Gia Long
During the 17th and 18th centuries, the Cambodian empire had been in decline and Vietnamese people migrated south into the Mekong Delta, which had previously been Khmer territory. Furthermore, Cambodia had been periodically invaded by both Vietnam and Siam. Cambodia lurched uneasily between both poles of domination as dictated by the internal strife of her two larger neighbors. In 1796, Ang Eng, a pro-Siamese king had died, leaving Ang Chan, who was born in 1791. When Gia Long unified Vietnam, Eng was given investiture by Siam in order to hold out Vietnamese influence, but in 1803, a Cambodian mission paid tribute to Vietnam in attempt to placate Gia Long, something that became an annual routine. In 1807, Ang Chan requested formal investiture as a vassal of Gia Long. Gia Long responded by sending an ambassador bearing the book of investiture, together with a seal of gilded silver. In 1812, Ang Chan refused a request from his brother Ang Snguon to share power, leading to a rebellion. Siam sent troops to support the rebel prince, hoping to enthrone him and wrest influence from Gia Long over Cambodia. In 1813, Gia Long responded by sending a large military contingent that forced the Siamese and Ang Snguon out of Cambodia. As a result, a Vietnamese garrison was permanently installed in the citadel at Phnom Penh, the Cambodian capital. Thereafter, Siam made no attempts to regain control of Cambodia during Gia Long's rule.