The emerging maritime boundary alignment between the Philippines and Vietnam is reshaping the security architecture of the First Island Chain.
For decades, Beijing has managed regional tensions by keeping disputes compartmentalized and treating frictions with Japan, pressure on Taiwan, and coercion in the South China Sea as separate, bilateral issues. That approach is eroding.
By grounding claims in the UN Convention on the Law of the Sea and reinforcing the 2016 arbitral ruling, regional states are narrowing the space for legal ambiguity. Paired with expanding trilateral coordination among the US, Japan and the Philippines, this shift strengthens Taiwan’s strategic position by hardening key maritime chokepoints and raising the cost of coercion.
Taiwan sits at the center of the First Island Chain, just north of the Luzon Strait and Bashi Channel — corridors critical for naval access between the South China Sea and the western Pacific. For years, overlapping and undefined maritime claims to Taiwan’s south created gray-zone space that Chinese forces could exploit.
A formal delimitation agreement between Manila and Hanoi would change that. Clearly defined, UNCLOS-consistent boundaries would establish internationally recognized coordinates, reducing room for competing interpretations. While such an agreement would not bind China, it would strip away much of the legal cover Beijing has used to justify persistent presence in disputed waters.
For Taiwan, that clarity supports deterrence by denial. Defined boundaries constrain efforts to normalize military activity under vague administrative claims, thereby imposing higher operational and political costs on any expansion of Chinese naval activity near the island’s southern approaches.
Trilateral sea denial
Legal alignment is increasingly matched by hard-power coordination. The US, Japan and the Philippines have moved beyond episodic exercises toward a more persistent, integrated posture focused on sea denial in the Luzon Strait.
US forces have expanded access to bases in northern Luzon and Batanes under the Enhanced Defense Cooperation Agreement, placing capabilities within roughly 150 kilometers of Taiwan. Japan, meanwhile, has strengthened missile and surveillance networks across its southwestern islands, including radar coverage from Yonaguni that monitors key approaches to the strait. Tokyo and Manila have also deepened defense ties through their Reciprocal Access Agreement and ongoing security cooperation.
Together, these measures signal credible commitment. Forward-deployed assets, shared infrastructure and tighter coordination indicate that any cross-strait contingency would likely trigger a coordinated response along a contiguous line stretching from Japan through the Philippines.
Such a posture complicates attempts by China’s military to impose a blockade or execute an amphibious operation.
This shift comes amid intensified Chinese activity across the region. Maritime authorities have expanded enforcement operations around Taiwan, including inspections of foreign vessels east of the island that analysts say blur the line between regulation and coercion.
High-stakes incursions
Near Japan, Chinese ships continue regular incursions around the Senkaku Islands. In the South China Sea, confrontations with Philippine and Vietnamese vessels have included water cannons, lasers and blocking maneuvers.
Regional responses are becoming more coordinated. Joint exercises have expanded into the Bashi Channel, and officials in Taipei have increased engagement with partners to align messaging and responses to gray-zone tactics.
As Southeast Asian states clarify their maritime boundaries, Taiwan faces pressure to update its own legal posture. One option is to limit its claims around Taiping Island to a 12-nautical-mile (22.2 km) territorial sea, aligning with the 2016 arbitration ruling that classified the feature as a “rock” without an exclusive economic zone.
Such a move would not resolve sovereignty disputes, but it could ease tensions with neighbors. The Philippines and Vietnam could acknowledge Taiwan’s administration of Taiping’s immediate waters without compromising their broader 200-nautical-mile claims.
Law as leverage
A similar understanding between Taipei and Hanoi could reduce friction in the Spratly Islands by pairing Vietnam’s broader claims with Taiwan’s limited, clearly defined zone.
By narrowing its claims, Taiwan would signal that its posture is defensive rather than expansionary. That, in turn, would help isolate Beijing’s sweeping historical assertions and reinforce a rules-based framework for managing disputes.
China would almost certainly condemn such steps and could respond with intensified gray-zone pressure, particularly around Taiping. But the broader trajectory is clear: maritime delimitation is becoming a tool of collective security.
Replacing ambiguity with defined legal boundaries, backed by coordinated military posture, reduces opportunities for coercion and strengthens deterrence. In that environment, Taiwan is not isolated at the edge of the system but increasingly integrated into a wider defensive network shaping the future of the Indo-Pacific.
