Throughout the second half of the nineteenth century and the early twentieth century, European colonial powers drew borders across Southeast Asia through treaties and maps. These documents were often drafted without a high degree of precision. They were also negotiated without the participation of the local kingdoms whose territories were being divided.
More than a century later, those documents have still not been retired. They remain the primary legal references whenever two countries dispute a piece of territory, even when such disputes lead to armed conflict.
Two cases in Southeast Asia clearly illustrate this pattern. The first is the 1891 London Convention, which divided Borneo between Britain and the Netherlands. The second is the 1904-1907 Franco-Siamese treaties, which established the border between Cambodia and Thailand.
Both agreements continue to be used by the International Court of Justice (ICJ) in resolving modern territorial disputes. One of them has even contributed to the deadliest armed conflict in Southeast Asia in more than a decade.
Malaysia and Indonesia: A Border Drawn Across Borneo from Afar
On June 20, 1891, Britain and the Netherlands signed the London Convention. The treaty established the boundary between British North Borneo and Dutch Borneo. Most of the border followed the mountain ridges that form the island's principal watershed.
However, the treaty did not explicitly define the status of two small islands off Borneo's eastern coast: Sipadan and Ligitan.
More than a century later, this ambiguity became the central issue in the territorial dispute between Indonesia and Malaysia before the ICJ. On December 17, 2002, the ICJ ruled by a vote of 16 to 1 that both islands belonged to Malaysia.
Indonesia based its claim on the 1891 Convention. The Court, however, found that the treaty did not clearly determine the status of Sipadan and Ligitan.
Because the treaty was inconclusive, the ICJ based its judgment on effectivités, or evidence of effective occupation and administration. The Court concluded that Malaysia had the stronger claim because of its administrative record.
This included regulating turtle egg collection, establishing a bird sanctuary on Sipadan, and constructing lighthouses on both islands in 1962 and 1963. The 1891 Convention also left another unresolved issue on Borneo, even after the Sipadan-Ligitan case had been settled.
Indonesia and Malaysia finally signed a memorandum of understanding on February 18, 2025. The agreement was endorsed during the 45th session of the Joint Indonesia-Malaysia Boundary Committee (JIMBC) and publicly announced in April 2026.
The agreement resolved the long-standing land boundary dispute on Sebatik Island, whose southern part belongs to Nunukan Regency in Indonesia and whose northern part belongs to the Malaysian state of Sabah.
Under the agreement, 127.3 hectares of land that had previously been administered by Malaysia were officially recognized as Indonesian sovereign territory. At the same time, about 4.9 hectares of Indonesian territory were transferred to Malaysia. The adjustment covered approximately 23 kilometers of the two countries' shared land border.
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Cambodia-Thailand: The Court of Justice Steps In Time and Again
Elsewhere in Southeast Asia, France and Siam signed a series of treaties in 1904 and 1907. These agreements established the border along the watershed of the Dangrek Mountains between what are now Cambodia and Thailand.
According to International Boundary Study No. 40 published by the U.S. Department of State, the mixed delimitation commission tasked with drawing the official maps did not always follow the actual watershed. In several areas, the maps deviated from the true divide. The location of Preah Vihear Temple was one of those deviations.
The same 1907 treaty also returned Battambang, Siem Reap, Serei Saophoan, and Preah Vihear Temple to Cambodia. According to Encyclopaedia Britannica, this outcome remains a source of resentment in Thailand to this day.
After Cambodia gained independence in 1953, Thai forces reoccupied Preah Vihear Temple. Cambodia then brought the dispute before the International Court of Justice (ICJ) in 1959.
In 1962, the ICJ ruled that the temple belonged to Cambodia. The Court based its decision on the 1907 delimitation commission map. It argued that Thailand had implicitly accepted the map by not formally protesting it for decades.
However, the ruling left one important issue unresolved. The status of the land surrounding the temple remained ambiguous.
That ambiguity later triggered border clashes between 2008 and 2011. The conflict began after Cambodia registered Preah Vihear as a UNESCO World Heritage Site, and Thailand objected. The fighting killed at least 20 people.
Cambodia returned to the ICJ seeking clarification. In 2013, the Court reaffirmed that both the temple and its surrounding area were under Cambodian sovereignty.
A 118-Year-Old Map Still Claims Lives
According to Encyclopaedia Britannica, these long-standing tensions erupted into a major armed conflict once again in 2025.
The crisis began with an exchange of gunfire near Preah Vihear Temple on May 28, 2025. One Cambodian soldier was killed. The incident was followed by a series of diplomatic and military retaliations.
By late July, fighting had spread to at least 12 locations along the border. The clashes involved air strikes, artillery fire, and landmines.
The first ceasefire was reached on July 28, 2025, in Putrajaya, Malaysia. It was facilitated by Malaysia and supported by the United States and China.
However, the ceasefire collapsed in November after another landmine incident. Fighting resumed in early December, including air and drone strikes. A second ceasefire was finally reached on December 27, 2025.
Overall, the conflict killed more than 100 people and displaced more than half a million civilians in both countries. Its immediate cause was a modern military escalation. Yet its legal and technical roots could still be traced back to a single colonial-era map drawn 118 years earlier.
