Siamese–Vietnamese War (1841–1845)

Bodindecha sent troops led by his son Phra Promborrirak and his brother-in-law Chao Phraya Nakhon Ratchasima Thongin from Battambang to lay siege on Pursat (Hải Tây), which was held by Vietnamese forces, in November 1840. Bodindecha also sent another force led by Phraya Rachanikul to take Kampong Svay (Hải Đông), which was defended by Đoàn Văn Sách. The Siamese was able to take Kampong Svay, however, Trương Minh Giảng retook Kampong Svay and proceeded to Pursat. Bodindecha then negotiated a peaceful surrender from the Đề đốc of Pursat before Trương Minh Giảng could reach him. Emperor Minh Mạng sent Phạm Văn Điển to reinforce but Minh Mạng himself died in February 1841. The new emperor, Thiệu Trị, changed the Vietnamese policy towards Cambodia and ordered the retreat of all Vietnamese forces from Cambodia. By October 1841, the Vietnamese had retreated to An Giang Province, where Trương Minh Giảng died, by suicide according to Thai and Cambodian sources. The retreat of the Vietnamese exposed Cambodia to Siamese domination. The Vietnamese had brought the defected Prince Ang Em to An Giang to rally Cambodian loyalty so Bodindecha also sent his son Phra Promborrirak to lead Prince Ang Duong to Oudong.

After the Siamese success in Cambodia, King Rama III ordered that the Vĩnh Tế Canal should be filled and destroyed. Bodindecha pledged to the king that the Vĩnh Tế Canal was guarded by Vietnamese forces in Hà Tiên and An Giang and the king should dispatch more troops to attack these provinces. The king then sent his half-brother Prince Isaret (later Vice-king Pinklao), accompanied by Chuang Bunnag (son of Phraklang, later Somdet Chao Phraya Sri Suriyawongse) and five brigantines, to attack Hà Tiên, called Banteay Mas in Thai and Cambodian. The king also sent another land force led by Chao Phraya Yommaraj Bunnag and Prince Ang Duong to attack An Giang Province.

Cô Tô mountain in modern Tri Tôn district, An Giang Province.

The fleet of Prince Isaret and Chuang Bunnag arrived at Phú Quốc in January 1842. Prince Isaret stayed on the island while ordering Chuang Bunnag to attack Hà Tiên. Chuang Bunnag led the Siamese brigantines to attack Hà Tiên while also sending a Cambodian force to take Cô Tô mountain. The Siamese canons shelled Hà Tiên intensely. Đoàn Văn Sách the defender of Hà Tiên requested for reinforcements so that the city did not fall. After a whole week of attacks, Chuang Bunnag was still unable to take Hà Tiên. Chuang then visited Prince Isaret at Phú Quốc, who decided to retreat due to the overwhelming Vietnamese numbers and the unfavorable winds. Nguyễn Tri Phương led the Vietnamese forces to capture Cô Tô from the Cambodians. Prince Isaret and Chuang Bunnag then led the Siamese forces to retreat to Chantaburi.

At the An Giang front, Chao Phraya Yommaraj Bunnag and Prince Ang Duong had led Siamese armies in January 1842 to take Vĩnh Tế Canal and An Giang province, penetrating into Hậu Giang Province. Nguyễn Công Nhân was unable to repel Siamese attacks and Thiệu Trị sent Tôn Thất Nghị to reinforce. Phạm Văn Điển, the governor of An Giang and Hà Tiên Provinces (Tổng đốc An Hà), also joined in An Giang but himself died of illness in April 1842[4]. Tôn Thất Nghị and Nguyễn Công Nhân pushed the Siamese back. The Siamese was defeated at Châu Đốc, suffering heavy losses, and retreated to Phnom Penh. Nguyễn Công Nhân was made Tổng đốc An Hà, the governor of An Giang and Hà Tiên Provinces.

Famine and diseases ravaged Cambodia from 1842-43 and the wars came to halt as both the Siamese and Vietnamese were exhausted of manpower and resources. On the Siamese side, Prince Ang Duong and his guardian Phra Promborrirak stationed at Oudong with support of Bodindecha at Battambang. On the Vietnamese side, Prince Ang Em was brought from Huế to stay with Nguyễn Tri Phương, who held the post of Tổng đốc An Hà, at Châu Đốc. However, Prince Ang Em died in March 1843, leaving only a princess. There was then no male Cambodian princes left for the Vietnamese. Bodindecha returned to Bangkok in 1845.

In 1845, some Cambodian nobles in Prince Ang Duong's court again began to seek Vietnamese assistance. Emperor Thiệu Trị organized the new offensives into Cambodia in three groups[5] with Võ Văn Giải, the Tổng đốc of Gia Định and Biên Hòa Provinces, as supreme commander. Nguyễn Văn Hoàng, the Đề đốc of An Giang, would led the Vietnamese fleet from Tân Châu upstream the Bassac River to attack Ba Phnum. Doãn Uẩn, the Tuần phủ of An Giang, from Thông Bình would proceed through Kampong Trabaek. Both armies would meet at Ba Phnum and go to attack Phnom Penh. The third army led by Nguyễn Công Nhân from Tây Ninh would floow first two armies into Cambodia.

After Prince Ang Duong had his eleven Vietnam sympathizers executed in May 1845, the Vietnamese began their advances. Nguyễn Văn Hoàng marched along the Bassac River and after defeating the Cambodians at Preak Sambour proceeded to Ba Phnum. Doãn Uẩn captured Kampong Trabaek and stationed at Khsach Sa. Bodindecha hurriedly marched to Battambang and to Oudong to defend. Nguyễn Văn Hoàng and Doãn Uẩn converged at Ba Phnum. Võ Văn Giải went from Saigon to Ba Phum to command forces. Nguyễn Tri Phương led armies from Châu Đốc to reinforce at Ba Phnum. Nguyễn Tri Phương and Doãn Uẩn then led forces from Ba Phnum to attack Phnom Penh in September 1845. Phnom Penh was defended by Phra Promborrirak, Bodindecha's son. Nguyễn Tri Phương managed to capture Phnom Penh. Phra Promborrirak and Siamese forces suffered heavy losses and retreated to Oudong.

Nguyễn Văn Chương led Vietnamese troops to lay siege on Oudong. Nguyễn Tri Phương stationed at Ponhea Leu to the south of Oudong while Doãn Uẩn stationed at Kampong Luong to the north with Bodindecha on his defensive. After some fightings, both sides agreed to negotiate. Doãn Uẩn requested that Prince Ang Duong should send a mission to Huế to apologize and submit to Vietnamese rule. After five months, Nguyễn Tri Phương and Doãn Uẩn lifted the siege and returned to Phnom Penh in November 1845.