The Cambodia-Vietnam maritime boundary dispute mirrors a global pattern of nations vying for control of strategic waters, echoing historical conflicts like the centuries-long disputes over the North Sea between Britain and Norway, or the centuries-long rivalry between Spain and Portugal over control of the Atlantic. These aren’t just lines on a map, but battles for valuable resources, economic dominance, and national pride. Just as those historical conflicts were fueled by imperial ambitions and economic interests, the Cambodia-Vietnam dispute is rooted in historical animosities, fueled by the legacy of French colonialism and Cold War-era friction.

This struggle over maritime borders, like the simmering tensions in the Gulf of Thailand or the fierce rivalries over the Spratly and Paracel Islands, is a high-stakes game of power and diplomacy where the future wealth of the region hangs in the balance. So, let’s dive into the depths of this maritime conflict and uncover what’s really at stake.

Dispute Overview

The dispute between Cambodia and Vietnam centers on overlapping claims in the resource-rich Gulf of Thailand, an area known for its untapped oil and gas reserves, as well as essential fishing grounds. Thailand currently operates 34 active petroleum exploration and production projects across 47 fields, with a daily production capacity of approximately 558,000 barrels of oil equivalent. This includes 2.4 billion cubic feet (bcf) of gas per day and 75,000 barrels per day (bpd) of condensate, alongside crude production of around 70,000 bpd, according to the Thai Department of Mineral Fuels.

The Gulf’s significance is highlighted by its location, bordering Cambodia, Vietnam, Thailand, and Malaysia, making it a critical waterway for trade, energy, and fisheries that support the economies of all four countries. In contrast, Cambodia’s gas consumption is minimal, as the country lacks gas-to-power plants despite having six offshore oil and gas blocks that typically do not produce fossil fuels. In 2021, Cambodia’s power mix comprised 41 percent hydropower, 41 percent coal, 8 percent fuel oil, and 6 percent solar.

For both Cambodia and Vietnam, the maritime boundary is not merely a matter of drawing lines in the sea; it represents a competition for access to a larger share of maritime resources. While the dispute may be quieter than the South China Sea conflict, it carries substantial implications for energy security and geopolitical dynamics in Southeast Asia.

Historical Background

Colonial Era

The historical backdrop of the Cambodia-Vietnam maritime boundary dispute is rooted in the late 19th and early 20th centuries during the height of French colonial rule. From 1863, Cambodia became a French protectorate, while Vietnam was already under French control since the mid-19th century. Vietnam, divided into the regions of Cochinchina, Annam, and Tonkin, was more directly integrated into French colonial administration, while Cambodia maintained some nominal independence under the French Indochina system. Significant boundary issues arose in the 1930s when France began establishing administrative divisions between the two territories, but these efforts were largely incomplete and unclear, especially concerning the maritime boundaries.

The Franco-Siamese treaties of 1904 and 1907 also played a role, shifting territories between Cambodia and what is now Thailand, yet leaving Vietnam’s maritime boundary with Cambodia unresolved. After both nations gained independence—Vietnam in 1954 following the First Indochina War, and Cambodia in 1953—the colonial-era treaties and administrative decisions became points of contention. The lack of specific, clear maritime demarcation during the colonial period led to disputes over the Gulf of Thailand, with both Cambodia and Vietnam claiming overlapping areas, especially after the discovery of oil and gas reserves in the region during the 1970s.

Post-Independence Period 

Following their respective independence (Cambodia in 1953 and Vietnam in 1954) the maritime boundary dispute between the two nations began to intensify. Early tensions were shaped by a combination of historical grievances and differing interpretations of colonial-era boundaries. The Vietnam War (1955–1975) further complicated matters, as Cambodia’s neutral stance and eventual involvement in the conflict created a tense atmosphere that postponed any attempts at resolving maritime issues.

One of the first major points of contention occurred in the late 1960s and early 1970s, as both nations started to explore the Gulf of Thailand for potential oil and gas reserves. With no clear maritime boundary in place, overlapping claims led to diplomatic friction. By the mid-1970s, after the Vietnam War, tensions boiled over when the newly unified Socialist Republic of Vietnam and the Khmer Rouge regime in Cambodia clashed, leading to open conflict.

A key incident was the 1975-1979 Cambodian-Vietnamese War, when Vietnam invaded Cambodia and overthrew the Khmer Rouge regime, installing a pro-Vietnamese government. This conflict not only worsened relations but also delayed any meaningful negotiations over maritime boundaries. During this period, Vietnam gained de facto control over several disputed islands in the Gulf of Thailand, increasing Cambodian resentment.

In the 1980s and early 1990s, as both countries began to recover from decades of war, the dispute remained unresolved, though diplomatic efforts were occasionally attempted. The tensions from these early post-independence conflicts shaped the trajectory of the dispute, with both nations remaining locked in a struggle for control over the valuable waters of the Gulf of Thailand.

Legal and Political Developments

In an effort to resolve their maritime boundary disputes through bilateral negotiations, Vietnam and Cambodia made multiple attempts after decades of hostility. The maritime aspect remained especially difficult because of strategic and economic importance. 1982 saw the signing of one of the first official accords as part of the efforts to restore relations following the Cambodian-Vietnamese War. This signaled the start of a more formalized diplomatic dialogue between the two countries about their maritime boundaries. In 2005, both nations signed agreements outlining their intentions to cooperatively explore and develop resources in the overlapping areas of the Gulf of Thailand while negotiations were ongoing, as part of further efforts to resolve the conflict.

Following Vietnam’s occupation of Cambodia, a treaty was signed in 1982 in an effort to formally define the borders between the two countries. Although it had opportunities for additional discussions on maritime boundaries, the main focus of this agreement was land borders. The validity of this pact has been called into question, nevertheless, with some in Cambodia contending that it was signed in favor of Vietnam because of the military occupation.

In spite of this, tensions over maritime rights persisted, and it set the stage for more talks. Regarding maritime disputes, the 2005 deal marked a significant turning point in bilateral ties. Both countries consented to a cooperative development plan in disputed waters, especially in regions thought to contain significant gas and oil deposits. 

In the absence of a definitive maritime boundary settlement, the agreement was considered a stopgap measure. It placed more emphasis on collaboration than rivalry in an effort to reduce hostilities by enabling both nations to gain economically. However, some Cambodians perceived this deal as a surrender to Vietnam’s increased power because it did not settle the fundamental sovereignty problem. 

Key Areas of Dispute

According to UNCLOS, each country is entitled to a 200-nautical mile EEZ from their coastline, but due to the proximity of borders, the claims overlap, creating zones of contention. For example, Koh Tral, also known as Phu Quoc Island, is a significant issue in the dispute over the maritime boundary between Cambodia and Vietnam. Koh Tral is within the overlapping claims of Vietnam and Cambodia, although being better known by its Vietnamese name, Phu Quoc Island. There has been a great deal of conflict over its inclusion in Cambodia’s territorial claims.

For both countries, Phu Quoc Island is strategically and historically significant. Its location in the Gulf of Thailand and its significance to Vietnam’s tourism sector make it an important military and economic asset for the nation. Since the French colonial era, the island has historically been a part of Vietnam’s government system. The island is essential to Cambodia’s nautical interests because of its advantageous location, which gives it access to resource-rich waters and important shipping lanes.

Military and Diplomatic Efforts

Military clashes between Cambodia and Vietnam in the 1970s and 1980s involved skirmishes over disputed areas like Koh Tral (Phu Quoc Island). These fights increased regional tension and hurt local fishing communities, disrupting lives and economies. Cambodia, with about 120,000 active military personnel and a smaller navy, faces a stark contrast with Vietnam, which has 482,000 active troops and a larger, more advanced navy. This military imbalance has heightened the stakes of the dispute, affecting both regional security and civilian lives. Although diplomatic talks have made significant progress, they remain incomplete

Recent Developments

Vietnam and Cambodia have maintained their diplomatic ties in recent years, holding sporadic high-level discussions with the goal of settling their maritime boundary issue. In an effort to keep the lines of communication open even in the face of persistent differences, efforts have included talks on cooperative resource management and confidence-building strategies. The disagreement has not been resolved as of 2024. Although no new, substantial agreements have been achieved, both countries have indicated that they are eager to carry on with talks. As long as negotiations on a definitive boundary demarcation continue, both nations may cooperatively use resources in disputed areas thanks to the continued validity of the 2005 joint development agreement.

End Note

In essence, the Cambodia-Vietnam maritime dispute is deeply rooted in historical grievances, colonial legacies, and the strategic importance of the resource-rich Gulf of Thailand. While both nations have made efforts through diplomatic negotiations and agreements, such as the 2005 joint development accord, to manage their overlapping claims, tensions persist. The dispute, though less prominent than other regional conflicts like the South China Sea, carries significant implications for energy security, national sovereignty, and regional stability.

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