The third point in Nguyen Van Thanh’s message of 1812 had to do with the situation on the Cambodian border, which had just erupted into a new crisis. The competition between the Siamese and Vietnamese for control of Cambodia was in abeyance during the lifetime of Gia Long’s wartime ally, King Chakri, known posthumously as Rama I. Rama I kept a tight grip on his Khmer vassal and Gia Long did nothing to contradict this. In 1799, Bangkok had called up a Khmer army and sent it with Siamese and Laotian armies to assist Gia Long against his Tay Son enemies during the final campaigns of the Vietnamese wars. But, when the commander of the Khmer army returned with what appeared to be an excessively friendly attitude toward the Vietnamese, he was exiled to Bangkok. The Khmer king Ang Eng had died in 1796, and was succeeded by his 7-year- old eldest son, Ang Chan. A Khmer regent governed Cambodia under Siamese supervision. In 1806, the regent died, and Ang Chan, then 16 years old, was crowned in Bangkok before being sent to the Khmer throne in Oudong. The presence of a crowned king in Cambodia along with increasing slack in the Siamese presence during Rama I’s declining years opened a possibility for Ang Chan and the Khmer royal court to claim a larger measure of autonomy. After Rama I’s death in 1809, the new Siamese king, Rama II, aiming to exercise his ascendancy in Cambodia, sought to undermine Ang Chan’s position. He affirmed the finality of the Siamese annexation of the Khmer provinces of Battambang and Siem Reap in the northwest and looked for ways to extend Siamese influence into the adjacent provinces of Pursat and Kompong Svay. The governor of Kompong Svay, a man named Moeung, was close to the Siamese and had been forced to flee to Bangkok in 1808 when he refused to acknowledge Ang Chan’s authority, an event that reportedly angered Rama II.
In 1810, Rama II sent Moeung back to his post in Kompong Svay, but Moeung still refused to swear allegiance to Ang Chan. When Ang Chan had Moeung assassinated, Rama II decided to teach the young Khmer king a lesson by engineering a rebellion led by Ang Chan’s younger brother, Ang Snguon. Early in the year 1812, Ang Snguon stood up against his elder brother in Pursat, about 130 kilometers northwest of Oudong, where a Siamese army arrived to champion his cause. Ang Chan, unwilling to bow to Siamese efforts to weaken his authority and to deprive him of territory, but lacking the military strength to challenge Siam, sent envoys to Gia Dinh requesting Vietnamese assistance. The old story of feuding Khmer princes and Siamese–Vietnamese contention was about to begin again.
Unable to stand up to the Siamese advance, Ang Chan fled to Gia Dinh at the beginning of 1813. Gia Long, no longer confined by the scruples of friendship toward his deceased wartime ally, Rama I, took time to mobilize a large army and placed it under the command of Le Van Duyet. In late spring of 1813, Le Van Duyet escorted Ang Chan into Cambodia but allowed the Siamese time to withdraw. The Siamese were unprepared to oppose such a large Vietnamese force and acquiesced in the Vietnamese restoration of Ang Chan. Ang Snguon took refuge in Bangkok. Le Van Duyet built strong defenses for Phnom Penh, where he installed Ang Chan with a Vietnamese garrison, then withdrew his army back into Vietnam. Rama II had overplayed his hand in Cambodia and provoked into reality a situation that he had been determined to avoid. Gia Long was characteristically decisive and planned carefully for success.
Gia Long enforced his authority as Ang Chan’s suzerain with the principle of maintaining boundaries between peoples. When Siamese envoys arrived by sea at Hue in 1815 and requested permission to return to Bangkok via Cambodia to pay their respects to the Khmer king, Gia Long refused, deter- mined to offer no opportunity for Siamese intrigue to germinate. Later in the year, when he learned that large numbers of Vietnamese fugitives and adven- turers were going to Cambodia and causing trouble, he instructed Ang Chan to expel all Vietnamese back into Vietnam except for soldiers assigned to garrison duty and merchants with passports. At the end of the year, when he learned that a Khmer officer had provoked a border incident with the Siamese, Gia Long instructed Ang Chan to punish him. In 1818, Gia Long ordered officials to encourage Chams, Chinese, and Khmers to settle on uninhabited land in the Mekong plain and to prevent Vietnamese from bothering them
. Gia Long’s policy toward Cambodia was to minimize interference in Khmer affairs and to encourage a sense of well being among the Khmer people while maintaining the upper hand in regard to Siam. In late 1818, in response to Khmer complaints about the Vietnamese official serving as “protector” at Phnom Penh, Gia Long replaced the man, saying, “The government must be in harmony with the feelings of the people. If the [Khmer] people do not want something, it cannot be forced on them. If something is forced on them, we will provoke hatred on the border.” Such high-sounding sentiments did not always translate from the court at Hue to Vietnamese administrators in the Mekong plain, but they had been typical of Gia Long’s attitude toward governing the diverse populations of the south and of government in general since the 1780s. He never showed interest in changing the way people lived so long as they were obedient to him and did not harm others.