Border flare-ups between Thailand and Cambodia seem endless: Can Asean do something?

Fighting erupted last week along their shared frontier and so far has killed at least 11 people. Both sides have exchanged artillery fire and Thailand carried out air strikes with F-16 jets after accusing Cambodia of firing rockets into civilian areas.

The neighbours have clashed repeatedly before, most recently in July, when dozens were killed and tens of thousands displaced. A fragile ceasefire—brokered in part with Trump’s self-touted involvement—has collapsed, and there is no clear diplomatic off-ramp in sight.

Washington is concerned by the continued clashes and casualties along parts of the 800km border. US secretary of state Marco Rubio urged both sides to immediately cease hostilities and to return to the measures outlined on 26 October in the Kuala Lumpur Peace Accords, which were one of Trump’s more prominent successes in branding himself a global peacemaker.

But a return to calm looks unlikely, at least in the near term. Both governments are leaning on deeply rooted nationalist sentiment to shore up support. The hardened rhetoric lays the groundwork for a prolonged and destructive conflict.

Like many disputes in Asia, this one is a hangover from Western imperialism, though the roots go back further. The once-powerful Khmer Empire declined and what was left was being nibbled away for centuries by the ascendant Siam in the west and what is now Vietnam in the east.

In colonizing Indochina, France made “the land in-between” a protectorate in 1863 and expanded its shrunken frontiers. Siam, now Thailand, remained independent despite pressure from France and also Britain, which ruled over Burma and India.

A Franco-Siamese treaty signed in 1904 proposed the frontier would follow the watershed line of the Dângrêk Mountains. But another map produced by French officials in 1907 deviated from that, placing key landmarks, including the 11th-century Preah Vihear temple, on the Cambodian side. Thailand later argued the map was never properly approved, while Cambodia maintained it was binding because Siam did not formally object at the time.

In 1962, the International Court of Justice ruled in favour of Phnom Penh, a decision it reaffirmed in 2013. But the surrounding border area was never fully demarcated, laying the groundwork for recurrent tensions.

The danger this time is how much more firepower is being used. Thailand’s forces are far larger and better equipped; the country is a longstanding ally of the US. Much-poorer Cambodia fields a smaller military and relies on Chinese assistance.

The risks of escalation cannot be overstated. The Association of Southeast Asian Nations (Asean) has historically avoided wading into the affairs of other members because of its policy of non-interference.

But fighting between two member countries cannot but be of concern. As it intensifies, voices within the region are calling for stronger intervention.

This year’s chair, Prime Minister Anwar Ibrahim of Malaysia, says he’s ready to help (as he did with the Trump deal) and is urging de-escalation. His efforts are sensible, but ultimately insufficient.

It’s a longstanding complaint, but the bloc has to stop managing crises and start resolving them. Installing an independent observer mission along the border and creating a mechanism to verify and publicly report violations would be a start towards real accountability. Some Asean states have been involved in monitoring the ceasefire, but that’s a long way from enforcement.

Resolving the border dispute would be even harder. It would need buy-in from both governments. Any demarcation process should be conducted by neutral surveyors, anchored in the existing world court rulings. Both Bangkok and Phnom Penh would need to commit publicly to accept the outcome and refrain from inciting nationalist sentiment further.

This won’t be easy. Both sides have compelling domestic incentives to let the fighting go on. Thai Prime Minister Anutin Charnvirakul is using the conflict to tap nationalist sentiment before an election as early as 2026. Cambodia’s most influential politician, Hun Sen, has exploited the violence to reassert himself in politics, portraying himself as a national defender.

Until Southeast Asia confronts its unresolved borders with some real determination, the war over a line on a map will remain a recurring threat the region can no longer afford. ©Bloomberg

The author is a Bloomberg Opinion columnist covering Asia politics with a special focus on China.