Cambodia–Vietnam border
A border that crosses the flat, flooded Mekong delta in straight lines and canals, then climbs into forested highlands to Laos. It is a French colonial line drawn across lands that Khmer and Vietnamese both settled, and in Cambodia it remains politically charged: a 2019 treaty fixing most of it was attacked by the opposition as a loss of territory.
Written from cited sourcesLast checked 29 September 2026Suggest a correction
See it on the globe:Cambodia →Vietnam →
Delta and highlands
In the south the border crosses the Mekong delta, where hardly any point is more than 20 feet above sea level and much of the land floods every year. With few natural features to follow, it runs in straight lines between villages and along the Vinh Te canal from Ha Tien toward the Mekong. North of Tay Ninh the land rises into the forested plateaus of the Annamite chain, home to highland peoples on both sides.[1]
The line is not an ethnic divide. After 1700 Vietnamese peasants moved into the Mekong lowlands, then Khmer territory, and the delta became Vietnamese in character; Khmer still live on the Vietnamese side, and Vietnamese in Cambodia along the Parrot’s Beak and near the Mekong.[1]
A French line
France took Cochin China from 1862 and made Cambodia a protectorate in 1864. A convention of 1870 and an agreement of 1873 outlined the southern border, and decrees of 1899 and 1904 split the highlands between Annam and Cambodia. By 1976, the study notes, the two independent states had signed no boundary treaty; Cambodia’s governments at times raised claims to “lost” lands in the delta with large Khmer minorities.[1]
Offshore, in 1939 Governor General Jules Brévié divided the disputed Gulf of Thailand islands for administration only, leaving sovereignty open. Cambodia later claimed more islands, apparently by confusing the French grad with the degree when reading the line’s angle.[1]
“the question of the islands of the Gulf of Siam, the possession of which is disputed between Cambodia and Cochin China”[1]
The 2019 treaty
In October 2019 Prime Minister Hun Sen and his Vietnamese counterpart signed a supplementary treaty confirming a 1985 protocol and a 2005 agreement, which fixed 84 percent of the land border: 315 of 375 markers were in place. Cambodia’s parliament called it an unprecedented achievement. Opposition figures called it unconstitutional, arguing that the 1985 treaty had been signed while Vietnamese troops occupied Cambodia and that it cost Cambodia territory.[2]
Timeline
1864
French protectorate
France takes Cambodia under its protection.[1]
1873
Southern line
An agreement outlines the border near Ha Tien.[1]
1939
Brévié line
The Gulf islands are divided for administration.[1]
1985
Delimitation treaty
A delimitation treaty is signed; the 2019 treaty builds on it.[2]
20195 December 2019
Supplementary treaty
Cambodia’s king promulgates the 2019 treaty.[2]
More borders
Cambodia
Vietnam
Sources
- [1]International Boundary Study No. 155: Cambodia – Vietnam Boundary · US Department of State, Office of the Geographer · 5 March 1976
- [2]Cambodia’s King Signs Controversial Treaty Defining Border With Vietnam · Radio Free Asia · 20 December 2019
Map geometry from Natural Earth via world-atlas. See credits.