At war since 1352, the ancient kingdoms of Ayutthaya and Angkor Wat are modernly known as Thailand and Cambodia [1]. Much of their dispute today is over borders, but the two kingdoms were once borderless mandala states that fought for dominance in Mainland Southeast Asia. Such history is critical to understanding their ongoing conflict, which has left hundreds dead and half a million civilians displaced [2, 3]. Yet President Trump believed that with a little pressure, he could get them to sign a ceasefire. Sign they did, but the agreement never had a chance for success due to the historical context of the border dispute and domestic political escalation.
Southeast Asian political city-states were shaped after the Hindu mandala, consisting of decreasing spheres of influence with a “God-King” (devaraja) at the center [4]. These mandala states were constantly at war with one another, not over land, but over people, whom they brought back to replenish their city-centers [5]. As a result, the people of Southeast Asia are highly intermixed, with many overlapping cultural claims. When the European powers arrived, they were consistently confused, as European states established strict borders while states in the mandala system did not [6].
Another key aspect is that mandala states’ “alliances” were between rulers rather than states, and thus were fluid [7]. People caught in the middle often switched sides based on who they predicted would win battles, essentially, who appeared to be the stronger power.
The Khmer Empire, centered in Angkor Wat, dominated large parts of Thailand, Laos, Cambodia, and Vietnam from the 9th to 15th centuries [2]. As their empire declined, the kingdom of Ayutthaya, primarily in modern-day Thailand, rose to challenge them [2].
The French, who colonized Cambodia, drew the modern border in 1907 under the Franco-Siamese Treaty [2]. It was supposed to follow the watershed of the Dangrek Mountains [6]. At the time, the King of Siam, previously known as Ayutthaya, had little understanding of borders and what they meant. The French, by contrast, were intentional in trying to “retrieve” a lost history and expand their colonial domain. They deliberately selected historical sites to remain in Cambodia, resulting in the drawn border deviating significantly from the watershed agreed upon in the treaty [8, 9]. The treaty ceded many border areas to Cambodia that were later retaken by the Thai during WWII, only to soon be returned to France as a condition for Thailand’s entry into the UN [2]. Since then, the International Court of Justice rulings repeatedly backed Cambodia’s claims to various border areas [4].
The border conflict is primarily over competing claims to the ancient temples of Preah Vihear and Ta Muen Thom, both with roots in the ancient Khmer kingdom of Angkor [2, 9]. From 2008 to 2011, skirmishes along the border between Cambodia and Thailand erupted following Cambodia's application for UNESCO World Heritage status for the Preah Vihear temple [2]. On May 28, 2025, a skirmish near the same temple resulted in the death of a Cambodian soldier and reignited violence [2].
Arguments regarding the border temples could be interpreted as religious in nature. However, it is more accurate to view them as part of a broader struggle over who can legitimately claim the history of the ancient Khmer Empire [10, 11, 12]. Today, the Thai assert that aspects of Thai culture originate from Khmer traditions [2]. Yet the majority of ethnic Khmer live in Cambodia, and the ancient city of Angkor Wat even appears on Cambodia’s national flag [4].
Further domestic complications arose when a conciliatory phone call between Thai Prime Minister Paetongtarn Shinawatra and former Cambodian Premier Hun Sen was leaked, likely by Hun Sen himself, revealing Paetongtarn referring to Cambodia’s strongman as “uncle” [13, 14]. PM Paetongtarn attempted to explain the phone call as part of “negotiations,” but she lost public trust and was removed by a judge [14, 15]. Nationalist sentiment constrains political actors and continues to fuel disagreements on both sides [3, 9, 10, 12].
Following an emergency meeting of the United Nations Security Council in July 2025, President Trump announced that Cambodia and Thailand had “agreed to work toward a ceasefire” [2]. The key words are “work toward.” Fighting continued despite multiple ceasefire agreements. It seems evident that the fluidity inherited from their mandala past shapes their current political behavior. Under the mandala framework, it is logical to sign a ceasefire in the presence of a greater power, such as the United States, and then resume business as usual.
President Trump believed he could solve wars through personal diplomacy, but history runs deeper than 2026. He tried nonetheless, promising a $45 million U.S. aid package focused on border stabilization, unexploded ordnance clearance, and anti-trafficking operations, while also threatening to halt trade talks with the two countries unless the conflict was resolved [16, 17].
A more solidified ceasefire was signed in December 2025 and has largely held through the time of writing, though small skirmishes occurred in January, and Cambodia recently reported continual Thai occupation of Cambodian territory [18, 19]. Yet for the same reasons as before, it too appears destined to fail. The recurring instability is not simply a product of poor diplomacy but of centuries of political tradition. Both Thailand and Cambodia still operate, consciously or not, within the logic of the mandala, in which alliances are fluid, power is relational, and deference to greater powers is strategic rather than permanent. Until these historical patterns are addressed, any ceasefire will be insufficient to address an older, ongoing contest for influence in Southeast Asia.
