In the early 1830s a rebellion broke out in the Mekong Delta. The Siamese sent troops to support it, and then in 1834 the Vietnamese (i.e., the Nguyễn Dynasty), pushed the Siamese back. Afterwards, they tried to control the area of Cambodia, and did so until a major rebellion broke out at the end of 1840.
This period from 1834 to 1840 is referred to as “the Vietnamese annexation of Cambodia.” One of the first people to write in English about this period was historian David Chandler in his 1973 PhD dissertation, “Cambodia Before the French: Politics in a Tributary Kingdom, 1794-1848.”
In writing about that period, Chandler relied heavily on a Vietnamese source, the Đại Nam thực lục (Veritable Records of Đại Nam); a collection of Nguyễn Dynasty documents. Chandler praised this work in his dissertation, saying that “For several stretches in the early nineteenth century” this was “the most detailed and accurate source” for what transpired in Cambodia (13).