Vietnam Deposits Territorial Sea Baseline Chart for Gulf of Tonkin with UN

Vietnam has officially submitted a chart and a list of geographic coordinates defining the baseline for its territorial sea in the Gulf of Tonkin to the United Nations, marking a significant step in asserting its maritime boundaries under international law. The announcement, made by UN Secretary-General António Guterres on 12 March 2025, confirms that Vietnam deposited these documents on 7 March, pursuant to the 1982 United Nations Convention on the Law of the Sea (UNCLOS). This move, detailed on the UN’s website two days later, underscores Hanoi’s commitment to clarifying its maritime claims in a region long marked by overlapping territorial disputes.

The submission includes a chart illustrating the straight baselines and outer limits of Vietnam’s territorial sea, alongside precise geographic coordinates for the baseline along its mainland territory in the Gulf of Tonkin. This area, situated in the northwestern part of the South China Sea, has historically been a flashpoint for tensions, particularly between Vietnam and China, due to competing claims over maritime boundaries and resources. Vietnam’s latest action, accompanied by a government statement issued on 21 February 2025, will also be featured in the upcoming issue of the UN’s ‘Law of the Sea Bulletin,’ further formalising its position on the global stage.

A Strategic Assertion of Sovereignty

Vietnam’s deposit of these documents is more than a procedural formality; it is a calculated assertion of sovereignty in a geopolitically sensitive region. The Gulf of Tonkin, bordered by Vietnam to the west and China to the north, holds strategic importance for trade routes, fisheries, and potential energy reserves. Under UNCLOS, a coastal state may define its territorial sea—extending up to 12 nautical miles from its baseline—where it exercises full sovereignty over the waters, seabed, and airspace. By depositing its baseline chart with the UN, Vietnam seeks to establish a clear legal foundation for its maritime jurisdiction, a move that could influence future negotiations or disputes.

This step comes at a time when maritime tensions in the broader South China Sea remain high. While the Gulf of Tonkin is distinct from the more contested areas like the Paracel and Spratly Islands, where Vietnam, China, and several other nations have overlapping claims, it is not immune to friction. A 2000 agreement between Vietnam and China on maritime delimitation in the Gulf of Tonkin was a rare diplomatic success, establishing a shared boundary for territorial waters and exclusive economic zones (EEZs). However, implementation challenges and differing interpretations of the agreement have occasionally strained relations, particularly over fishing rights and resource exploration.

Analysts suggest that Vietnam’s submission to the UN could serve multiple purposes. Firstly, it reinforces Hanoi’s adherence to international law, positioning the country as a responsible actor in maritime governance. Secondly, it provides a documented basis for defending its claims against potential encroachments. “This is a proactive measure by Vietnam to safeguard its interests in the Gulf of Tonkin,” said Dr. Le Thi Minh, a maritime law expert at Hanoi National University. “By formalising its baselines under UNCLOS, Vietnam is creating a legal shield that can be invoked in diplomatic or judicial forums if disputes arise.”

Regional Implications and Potential Reactions

The implications of Vietnam’s action extend beyond its borders, potentially eliciting responses from neighbouring states, particularly China. While the 2000 delimitation agreement reduced the scope for direct conflict in the Gulf of Tonkin, Beijing has historically been sensitive to unilateral moves by other claimants in the South China Sea region. Although Vietnam’s submission pertains specifically to its mainland territorial sea and does not directly challenge the agreed boundary, it may still be interpreted as a broader signal of Hanoi’s intent to assert its maritime rights more robustly.

If China or other regional actors were to contest the submitted baselines—though there is no immediate evidence to suggest they will—it could reignite dormant tensions. Conditional analysis indicates that, if Beijing perceives this as a precursor to further Vietnamese claims in disputed areas, diplomatic exchanges or even maritime incidents could follow. However, such outcomes remain speculative, and no adverse inferences should be drawn at this stage. Vietnam’s Foreign Ministry has yet to comment on the submission, but past statements have consistently emphasised the country’s commitment to resolving maritime issues through dialogue and adherence to international law.

Beyond China, other South East Asian nations with interests in UNCLOS compliance may view Vietnam’s move as a precedent. Countries like the Philippines and Malaysia, which have their own maritime disputes with China, could draw inspiration from Hanoi’s approach to formalising claims through UN mechanisms. This could, in turn, encourage a more coordinated regional stance on maritime governance, though such cooperation remains uncertain and contingent on broader geopolitical dynamics.

Legal and Technical Dimensions

From a legal perspective, Vietnam’s submission aligns with Article 16, paragraph 2 of UNCLOS, which requires coastal states to deposit charts and coordinates defining their territorial sea baselines with the UN Secretary-General. This ensures transparency and provides a public record that other states can reference. The baseline is a critical reference point: it determines not only the territorial sea but also the starting line for measuring the 200-nautical-mile EEZ and, where applicable, the continental shelf, both of which grant states rights to resources like fish stocks and hydrocarbons.

The technical details of Vietnam’s submission, as published on the UN website, include a chart and a list of coordinates specific to the Gulf of Tonkin. While the exact content of the chart is not publicly detailed in the initial announcement, its focus on the mainland territory suggests it does not extend to Vietnam’s claims over disputed islands like the Paracels, which are also claimed by China and Taiwan. This distinction is crucial, as it limits the scope of potential contention while still affirming Vietnam’s coastal rights.

One technical challenge in defining baselines in the Gulf of Tonkin lies in the region’s geography. The gulf features a complex coastline with numerous bays, inlets, and islands, which can complicate the drawing of straight baselines under UNCLOS rules. Vietnam’s 21 February statement, to be published in the UN bulletin, likely addresses how these baselines were determined, ensuring compliance with international standards. Should discrepancies or ambiguities arise in the interpretation of these coordinates, they could become points of contention in the future, though no such issues have been raised as of now.

Broader Context of Vietnam’s Maritime Strategy

Vietnam’s deposit of its baseline chart must also be viewed within the context of its broader maritime strategy. Over the past decade, Hanoi has increasingly prioritised modernising its navy and coast guard, enhancing its capacity to monitor and defend its maritime zones. At the same time, it has pursued diplomatic avenues to strengthen its position, including active participation in ASEAN discussions on a Code of Conduct for the South China Sea and bilateral engagements with partners like the United States, Japan, and the European Union.

This latest action reflects a dual approach of legal fortification and international cooperation. By aligning its maritime claims with UNCLOS and making them public through the UN, Vietnam not only bolsters its domestic legal framework but also garners international legitimacy. This is particularly important for a country that relies heavily on maritime trade and fisheries for its economy—sectors that are directly impacted by the security and stability of its waters.

Moreover, Vietnam’s move may resonate with domestic audiences as a demonstration of national resolve. Maritime sovereignty is a deeply emotive issue in Vietnam, tied to historical narratives of resistance against external powers. While the government has not framed the UN submission as a confrontational act, it is likely to be perceived by many Vietnamese as a firm step in protecting national interests.

Looking Ahead: Stability or Friction?

As Vietnam’s baseline chart becomes part of the UN’s official record, the immediate focus will be on whether this action contributes to stability or introduces new friction in the Gulf of Tonkin. On one hand, transparency in maritime claims can reduce misunderstandings and prevent accidental escalations. On the other, the geopolitics of the South China Sea region are rarely straightforward, and even procedural steps can be politicised.

Conditional analysis suggests that if Vietnam’s submission is met with silence or tacit acceptance by neighbouring states, it could pave the way for more constructive dialogue on maritime boundaries in the gulf. Conversely, if it prompts objections or counter-claims—though there is no evidence to suggest this is imminent—it may complicate regional dynamics. For now, disclaimers apply: these scenarios are speculative, and no adverse inferences should be drawn about the intentions or responses of other parties.

What is clear is that Vietnam has taken a significant step in asserting its maritime rights under international law. By depositing its territorial sea baseline chart with the UN, Hanoi has laid down a marker of its sovereignty in the Gulf of Tonkin, one that could shape the legal and diplomatic contours of its maritime future. As the region watches, the hope is that such actions foster clarity and cooperation rather than contention, though only time will reveal the full impact of this development.