Cambodian rebellion (1840)

The Cambodian uprising of 1840 was a general uprising of Khmer people against direct Vietnamese rule in September 1840. In response to Siamese invasion of Cambodia and Southern Vietnam in 1833, the Vietnamese Nguyen dynasty had annexed Cambodia into Trấn Tây Province in 1835 with Ang Mey as puppet female ruler under Trương Minh Giảng the Vietnamese Governor-General of Trấn Tây or Cambodia, imposing direct rule, assigning Vietnamese administrators over the local indigenous Khmer aristocracy, pursuing Vietnamese-led economical and agricultural expansion and ethnocultural assimilation policies. Vietnamese rule over Cambodia was established with the consent of the pro-Vietnamese faction in Cambodia in order to repel and eliminate any Siamese influence or incursions into Cambodia but friction between the Khmers and the Vietnamese eventually led to the general uprising against Vietnam.

Not all of Cambodia was under Vietnam's Trấn Tây Province. Siam had earlier appropriated Northwestern Cambodia, including Battambang and Siemreap, into its own direct rule in 1794. The Siam-held Northwestern Cambodia, known in Thai historiography as Inner Cambodia, centered on Battambang, had been serving as the base for expansion of Siamese interests in Cambodia. Through this period of Vietnamese rule, the Siamese, from Battambang, had been looking for a ripening opportunity to reassert their domination over Cambodia. Defection of Cambodian Prince Ang Em from Siamese to Vietnamese side in 1839, execution of pro-Siamese Cambodian Princess Ang Ben in 1840 and deportation of Prince Ang Em and high-ranking Cambodian ministers to Huế dissatisfied Khmer people, who viewed the Vietnamese as eroding their cultural identity and traditional governance. This Cambodian uprising against Vietnam in 1840 enabled Siam to push forward its candidate Prince Ang Duong for the Cambodian throne, instigating the Khmers to rise against the Vietnamese.

Cambodian uprising of 1840 undermined Vietnamese position in Cambodia, allowing the Siamese armies under Chaophraya Bodindecha to march into Cambodia in November 1840 to restore Siamese rule, leading to the Siamese–Vietnamese War of 1840–1842. In spite of their precarious position, the Vietnamese still hold out against the invading Siamese. Death of the Vietnamese Emperor Minh Mạng in 1841 spelt the end to Vietnamese rule in Cambodia as the new Vietnamese ruler Thiệu Trị favored withdrawal of Vietnamese troops from Cambodia. Thiệu Trị ordered the general retreat of the Vietnamese from Cambodia in late 1841, thus ending the six-year direct rule of Vietnam over Cambodia. Vietnamese withdrawal allowed the Siamese to take control over Cambodia, bringing Cambodia into another period of Siamese domination.