The Siamese–Vietnamese wars were a series of armed conflicts between the Siamese Ayutthaya Kingdom and Rattanakosin Kingdom and the various dynasties of Vietnam mainly during the 18th and 19th centuries. Several of the wars took place in modern-day Cambodia.
The political, dynastic, and military decline of the Khmer Empire after the 15th century, known as the Post-Angkor Period, left a power vacuum in the Mekong floodplains of central Indochina. United under strong dynastic rule, both Siam to the west and Vietnam to the east sought to achieve hegemony in the lowland region and the Lao mountains. The Siamese introduced — and Vietnam soon followed — the hostage system for Cambodian royals, who were relocated to their courts, actively undermining royal affairs and shaping future Cambodian policies.[a] Eventually, territory was annexed by both powers, who conceived, maintained and supported their favorable Cambodian puppet kings. Actual combat mainly took place on Cambodian territory or on occupied lands. The 19th-century establishment of French Indochina put an end to Vietnamese sovereignty and to Siamese policies of regional expansion. Subsequent clashes of the two countries were not caused by regional rivalry, but must be viewed in the context of the 20th-century imperial policies of foreign great powers and the Cold War.[2][3][b]
Prelude
The roots of the conflict started at the beginning of the 14th century, when Tai people busily expanded their states and came to clash with established Vietnamese state in the east. During the latter centuries, as the Vietnamese expanding southward to the lower Mekong, they came to conflict with Cambodia and the Siamese state.[5]
Two large Siamese forces invade Cambodia in an effort to help Prea Srey Thomea regain the throne. One Siamese army is badly beaten by the Cambodians and their Vietnamese allies at the Battle of Bantea Meas. The Second Siamese army captures the Cambodian capital of Udong where the Vietnamese supported Cambodian king switches allegiance to Siam. Vietnam loses the suzerainty of Cambodia but annexes several border provinces of Cambodia.
In 1769, King Taksin of Siam invaded and occupied portions of Cambodia. The following year a proxy war between Vietnam and Siam erupted in Cambodia when the Nguyễn Lords responded by attacking Siamese cities. At the outset of the war, Taksin advanced through Cambodia and placed Ang Non II on the Cambodian throne. The Vietnamese responded by recapturing the Cambodian capital and installing Outey II as their preferred monarch. In 1773, the Vietnamese made peace with the Siamese in order to deal with the Tây Sơn rebellion, which was a result of the war with Siam. Two years later Ang Non II was proclaimed the ruler of Cambodia.
^At the time of the invasion one group of the royal family, the reigning king and two or more princes, escaped and eventually found refuge in Laos, while another group, the king's brother and his sons, were taken as hostages to Ayutthaya.[1]
^Laos and Cambodia had been Siamese vassal states since Ayudhya times.[4]
