Along the Laos-Vietnam border, 1,150 of its total 1,324 miles, or about 87 percent, follows the crest of the Annamite Mountains. This ridge separates rivers flowing into different valleys.
The same pattern appears along the Myanmar-Thailand border, the Cambodia-Thailand border, and the Indonesia-Malaysia border on Borneo. Instead of following straight lines based on latitude and longitude, these boundaries trace mountain ridges known as watersheds, or water divides.
Historian Kyle Gardner, in his paper "Moving Watersheds, Borderless Maps, and Imperial Geography in India's Northwestern Himalaya," describes this approach as the "water-parting principle." British colonial officials believed that watershed boundaries produced borders that were precise, easy to map, and less likely to become disputed.
This principle became the dominant approach to drawing national borders in mountainous regions throughout the 20th century.
Here are four ASEAN borders that clearly illustrate this principle. One of them even became the subject of an international dispute before the International Court of Justice.
A Legacy of River Kingdoms: Laos and Vietnam
According to International Boundary Study No. 35 published by the U.S. Department of State, the highlands of Southeast Asia historically served as buffer zones between kingdoms centered in major river valleys.
The Khmer, Annamese, and Thai kingdoms competed for influence in these fertile lowlands. Meanwhile, the mountain ranges between them were rarely brought fully under the control of any single kingdom. As a result, watersheds naturally became boundaries long before the arrival of European colonial powers.
Evidence of this emerged during a territorial dispute in 1958. The Lao government presented documents stating that the earliest treaty between the King of Annam, present-day Vietnam, and the Kingdom of Lan Xang, present-day Laos, dated back to 1364. The treaty established the boundary along the watershed.
The document described the border as following "the parting of the waters." France later reaffirmed this boundary through a decree issued by the Governor-General of Indochina on October 2, 1916. The decision came after several Vietnamese settlements had expanded west of the watershed.
Treaty Language That Left No Doubt: Myanmar and Thailand
Along the approximately 1,118-mile border between Myanmar, then known as Burma, and Thailand, the watershed principle was written directly into the treaty itself.
The Convention signed in Bangkok in 1868 between the Governor-General of India and the King of Siam stated that the boundary would follow the Moei River to its source in the Pa-wan Mountains. From there, it would continue along the main mountain ridge that forms the watershed across the peninsula until reaching the Andaman Sea.
The treaty explicitly described the route as "then along the Pa-wan range to the main watershed."
According to International Boundary Study No. 63, most of this border follows mountain ridges and the principal watershed. Rivers defined by the thalweg principle make up only the sections that cross broad valleys, including the Salween River and the Nam Mae Moei River.
When Maps Departed from the Watershed: Cambodia and Thailand
The 499-mile border between Cambodia and Thailand was established through a series of Franco-Siamese treaties signed in 1904 and 1907. Of its total length, 326 miles, or about 65 percent, follows the watershed of the Dangrek Mountains. Another 130 miles follows rivers, while the remaining 43 miles consists of straight-line segments.
These treaties also became the foundation of the Preah Vihear Temple dispute, one of Southeast Asia's most well-known border conflicts.
According to International Boundary Study No. 40, Thailand argued that the temple lies north of the watershed defined in the 1904-1907 treaties. Cambodia, however, based its claim on the map produced by the boundary delimitation commission. At several points, the map deviated from the actual watershed.
Preah Vihear was one of those locations.
In 1962, the International Court of Justice (ICJ) ruled that the commission's map, rather than the watershed itself, was legally binding. As a result, sovereignty over the temple was awarded to Cambodia.
Thailand accepted the ruling in practice. However, at the time the study was written, it continued to maintain formal reservations regarding the legal basis of the decision.
A Colonial Legacy on Borneo: Indonesia and Malaysia
Beyond mainland Southeast Asia, the same principle was applied to the only land border between Indonesia and Malaysia. Stretching 1,107 miles across the island of Borneo, it separates the Indonesian provinces of West Kalimantan and East Kalimantan from the Malaysian states of Sarawak and Sabah.
According to International Boundary Study No. 45, the boundary was established through a series of pre-independence agreements between Britain and the Netherlands. One of the most important was the London Convention of June 20, 1891, which explicitly stated that "the boundary followed the main watershed of the principal rivers" until Tanjung Datu on the island's western coast.
With the exception of two small sectors near Sabah and southwest of Kuching, where rivers serve as the boundary, the rest of the border is marked entirely by mountain ranges that form the principal watershed.
The pattern is virtually identical to the one established centuries earlier in the river valleys of Indochina, on the opposite side of the South China Sea.
