] 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.^[20][21][5]

Siamese offensives of 1840–1842

Map of army routes during the Siamese offensives of 1840–1842. Red represents Siamese routes. Yellow represents Vietnam and Cambodia.

Cô Tô mountain in modern Tri Tôn district, An Giang Province.

In November 1840, the Siamese warlord Chaophraya Bodindecha sent troops led by his son Phra Phromborrirak and his brother-in-law Chao Phraya Nakhon Ratchasima Thongin from Sisophon to lay siege on Pursat, which was held by Vietnamese forces. The Siege of Pursat was a success, as the Siamese army moved south to attack Phnom Penh.

Bodindecha also sent forces, led by Phraya Ratchanikul, to take Kampong Svay, which was occupied by Đoàn Văn Sách. The Siamese took Kampong Svay but were defeated by Trương Minh Giảng at the Battle of Chikraeng. Phraya Ratchanikul's army was cut off and had to retreat with heavy losses. Bodindecha then negotiated a peaceful surrender with Võ Đức Trung^[10] the military commander of Pursat on December 31, 1840, before Trương Minh Giảng could reach him.

Emperor Minh Mạng, who had sent reinforcements under Phạm Văn Điển, died after a fall from a horse in February 1841. The new emperor, Thiệu Trị, reversed Vietnamese policies on Cambodia and ordered the retreat of all Vietnamese forces. By October 1841, the Vietnamese had retreated to An Giang Province. Viceroy Trương Minh Giảng evacuated Phnom Penh, retreating to Châu Đốc. Eventually, Trương Minh Giảng committed suicide to take responsibility for the loss of Cambodia.

The Vietnamese had taken the defected Prince Ang Em to An Giang to rally Cambodian support. However, Bodindecha was now unopposed and sent his son Phra Phromborrirak to help Prince Ang Duong to the throne in Oudong and to massacre all remaining Vietnamese people still dispersed in Cambodia.^[1]

Interbellum (1842–1845)

Famine and diseases ravaged Cambodia from 1842 to 1843, and the war came to a halt since both warring parties had been exhausted of manpower and resources.

Prince Ang Duong and his guardian Phra Phromborrirak took position at Oudong and were supported by Bodindecha at Battambang. Vietnamese emperor Thiệu Trị allowed Cambodian royalties Ang Em, Ang Mey, other princesses and Cambodian nobles including Tolaha Long to return to Cambodia.^[10] They came from Huế and joined Nguyễn Tri Phương at Châu Đốc. However, Prince Ang Em died in March 1843 and left only Princess Ang Mey under Vietnamese control. Bodindecha returned to Bangkok in 1845, along with him Chauvea Tolaha Prom the new Cambodian Prime Minister went to Bangkok to offer tributes from Ang Duong to King Rama III.^[10]

Vietnamese offensives of 1845

The Siamese campaigns of 1841 had failed to bring about lasting peace but greatly devastated and depopulated large areas of central, south, and southeastern Cambodia, which antagonized many Cambodians. As soon as Bodindecha had returned to Siam, in 1845, several noblemen in Prince Ang Duong's court at Oudong expressed their desire to seek an allegiance with Vietnam rather than Siam. In May 1845, a group of eighteen Cambodian Oknha noblemen, led by Oknha Chakrey Mei, plotted to overthrow Ang Duong in favor of Ang Mey.^[10] However, the plan was quickly caught by Ang Duong and most of the conspirators including Chakrey Mei were executed with few managed to flee. This event stirred up new round of tension between Siam and Vietnam on Cambodia. Upon learning of the abortive plot, Emperor Thiệu Trị launched an offensive into Cambodia in three groups with Võ Văn Giải, the governor of Gia Định Province and Biên Hòa Province as supreme commander;^[5]

• Nguyễn Văn Hoàng, the admiral of An Giang, led a Vietnamese fleet from Tân Châu upstream the Bassac River to attack Ba Phnum.
• Doãn Uẩn, the commander of An Giang, would proceed through Kampong Trabaek District. Both armies would meet at Ba Phnum and jointly attack Phnom Penh.
• The fleet, led by Nguyễn Công Nhân from Tây Ninh, would follow and reinforce the first two armies.

After Prince Ang Duong had the outspoken Vietnamese sympathizers at his court executed in May 1845, the armies began to advance in July 1845. Nguyễn Văn Hoàng marched along the Bassac River and, after he had defeated a Cambodian contingent at Preak Sambour, proceeded to Ba Phnum. Doãn Uẩn captured Kampong Trabaek and set up camp at Khsach Sa. Chaophraya Bodindecha left Bangkok on July 25, 1845, with his forces and hurriedly marched via Battambang to Oudong to defend the Cambodian royal capital.

Nguyễn Văn Hoàng and Doãn Uẩn converged at Ba Phnum. Võ Văn Giải arrived from Saigon at Ba Phum to command forces and Nguyễn Tri Phương led reinforcement troops from Châu Đốc to Ba Phnum. Nguyễn Tri Phương and Doãn Uẩn attacked Phnom Penh in September 1845. Phnom Penh was defended by Phra Phromborrirak, Bodindecha's son and 5,000 Siamese troops. Taking advantage of high-water inundation surrounding the city,^[10] Nguyễn Tri Phương managed to capture Phnom Penh with his fleet on September 11, 1845, and the Thai suffered 600 killed while 1,400 Cambodians were killed.^[23] Phra Phromborrirak and the Siamese forces retreated to Oudong.^[24] Ang Mei and other Cambodian princesses including Ang Duong's mother were moved from Châu Đốc to Phnom Penh to rally the Cambodians on Vietnamese side.^[10]

Nguyễn Văn Chương led about 20,000 Vietnamese troops and 1,000 warships, divided into many smaller forces, expecting to attack and besiege Oudong from all directions, which was defended by Bodindecha and Duong.^[25] Meanwhile, Thieu Tri escorted Queen Mey and the Cambodian regalia back to Phnom Penh, planning in order to gain support from the Cambodian population. Nguyễn Tri Phương was stationing at Ponhea Leu, south of Oudong, and Doãn Uẩn was stationing at Kampong Luong, to the north.

The Vietnamese commander, Chương, suddenly fell very ill as the Vietnamese captured the port of Kampong Luong on the Tonle Sap River in October. Therefore, he ordered the army to retreat and cancel the campaign. Other units began panicking and being demoralized, and the Thai attacked them in Kampong Luong, taking many casualties and prisoners.^[26] After five months, Nguyễn Tri Phương and Doãn Uẩn lifted the siege and returned to Phnom Penh in November 1845.^[1] In December, after some further minor clashes between Siamese-Cambodian forces with the Vietnamese command post in Kampong Luong, both sides agreed to negotiate.^[27] Doãn Uẩn requested for Prince Ang Duong to send a mission to Huế, to apologize, and to submit to Vietnamese rule.