This week marks a year since China abruptly declared an Air Defense Identification Zone (ADIZ) covering a large area of the East China Sea, including islands the legal possession of which China disputes with Japan. Especially because China has sent signals that it might enact another such zone in the South China Sea, this anniversary provides a moment to reflect on the international legality of the East China Sea ADIZ and international rules applicable to it. In general, China’s establishment of an ADIZ is not per se illegal as a matter of international law; however, the requirements China has declared for its East China Sea ADIZ are much broader than recent customary practice by others and China could enforce it in particular ways that would violate international law. Because international law does not yet have much to say about ADIZs, the practices worked out between China, the United States, Japan and others in this case will serve as important baselines for future ADIZs in Asia.

International law governing the status and limits of ADIZs – in which aircraft are usually required to submit flight plans and report their locations to national air traffic control – is not clear in any detailed way, though some well-established principles apply. Approximately two dozen countries (including the United States, Japan and Korea) maintain some form of ADIZ, generally for the purpose of defending against hostile intrusions of airspace or de-conflicting air traffic. So long as these zones are immediately adjacent to states’ own airspace, they are not very controversial. Indeed, by establishing clear patterns of behavior and requiring communication, they can be stabilizing. With no treaty that directly authorizes or regulates them and so little consistent state practice, however, it is hard to discern clear-cut and widely-recognized international rules for ADIZs.

Although there is no prohibition on ADIZs per se, they may be used in ways that violate other international legal provisions. The specific identification requirements declared by China go beyond typical ADIZs in that they apply to aircraft flying through the zone but not entering Chinese airspace (or even disputed airspace), and they apply to commercial aircraft as well as state (including military) aircraft, which are usually considered immune from regulation by other states.

An especially significant legal concern is that if China enforces its ADIZ in ways that prevents other states from freely transiting that airspace, it would violate freedom of overflight rights on the high seas. Customary international law and Article 87 of the UN Convention on the Law of the Sea (or UNCLOS, to which China is a party) guarantee such overflight rights. In announcing its ADIZ a year ago, the Chinese defense agency implementing it declared that Chinese armed forces would adopt “defensive emergency measures” that did not cooperate with its administration of the zone. Such aggressive statements raised concerns that Chinese air forces might interfere with other states’ entitled overflight of international waters. So far, China’s enforcement practices are worrisome for their heightened risk of accidents or inadvertent crisis escalation – especially given overlapping territorial claims, and even overlapping ADIZs, in China’s ADIZ area – but it is debatable whether and to what extent they illegally interfere with free overflight.

Aggressive Chinese statements about “defensive measures” also contribute to perceptions that by instituting a consistent military presence there and by casting it in these terms, China may be attempting to establish over time a stronger claim to ownership over the contested islands below. That is, it could be to create facts in the sky to advance its bargaining position over disputed territory below. There is no basis in international law to derive rights to land by control of the skies above it, however.

The greatest danger of China’s move is not that it significantly reshapes the core territorial disputes. It is that expanded Chinese military patrols in the air, like its activities at sea, in such close proximity to those of rival players could lead to accidents, provocations, or miscommunications that might easily escalate. So as with maritime activities, for now diplomacy on air activities should focus on creating workable rules of the road and crisis communication mechanisms – and should not be distracted by claims that the ADIZ will substantially affect competing territorial claims. Non-binding codes of conduct would help mitigate dangers in the East China Sea ADIZ. They would also establish a template that could – in the absence of detailed, binding international law on this issue – be used elsewhere should China or other states establish additional zones, especially in other areas encompassing competing territorial claims like the South China Sea.

About Matthew Waxman

Matthew C. Waxman is Liviu Librescu Professor of Law at Columbia Law School, as well as Adjunct Senior Fellow for Law & Foreign Policy at the Council on Foreign Relations and Member of the Hoover Institution Task Force on National Security & Law. He previously served in senior positions in the U.S. State Department, Defense Department, and National Security Council.