In response to a May announcement that Japan and the Philippines would launch maritime boundary delimitation talks, China has deployed survey and law enforcement ships to conduct a series of unprecedented operations in the Philippine Sea. The scale and character of China’s response suggest that these operations are not just about sending a signal to Tokyo and Manila. Beijing may also see an opportunity to change the status quo by establishing a semi-permanent presence and exercising some form of jurisdiction east of Taiwan.
The Announcement and Beijing’s Response
On May 28, Japan and the Philippines unveiled a new set of initiatives underscoring their deepening strategic partnership. These included launching talks on an agreement to share classified military information and the transfer of soon-to-be-retired destroyers from Japan to the Philippine Navy. The two governments also agreed to begin negotiations on delimiting their overlapping maritime boundaries in the Philippine Sea. Fewer than 400 nautical miles separate Japan’s southwestern Ryukyu Islands from the Philippines’ northernmost Batanes Islands, producing overlapping exclusive economic zone (EEZ) and continental shelf claims under the United Nations Convention on the Law of the Sea (UNCLOS).
Beijing swiftly condemned the delimitation talks as “illegal, null, and void,” on the grounds that a large portion of the area in question also overlaps with maritime claims extending from Taiwan. Taipei has responded more cautiously, saying only that it should eventually be consulted and that its fishing rights must be safeguarded. While these overlapping claims originate from Japan’s southwest islands, it is worth noting that this does not involve the sovereignty dispute over the Senkaku Islands, which are administrated by Tokyo but also claimed by Beijing and Taipei. The overlap in question arises from the opposite-facing coasts of the Ryukyus and Batanes; the Senkakus lie farther to the northeast.
AIS tracks of two Chinese surveys. June survey: Donghai Jiu 113, Hai Xun 09, Hai Xun 08, Hai Xun 06. August survey: Xiang Yang Hong 03, China Coast Guard 1306, “ERP.”
Beijing followed its condemnation with a series of unprecedented, and at least partly illegal, operations by Chinese law-enforcement and survey vessels in waters east of Taiwan. Within days of Japan and the Philippines announcing the start of boundary talks, China’s Ministry of Transport carried out a “maritime traffic law enforcement and hydrographic survey operation” in those waters, deploying large patrol vessels including the 10,000-ton Hai Xun 09. During these operations, Chinese law enforcement hailed a reported 198 commercial vessels transiting the area and demanded their ports of call and crew details, which would be unlawful even if it had occurred in China’s EEZ. Weeks later, the Ministry of Natural Resources conducted a separate marine environmental survey that Chinese state commentary framed as building a fuller understanding of the country’s “jurisdictional areas.” Chinese vessels also reportedly operated near submarine-cable routes. By early July, Taiwanese officials warned that the sustained presence of mainland government vessels across portions of the First Island Chain appeared intended to establish a “new status quo.”
AIS tracks of the China Coast Guard 1306, 1401, 2304, 2305, and 2502
Commercial automatic identification system (AIS) data from Starboard Maritime Intelligence supports public reporting. Between June and August 2026, Chinese law-enforcement activity was concentrated in the area where all parties have overlapping claims. During that period, China Coast Guard vessels 2304 and 2502 accounted for much of the observable activity. The latest vessel active in the area has been by the China Coast Guard 1306, which has been on patrol since August 4 and was joined on August 14 by the research ship Xiang Yang Hong 3 to conduct a survey that remains ongoing as of the time of publishing. By contrast, virtually no Chinese law enforcement presence was observed on AIS in the same area between January and late May (before the announcement).
Assessing the Delimitation
Can two coastal states delimit a maritime boundary when a third state claims overlapping rights? A long history of state practice and international rulings say “yes.” Almost half of all delimitation cases involve overlapping claims by more than two states, but trilateral or multilateral boundary agreements are rare; most cases are resolved through a series of bilateral pacts.
Under UNCLOS Article 74.1, states are charged with delimiting their overlapping EEZs (and in Article 83.1, with identical language, their overlapping continental shelves):
The delimitation of the exclusive economic zone between States with opposite or adjacent coasts shall be effected by agreement on the basis of international law, as referred to in Article 38 of the Statute of the International Court of Justice, in order to achieve an equitable solution.
Most delimitation agreements are concluded through state-to-state negotiations, with no external body involved. However, UNCLOS Article 74.4 also provides that if no agreement can be reached within a “reasonable period of time,” states may resort to the compulsory dispute resolution mechanisms of UNCLOS, which generally result in the case being brought to an international court or tribunal. But whether concluded in or out of court, bilateral agreements are commonplace, even when the maritime areas being delimited involve the claims of a third state.
Agreements concluded bilaterally can vary in how they address third-party states with overlapping claims. Some delimit their claims with total disregard for those of third states, as Italy and Tunisia did in 1971 with respect to Malta. Some voluntarily restrict the area being delimited to avoid overlapping with an acknowledged third state. In such cases, the two states often extend their delimitation line only until it approaches the presumed “trijunction” – the eventual point of delimitation between their claims and that of the other party. For instance, Thailand and Vietnam in 1997 delimited their continental shelf boundary only up to the point where it reached the outer limit of Malaysia’s acknowledged claim. Both sides pledged to seek negotiations, so far unsuccessful, with Kuala Lumpur to demarcate the trijunction.
In rarer cases, two states have consulted a third party with overlapping claims during their bilateral talks, most famously Denmark and Norway with regard to Iceland in 1995. But in discussing that agreement, the UN Handbook on Maritime Boundary Delimitation pointedly affirms that “such a consultation did not constitute a ‘multilateralization’ of the negotiations and, in any case, there is no obligation to ‘multilateralize’ negotiations on maritime boundary delimitation,” warning that doing so could “jeopardize a difficult and sensitive bilateral negotiation.”
International tribunals have also consistently supported bilateral delimitation agreements in cases where third state claims are potentially involved, and have explicitly rejected objections related to third state claims in recent judgements.
In both Cameroon v. Nigeria (2002) and Nicaragua v. Colombia (2012), the International Court of Justice (ICJ) rejected requests by neighboring states (Equatorial Guinea and Costa Rica, respectively) to intervene on the basis that their maritime claims overlapped with the area being delimited and that a delimitation in the case at hand would prejudice their own “legal interests.” In Bangladesh v. Myanmar (2012), the International Tribunal for the Law of the Sea likewise rejected Myanmar’s argument that the tribunal could not delimit the overlapping continental shelf claims due to a potentially overlapping claim from India. In these and other such cases, the courts have commonly cited two principles in explaining their decisions to proceed with delimitation.
First, under international law, treaties and agreements between two states have no power over non-parties. This means that any bilateral delimitation line has no legal effect on the claim of a state not party to the agreement. This principle is articulated and reaffirmed in several conventions, including Article 34 of the Vienna Convention on the Law of Treaties and Article 296.2 of UNCLOS, where it directly applies the principle to decisions made by courts or tribunals under the convention’s dispute resolution mechanism.
Second, international judgements on maritime delimitation have consistently accounted for possible third state claims. When courts delimit these areas bilaterally, they generally draw the demarcation lines only in areas where third state claims are clearly not implicated. Beyond that, judgements usually describe the line of demarcation continuing along an established path “until it reaches the area where the rights of third States may be affected.” In this way, the court protects the possible claims of third states without defining their geographic extent.
In the context of this long record of bilateral delimitation, the Japan-Philippines talks are in no way unusual. That claims extending from the island of Taiwan overlap in part with those of the two negotiating states does not remove their obligations under UNCLOS to resolve their overlapping claims nor prevent them from doing so bilaterally. In following established practice, Manila and Tokyo are free to delimit the southwestern region of their overlapping claims beyond 200 nautical miles from Taiwan and then as far north and west toward the trijunction point of the EEZ from Taiwan as they wish. They could opt to emulate the practice of the courts, stopping precise delimitation somewhere short of the point of equidistance with Taiwan and agreeing the line will continue along that path until it hits some as-yet-to-be-determined trijunction.
But regardless of how the two sides approach the delimitation, any agreement reached will have no impact on legitimate EEZ and continental shelf claims made from Taiwan.
Assessing China’s Response
Given the normality of Japan-Philippine talks, the scale of China’s maritime operations in response have come as a surprise, and have been condemned by Australia, France, Germany, Taiwan, the United Kingdom, and the United States. These reactions have been driven by fears that China is using the Japan-Philippines talks as an excuse to augment its maritime encirclement of Taiwan and routinize unlawful enforcement operations in the high seas.
China has claimed that its operations are aimed at exercising jurisdiction over its maritime claims. The survey activity may be legally justifiable under this logic, as marine scientific research and geological surveys are exclusive rights of the coastal state within its EEZ and continental shelf. But much of the survey activity has fallen within 50 nautical miles of Taiwan, in areas largely irrelevant to the Japan-Philippines delimitation, suggesting they are as much about representing China’s claim to the island of Taiwan as they are about the maritime area being delimited by Manila and Tokyo. And though Beijing does have a history of using survey activity to demonstrate its maritime claims, including in the South China Sea, such operations have a dual use of collecting data on subsea environments that can inform undersea warfare capabilities. The undersea domain in the area surveyed would be a critical battleground in any Taiwan conflict scenario.
But perhaps the more serious normative concerns lie with Beijing’s interference with commercial traffic. While UNCLOS gives coastal states the right to protect ocean and seabed resources within their EEZs and continental shelves, these zones are otherwise treated as high seas where ships of all kinds are entitled to unrestricted freedom of navigation. Inspections of hundreds of commercial vessels that show no signs of exploiting or damaging the resources of the seabed or water column constitutes a violation of the rights of all user states. Paired with other Chinese violations of UNCLOS, from “historic rights” across the South China Sea to denying the high seas status of the Taiwan Strait, the operations in the Philippine Sea suggest Beijing seeks broad and unprecedented jurisdiction over what it calls its “near seas.”
These law enforcement patrols also add to growing fears that Beijing might pursue reunification with Taiwan through a blockade or quarantine. It is not hard to imagine how these operations, consisting of sustained law enforcement patrols monitoring and interdicting commercial traffic across an area covering several maritime approaches to Taiwan, could function as a rehearsal for a quarantine effort to test regional reactions and normalize a low level of coercion. In addition, the world has become acutely aware of the consequences of the interruption to commercial traffic due to the ongoing economic pain stemming from events in the Strait of Hormuz. Even if one accepted Beijing’s claim that the operations were squarely aimed at Japan-Philippines delimitation talks, the emergence of another instance of state-led interference with maritime commerce should be a worrying development.
Looking Ahead
Whatever the original intent, Beijing’s maritime forces are establishing a more sustained and assertive presence in the waters east of Taiwan. This matches patterns of Chinese behavior across other disputed maritime areas, where Beijing has frequently paired legal and diplomatic posturing with an expanded operational presence at sea that often outlasts the initial claimed trigger.
Maritime delimitation is rarely a quick process. With negotiation timelines for Japan and the Philippines likely to be measured in years rather than months, China’s justification for its law enforcement activities will enjoy an equally extended timeline. Whether it sustains these operations for that duration, or escalates them, will depend in no small part on the continued response of the international community—whose rights as user states of the maritime commons are at stake.
