During the visit of the Philippine President Ferdinand Romualdez Marcos Jr to Japan from May 26 to 29, both nations announced the start of bilateral negotiations on the delimitation of exclusive economic zones and continental shelves, directly targeting the maritime areas east of China's Taiwan Island.

This provocative move violates China's legitimate maritime rights and interests, and runs counter to the United Nations Convention on the Law of the Sea and basic norms of international relations.

The negotiations signify a deepening maritime security alliance and escalating geopolitical coordination between Japan and the Philippines.

Beneath the diplomatic surface lie clear strategic calculations. Japan is leveraging its alliance with the United States to break free from postwar pacifist constraints and expand its maritime control footprint across the Western Pacific.

The Philippines, meanwhile, is seeking external support to counterbalance China's legal sovereign claims in the South China Sea and the waters off eastern Taiwan.

By negotiating and disposing of maritime areas that are under China's jurisdiction, the two sides are attempting to undermine China's maritime rights and interests. The move is closely linked to the broader geopolitical pattern of the Western Pacific, and poses significant risks.

First, it threatens China's maritime rights and interests. Should Japan and the Philippines reach a formal delimitation agreement, they will unilaterally allocate rights for fishing, oil and gas exploration, marine scientific research and other maritime activities in the targeted waters. Such a deal would establish an illegal status quo, forcing China into a passive position and substantially driving up the cost of future rights protection. It would also undermine China's long-established lawful practices of maritime management, resource development and sustainable utilization in these traditional waters.

Second, the move fuels geopolitical tensions across the Taiwan Strait.

By focusing on the sea areas east of China's Taiwan island, Japan and the Philippines are pushing for the "internationalization" of the Taiwan question.

They aim to fabricate a false perception of maritime separation across the Taiwan Strait, collude with the separatist "Taiwan independence" forces on the island and disrupt China's national reunification process. Their geopolitical manipulation adds volatility to the regional security landscape.

Third, it endangers regional maritime governance. If this backdoor maritime delimitation by Japan and the Philippines is tolerated by the international community, it will set a dangerous precedent.

Other states may follow suit, forming small cliques to privately divide overlapping maritime domains. Such trends would marginalize the dispute settlement mechanisms under UNCLOS and weaken China's regional framework which prioritizes sovereignty and bilateral consultation in maritime disputes.

Fourth, it creates room for external powers to interfere in regional affairs. Both Japan and the Philippines are embedded in the US-led Asia-Pacific alliance system, and have received implicit support from Washington.

The US may try to take advantage of this development to scale up patrolling east of Taiwan and establish a formal trilateral maritime coordination mechanism with Japan and the Philippines to further compress China's strategic space in the Western Pacific.

China has solid legal grounds to reject these illegitimate negotiations. At the domestic level, China's Law on the Territorial Sea and the Contiguous Zone and Law on the Exclusive Economic Zone and the Continental Shelf explicitly stipulate that Taiwan is an inalienable part of China's territory, with all adjacent maritime areas under China's jurisdiction.

On the international level, China enjoys complete sovereign rights over the exclusive economic zone, as well as inherent jurisdiction over continental shelf exploration, exploitation and governance in these waters.

No third state is entitled to conduct private maritime delimitation, resource distribution or jurisdiction partition negotiations without approval by China.

Thus, the Japan-Philippines negotiations lack legal legitimacy.

Under the natural prolongation doctrine of UNCLOS, China's continental shelf extends to cover the sea areas east of Taiwan. China holds sovereign rights over the seabed, subsoil and natural resources of these waters.

UNCLOS also makes it clear that wherever there are overlapping sovereign claims involving a third party, relevant states shall not bypass the entitled sovereign state to separately finalize delimitation arrangements.

Only adjacent or opposite states can undertake maritime boundary negotiations without any third-party sovereign involvement. Therefore, the Japan-Philippines negotiations violate the core spirit of UNCLOS and have no international legal validity whatsoever.

Above all, national sovereignty is non-negotiable. As an administrative region of China, Taiwan has no independent maritime sovereignty.

Any state handling maritime affairs surrounding Taiwan must recognize the one-China principle and respect China's full sovereignty over Taiwan and its affiliated maritime spaces.

By excluding China and arbitrarily arranging marine affairs east of the Taiwan Strait, Japan and the Philippines are attempting to divide China's territorial and maritime sovereignty.

Their actions cross the red line of the one-China principle and China will never tolerate such infringement upon its core national interests.

Chang Yen-Chiang is a professor at the School of Law at Dalian Maritime University; Zhou Siyu is a postgraduate at the same school.

The views don't necessarily represent those of China Daily.

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