Cambodia–Thailand border

Khmer and Thai

From the 9th to the 13th century the Khmer empire, centred on Angkor, ruled much of the region. The Thai kingdoms that pushed south afterwards raided Angkor several times, and by the mid-19th century Siam and Vietnam were jointly controlling Cambodia through kings they chose.^[1]

France made Cambodia a protectorate in 1863, and in 1867 Siam recognised it while keeping the provinces of Battambang and Angkor. As France and Britain pressed on Siam from both sides, a convention of 1904 set the line along the watershed of the Dângrêk range, and a treaty of 1907 handed Battambang, Siem Reap and Sisophon to French Cambodia.^[1]

The line on the ground

From the Gulf of Thailand the border follows the watershed of the Cardamom Mountains through dense forest. North-west of Pailin it drops into the plain west of the Tonle Sap, following rivers and straight lines past Poipet, where the Phnom Penh–Bangkok road and railway cross. It then climbs the Dângrêk escarpment and runs east along its crest to the Laos tripoint.^[1]

Siam never forgot the lost provinces. In 1941, after France’s defeat in Europe and a brief war, a Japanese-brokered treaty gave them back to Thailand; after the Second World War the Washington Accord of 1946 returned them to France.^[1]

Preah Vihear

The Khmer temple of Preah Vihear stands on the edge of the Dângrêk escarpment. After Cambodian independence Thai police occupied it, and in 1959 Cambodia took the case to the International Court of Justice. Thailand argued the temple lay north of the watershed the treaties named; Cambodia relied on the delimitation commission’s map, which put it on the Cambodian side. On 15 June 1962 the court ruled that the map was valid and the temple Cambodian.^[1] Thailand accepted the ruling in practice but kept a vague reservation about it and about the colonial treaties, which it says it signed under duress.^[1]

In 2013, at Cambodia’s request, the court interpreted its judgment: the 1962 ruling had given Cambodia the whole promontory of Preah Vihear, and Thailand had been obliged to withdraw its forces from it.^[3]

“The Judgment of 15 June 1962 decided that Cambodia had sovereignty over the whole territory of the promontory of Preah Vihear”^[3]

War in 2025

In July 2025 the dispute turned into five days of fighting. Malaysia brokered a ceasefire, pushed through by US President Donald Trump’s threat to withhold trade privileges, and it was formalised at a regional meeting in Kuala Lumpur in October.^[4] Heavy fighting broke out again in early December. Only Thailand used airstrikes; Thai soldiers were wounded by what Thailand said were newly laid Cambodian mines, while Cambodia said they were left over from its civil war.^[4]

On 27 December 2025 the two defence ministers signed a new ceasefire at a border checkpoint, ending 20 days of fighting that had killed more than 100 people and displaced more than half a million.^[2][4] Both sides agreed to hold their troops in place, with ASEAN observers watching, and to resume demarcation.^[2] In April 2026 Cambodia, which says Thai forces have kept areas they captured, urged Thailand to restart border talks; Thailand said it had its own procedures to follow first.^[5]