The Cambodian Conflict of 1812–1813 was a dynastic conflict between the incumbent King Ang Chan II of Cambodia, who had support from the Vietnamese Nguyen dynasty under Emperor Gia Long, and his younger brother Prince Ang Sngoun, who had support from Siam under King Rama II of the Rattanakosin Kingdom, resulting in both the Siamese and the Vietnamese marching armies into Cambodia.
Previously, the young King Ang Eng of Cambodia was taken to Bangkok due to upheaval in Cambodia in 1782, where he lived as a hostage under Siam. King Rama I of Siam, through his agent the pro-Siamese Cambodian minister Chaophraya Aphaiphubet Baen, exerted Siamese domination over Cambodia in 1790. The Siamese king Rama sent Ang Eng out to rule Cambodia in person under Siamese domination in 1794 but also annexed Northwestern Cambodia, including Battambang and Siemreap, to be under Aphaiphubet as the first Siam-appointed governor of Battambang under Siam's direct rule. The Cambodian king Ang Eng died in 1797, leaving four young sons Ang Chan, Ang Sngoun, Ang Em and Ang Duong. King Rama I sent Ang Chan to rule as King of Cambodia under Siamese auspices in 1806. However, Ang Chan had negative experiences with the Siamese court and sought out for Emperor Gia Long of Vietnam to counter Siamese influences.
When King Rama I of Siam died in 1809, Ang Chan defied Siamese domination by refusing to attend the funeral of the Siamese king at Bangkok. The new Siamese king, Rama II, appointed Ang Sngoun, younger brother of Ang Chan, to the position of Viceroy of Cambodia without consulting Ang Chan, turning Ang Sngoun into a pro-Siamese rival of Ang Chan, who also became pro-Vietnamese as the Cambodian royal court polarized into Siam-aligned and Vietnam-aligned factions. Ang Chan ordered executions of two pro-Siamese Cambodian ministers in September 1810, prompting the Siamese king to send troops to the Siam-controlled Battambang in preparation to attack Cambodia. Ang Chan requested assistance from the Vietnamese Emperor Gia Long, who sent Nguyễn Văn Nhơn the governor of Saigon to bring forces to Longvek to guard Ang Chan. The standoff lasted four months until Nguyễn Văn Nhơn pulled off his troops back but the Siamese remained at Battambang.
Ang Sngoun rebelled at Pursat against Ang Chan in January 1812. King Rama II of Siam sent Chaophraya Yommaraj Noi to bring Cambodian–Siamese forces of 5,000 from Battambang into Cambodia to support Ang Sngoun in April 1812, prompting King Ang Chan to flee to Saigon, seeking refuge under protection of the Vietnamese. In the same time, Siam also sent a diplomatic mission to the Vietnamese Emperor Gia Long at Huế explaining the necessity of Siamese intervention in Cambodia – the notion that Gia Long did not agree. Siamese commander Yommaraj Noi occupied Oudong the Cambodian royal capital until early 1813 when he decided to retreat as he was not prepared for a long-term occupation of Cambodia. Before leaving, the Siamese destroyed and burnt down Oudong, which had been the royal capital of Cambodia for two centuries. The Cambodian princes Ang Sngoun, Ang Em and Ang Duong, younger brothers of Ang Chan, were all taken to Bangkok to live at Wang Chao Khamen or Cambodian palace quarters in custody. Gia Long had Lê Văn Duyệt the governor of Saigon parade King Ang Chan back to Cambodia in a grand showcase of Vietnamese military forces in May 1813, restoring Ang Chan to the Cambodian throne.
During this Cambodian dynastic conflict, Siam and Vietnam avoided direct confrontation. Gia Long, who had earlier spent some years in political refuge in Bangkok, was not poised to go against his former ally but Vietnam was becoming Siam's geopolitical opponent over domination of Cambodia. Siam also could not afford to engage in a full-scale war with Vietnam due to the ever-existing Burmese threat from the west. This new generation of Cambodian princely conflict developed simultaneously with the renewed Siamese–Vietnamese rivalry. King Ang Chan of Cambodia, however, preferred Vietnam as his hegemon. The events of 1812–1813 shifted Cambodia from the period of Siamese influence to the period of Vietnamese domination as Siam lost control over Cambodia. Vietnamese influence on Cambodia would be externally unchallenged for two decades until Siam invaded Cambodia again in 1833.
Background
Cambodian dynastic conflicts In 1769, King Taksin of Thonburi sent messages to King Ang Ton of Cambodia, urging him to send tributes to Siam and submit. King Ang Ton refused. In 1771, King Taksin ordered Phraya Yommaraj (later King Rama I) to lead troops of 10,000 men to invade Cambodia through Battambang, Siemreap and Pursat to attack Oudong and to bring the pro-Siamese Prince Ang Non to the Cambodian throne. King Taksin himself with the general Chen Lian (陳聯) led the fleet to attack Hà Tiên, leading to the Siamese-Vietnamese War (1771–1773). King Ang Ton fled to Saigon under the protection of the Nguyen Lord Nguyễn Phúc Thuần. Nguyễn Phúc Thuần sent the general Nguyễn Cửu Đàm to aid against the Siamese invasions. The Siamese were repelled with Prince Ang Non stayed behind at Kampot. Also in 1771, the Tây Sơn Rebellion arose against the rule of Nguyen Lords. King Ang Ton returned to Cambodia and negotiated with Ang Non. Ang Ton abdicated the Cambodian throne in 1775 in favor of Ang Non who became the new King of Cambodia. However, Ang Ton died in 1777, leaving Ang Non in full power in Cambodia. Cambodian nobles, led by Chauvea Tolaha Mou and his brother Oknha Decho Ten, were dissatisfied with King Ang Non's pro-Siamese stance. Tolaha Mou sought support from Nguyễn Phúc Ánh who had been fighting against the Tây Sơn from Saigon. Nguyễn Phúc Ánh sent Vietnamese army to support Tolaha Mou, who arrested and murdered King Ang Non in 1779. Tolaha Mou placed a four-year-old son of Ang Ton named Ang Eng as the new king of Cambodia with himself as the regent. King Taksin, upon learning about the Cambodian regicide, was furious at Oknha Yomreach Baen who was the protector of King Ang Non. Taksin ordered Yomreach Baen arrested and imprisoned at Thonburi but Chaophraya Chakri secured his pardon and release.
Cambodia under Siamese domination In 1782, Chaophraya Chakri became King Rama I of the Chakri dynasty. Oknha Yomreach Baen and Oknha Kralahom Pok successfully staged a coup and killed Tolaha Mou in 1782. However, the ensuing confusion and civil war in Cambodia prompted Yomreach Baen and Kralahom Pok to bring the young king Ang Eng and his elder sisters Princess Ang E and Ang Pen to Bangkok. King Rama I took care of King Ang Eng as his adopted son, while Princesses Ang E and Ang Pen became consorts of Prince Surasinghanat. In Cambodia, Decho Ten, the younger brother of Tolaha Mou, declared himself the Tolaha or regent and took power under the support of the Tây Sơn. King Rama I appointed Yomreach Baen as Chaophraya Aphaiphubet the regent of Cambodia. Chaophraya Aphaiphubet Baen managed to oust Decho Ten in 1790 and took control of whole Cambodia for Siam. King Rama I kept Ang Eng in Bangkok away from Cambodian political conflicts. Aphaiphubet Baen was the regent of Cambodia for twelve years until 1794 when King Rama I allowed Ang Eng to assume personal rule in Cambodia. King Rama I also rewarded Aphaiphubet Baen with the northwestern part of Cambodia including Battambang and Siemreap for Aphaiphubet Baen to govern under direct Siamese suzerainty, thus annexing those territories into Siam proper.
King Ang Eng died in 1796, leaving four sons Princes Ang Chan, Ang Sngoun, Ang Em and Ang Duong. Kralahom Pok, who had become Tolaha Pok, served as the regent of Cambodia until 1806 when he brought the four young Cambodian princes to visit King Rama I at Bangkok. Tolaha Pok fell ill and died at Bangkok. King Rama I installed Ang Chan as the new King of Cambodia in 1806. Emperor Gia Long of the Vietnamese Nguyen dynasty also invested Ang Chan as the King of Cambodia next year in 1807. The new king Ang Chan asked for the permissions for his aunts Princesses Ang E and Ang Pen to return to Cambodia. King Rama I refused, citing that the princesses were already mothers of daughters of the late Prince Surasinghanat. On an occasion, Ang Chan visited King Rama I before returning to Cambodia. However, Ang Chan entered the royal hall without permission and prerequisite ceremonies. King Rama I strongly rebuked Ang Chan in front of Siamese officials. In 1808, Oknha Decho Meng the governor of Kampong Svay rebelled against King Ang Chan.
King Rama I died in 1809. Chaophraya Aphaiphubet also died the same year. King Ang Chan did not attend the funeral of King Rama I at Bangkok and instead sent his younger brothers Ang Sngoun and Ang Em, along with Cambodian nobles Oknha Chakrey Pen and Oknha Kralahom Moeung, to go to Bangkok. King Rama II granted Ang Sngoun and Ang Em the titles of Uprayorach (viceroy) and Ouparach (deputy viceroy), respectively. In the same year, the Burmese invaded Phuket and King Rama II requested supporting troops from Cambodia to defend Bangkok. King Ang Chan, however, did not comply. Chakrey Pen and Kralahom Moeung, the two pro-Siamese Cambodian ministers, then organized troops to be sent to Bangkok without the permission of the Cambodian king. Ang Chan then had Chakrey Pen and Kralahom Moeung executed for sedition on September 14, 1810. Tension arose between Ang Chan and the Siamese court. Ang Chan sent Oknha Bovorneayok Sous to request military aid from the Vietnamese. Nguyễn Văn Nhơn the governor of Saigon led the Vietnamese troops of 1,000 men to take defensive position at Longvek against possible Siamese offensives. Prince Senanurak of the Front Palace ordered Phraya Rongmueang to station Siamese troops at Battambang. The Siamese-Vietnamese standoff lasted for four months until Nguyễn Văn Nhơn pulled the troops back to Saigon in January 1811 but Phraya Rongmueang remained in Battambang.
Siamese-Vietnamese standoff On 15 January 1812, Prince Ang Sngoun the Uprayorach and younger brother of Ang Chan, left the royal city of Oudong at night along with a group of pro-Siamese mandarins. Ang Sngoun rallied troops at Pursat. Ang Chan sent delegates to visit his brother Ang Sngoun at Pursat, urging him to return to Oudong but to no avail. Ang Chan then decided to ask for Vietnamese support again. Nguyễn Văn Nhơn the governor of Saigon sent Nguyễn Văn Thoại to lead Vietnamese forces of 500 men to station at Longvek. The Siamese court sent Chaophraya Yommaraj Noi to Battambang to lead the Siamese expedition into Cambodia to settle the princely struggle issues. King Ang Chan ordered his generals Oknha Bovorneayok Sous and Oknha Thommeadecho to defend Kampong Chhnang.
Chaophraya Yommaraj Noi sent his delegates to negotiate with Ang Chan at Oudong but Ang Chan gave no responses. Yommaraj Noi, along with Phraya Rongmueang at Battambang, decided to march the Siamese army of 5,000 men down, taking Prince Ang Sngoun from Pursat to attack Kampong Chhnang in April 1812, leading to the Battle of Kampong Chhnang. The Cambodians were outnumbered as Oknha Bovorneayok Sous and Oknha Thommeadecho sent a man to inform King Ang Chan at Oudong that the Siamese came in large numbers. Ang Chan then decided to leave Oudong on April 9, 1812, along with the royal family to take refuge in Phnom Penh. Nguyễn Văn Thoại provided boat vessels for the Cambodian king and his family to travel at Phnom Penh. Princes Ang Em and Ang Duong, two other younger brothers of Ang Chan, decided not to join the king in flight and defected and fled back to the Siamese on April 10. Nguyễn Văn Nhơn then invited Ang Chan to seek safety shelter in Saigon. The Cambodian king and his entourage reached Saigon on April 25, 1812.