Cambodian conflict (1812–1813)

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 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.