Tây Thành province

Tây Thành was formerly the 32nd province of Nguyễn Vietnam, encompassing what is now modern-day Cambodia. Its capital was the Cambodian capital of Phnom Penh (known to the Vietnamese as "Nam Vang"). It was a special province with a dual system, consisting of Cambodian monarchs who reigned as puppet rulers while a Vietnamese governor resided in Phnom Penh. The province was finally abolished in 1847 after peace talks between Thailand and Vietnam concluded dual-suzerainty over Cambodia.

Chronology
In 1834 the vassal king of Cambodia, Ang Chan II, was escorted back to Cambodia by Vietnamese troops following a three-year war. The Tây Thành province was formed in late 1834 after Vietnamese emperor Minh Mạng's edict that appointed General and the Resident-Superior of Cambodia Trương Minh Giảng as governor of Tây Thành province. Cambodian princess Ang Mey, daughter of king Chan, was crowned as de jure ruler of Cambodia in early 1835 with a Vietnamese-style ceremony. From 1835 to 1840, Minh Mạng began conducting the progress of what historians called The Vietnamization of Cambodia in order to assimilate the Cambodians into Vietnamese cultural sphere and Vietnamese ethnicity.

Frustrated with the slowness of his program, in June 1840 Minh Mạng demoted Mey and the other princesses. In August 1841 they were all arrested and deported to Vietnam along with the Cambodian royal regalia. Spurred by the death of Princess Ang Baen and the absence of their Queen Ang Mey, many Cambodian courtiers and their followers revolted against the Vietnamese rule. The death of Minh Mang in 1841 changed the situation. Vietnamese offices and soldiers in Tây Thành province were assassinated by Cambodian guerillas while the governor Trương Minh Giảng had been recalled by the newly crowned emperor Thieu Tri to deal with internal rebellions. After a failed attempt to place Prince Im on the Cambodian throne in late 1841, Trương Minh Giảng committed suicide.

The Siamese launched an offensive in late 1841 and took Phnom Penh with ease, but had to withdraw in 1844 to Udong. After having suppressed internal rebellions, the Vietnamese counterattacked and placed Queen Ang Mey on the throne in 1845. After reached a peace agreement with Siam, in June 1847 the Vietnamese court released all royal members of the Cambodian ruling family, transferred the government to the new independent Cambodian monarchy of king Ang Duong and withdrew their army from Cambodia, ending the Tây Thành province.