The escalating border conflict between Thailand and Cambodia in December 2025 represents a critical failure of regional diplomacy and an undermining of the ASEAN Centrality. Cambodia’s attempts to internationalize the dispute through the United Nations Security Council and threats of petitioning the International Court of Justice for provisional measures clearly run counter to the central ASEAN principle of regional autonomy and dealing with issues regionally.

By prioritizing nationalist optics over regional stability, Phnom Penh undermined a foundational norm of ASEAN which Acharya has noted formed the glue since 1967: regional autonomy. Two central aspects, which will be focus of this piece, are the mutual respect for sovereignty and most cherished norm of ASEAN not inviting external or third parties into regional disputes, instead settling issues within ASEAN.

The ‘Gentlemen’s Agreement’ and Fragile Peace

A primary driver of the hostilities is Cambodia’s breach of the 2011 ‘gentlemen’s agreement’ regarding the Ta Muen temple complex. This informal pact had successfully maintained the site as a demilitarized zone for over a decade. The 2011 verbal gentleman’s agreement was established as an important de-escalation mechanism following the military skirmishes of 2008–2011. Under the terms of this informal pact, both Thailand and Cambodia agreed that all soldiers would abandon their front-line military positions to create a demilitarized buffer. To maintain the site as a shared heritage zone, a specific compromise allowed for 5-7 unarmed soldiers from both countries to be stationed within the Ta Muen temple complex specifically to facilitate and assist tourists from their respective sides. These personnel were restricted to the temple grounds only between the hours of 9:00 and 15:00.

This arrangement represented a significant and reasonable concession from the Thai side. Despite the 2008-2011 conflict resulting in Thailand’s full and effective control over the entire Ta Muen complex, Bangkok chose to forgo exclusive military occupation in favor of deconfliction. By agreeing to these terms, Thailand formally recognized that the temple complex was located in disputed territory, demonstrating a commitment to the ASEAN spirit of peaceful coexistence over unilateral territorial assertion.

An important turning point in the 2025 escalation occurred in February, when the delicate peace at the Ta Muen Thom ruins was broken by a display of Cambodian nationalism. On February 13, 2025, a group of Cambodian civilians and soldiers entered the site and filmed themselves singing the Cambodian national anthem, a symbolic act of territorial claim that directly violated the 2011 gentlemen’s agreement. This informal pact had long restricted the site to joint religious use while prohibiting political or nationalistic demonstrations. The provocation was not an isolated civilian incident; it was actively encouraged by Cambodian officials, with Oddar Meanchey provincial governor Mean Chanyada later publicly urging citizens to “continue singing the anthem” at the temple.

The involvement of the Cambodian military further signaled that this was a state-sanctioned breach of the status quo. During the confrontation, a high-ranking Cambodian commander on-site challenged Thai troops to open fire, asserting that Thai forces had no right to be on the land. By instrumentalizing a shared cultural heritage site for nationalist theater, the Cambodian military leadership effectively dismantled the 2011 peace framework, setting the stage for the deadly military skirmishes that would erupt later in the year.

Violation of Sovereignty and the ASEAN Way

Hun Sen’s decision in June 2025 to release a private 17-minute audio recording of a conversation with Thai Prime Minister Paetongtarn Shinawatra represents one of the most brazen violations of the ASEAN non-interference principle in the bloc’s history. By making the recording public on his Facebook page, Hun Sen did not merely breach diplomatic protocol; he actively weaponized private state communications to fuel a political crisis in Bangkok.

This act directly led to the collapse of the Thai government and indirectly the eventual removal of the Thai Prime Minister by the Constitutional Court. Such actions directly contradict Article 2 of the Charter, which explicitly forbids members from breaching sovereignty of another ASEAN member. This interference transformed a localized border issue into a tool for domestic political leverage, making a diplomatic resolution all the more difficult.

Undermining ASEAN: Attempts to Internationalize the Dispute

Finally, Cambodia’s strategy of internationalization serves as a blow to regional autonomy and ASEAN Centrality. By bypassing ASEAN’s mediation mechanisms and seeking intervention from the United Nations Security Council and the International Court of Justice, Phnom Penh has signaled a lack of faith in the regional bloc. This maneuver violates the long-standing ASEAN norm of solving ASEAN security issues internally within the bloc.

One day after conflict broke out in July 2025, Cambodian diplomats submitted a letter asking the UNSC to discuss and include the border dispute onto the council’s agenda. In the run up to the July clashes, Cambodia had previously ‘threatened’ to petition the ICJ in an attempt to relitigate the Preah Vihear issue in June. The veiled threat was not followed through but nonetheless again threatened to breach regional autonomy by inviting third party intervention.

It can be argued that Cambodian actions were necessary to stave off war or that Phnom Phen had no other options. This argument does not hold up to scrutiny as the leaked phone call demonstrated PM Paetontarn’s willingness to deal and her government’s subsequent silence and inaction after the phone call leak. Hun Sen, personally, continued escalation, openly calling for regime change in Bangkok, noting “I hope there will be a new prime minister in Thailand who will come out and solve issues with neighboring countries, especially with Cambodia.” These acts in total constitute genuinely unprecedented breaches of the ASEAN Way and ASEAN’s raison d’etre of regional autonomy and ASEAN Centrality which have been ASEAN cornerstones since its founding.

Concluding Thoughts

The issue of blame or wrongdoing is peripheral to this discussion. Instead, the border dispute between Cambodia and Thailand has treaded into unchartered political space of one ASEAN member intervening in another’s internal affairs to the extent that it collapsed a government. Further, the cherished norm of ‘settling’ disputes without third party interference has been breached. The reasons for these actions are not relevant. Cambodia’s threatened ICJ case is not nearly as important as asking for intervention from the UNSC, which for the past four years subcontracted the Myanmar issue to ASEAN. This openly demonstrated to great powers and the international community that an ASEAN member was bypassing its regional organization and asking for external intervention. The damage to ASEAN’s reputation and credibility were not served well by Phnom Phen’s actions nor do they bode well for a future of increased great power competition and an America that has thrown global norms to the wind as it withdraws from international organizations.

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