An ADIZ is a unilaterally declared area of airspace beyond a state’s sovereign airspace (12 nautical miles from its coastline) within which aircraft intending to enter national airspace are required to identify themselves.

Declaring an ADIZ does not confer sovereignty over the airspace it covers, nor does it change its legal status. The purpose of an ADIZ is to give a state early identification of incoming aircraft, extending reaction time to assess and respond to potentially hostile activity, such as a hijacked civilian airliner or a military fighter or bomber.

The first ADIZ was established on 27 December 1950 by the United States in conjunction with Canada, shortly after President Harry S. Truman declared a national emergency due to the Korean War. It encompassed large swaths of airspace over the Atlantic, Pacific, and Gulf of Mexico, as well as internal areas of the US, and required all flights – domestic and foreign – to file flight plans and report their positions well before entering US airspace.

The concept was then adopted by allied countries Norway, Iceland, Japan, Taiwan, and South Korea also adopted ADIZs during the 1950s and 1960s, embedding the practice into broader national security architectures throughout the Cold War period. Today, around 20 to 25 countries operate some form of ADIZ, depending on how overlapping zones and zones established for limited purposes are counted.

While no treaty explicitly regulates the establishment or operation of Air Defence Identification Zones, the absence of sustained objections to their unilateral declaration suggests a degree of acceptance in state practice, allowing their existence to be treated as consistent with customary international law.

That said, while state may scramble interceptor aircraft to identify or monitor an aircraft that fails to comply with ADIZ procedures, non-compliance alone does not provide legal grounds to use force or engage the aircraft.

Due to the lack of formal legal enforcement instruments, the practical legitimacy of a given state’s ADIZ hinges on the consistency and credibility of its enforcement measures.

Click here for a full explanation of the legalities regarding ADIZs.

There is no set area an ADIZ covers by default, and the size of different states’ ADIZs varies widely. Typically, the size of an ADIZ is constrained by practical considerations such as local geography and a state’s capacity to enforce it:

Geography: Narrow landmasses, dense civil-aviation corridors, and proximity to neighbouring flight information regions can limit the size of an ADIZ, while long coastlines, wide maritime approaches, and greater strategic depth allow for more expansive ones.

Capacity: The ability to monitor and enforce an ADIZ is constrained by a state’s radar coverage, command-and-control systems, and interceptor availability. States with advanced surveillance networks, rapid response forces, and force projection capabilities can sustain much larger ADIZs than those without.

The line between routine air safety measures and instruments of strategic pressure can become blurred when it comes to ADIZs. Despite being neither sovereign airspace nor legally codified regimes, they offer states a means to shape behaviour, test boundaries, and impose costs without crossing clear thresholds that would trigger military or legal escalation.

This means they can be and are exploited as tools of greyzone warfare, particularly as a means of normalising control over an area and testing and degrading another state’s air defence capabilities:

States may use the act of establishing an ADIZ as a greyzone tactic – often part of a salami-slicing campaign – aimed at reshaping expectations and behaviour through declaration, procedure, and precedent. By declaring an ADIZ beyond its territorial airspace, a state projects administrative authority over a neutral area where it has no formal sovereignty – signalling an expectation that others will acknowledge its right to do so.

This forces third parties – particularly airlines and foreign militaries – to make compliance choices: filing flight plans, maintaining transponder use, and establishing radio contact may be framed as technical safety measures, but routine compliance gradually moves the baseline from freedom of navigation to conditional access, normalising the declaring state’s authority over the area.

ADIZ declarations are also used to test diplomatic thresholds. The scale, timing, and scope of a zone, and the reactions it provokes, help a state gauge how far it can push the limits of existing norms before being met with diplomatic pushback.

Once an ADIZ has gained recognition, it creates standing options for escalation. Procedures that are lightly applied in peacetime can be tightened during crises without the political cost of making a new announcement. In parallel, because ADIZs present as defensive measures, there is often an assumed expectation of compliance – meaning overflights that were once viewed as routine are suddenly judged as irresponsible or even provocative behaviour.

Incursions into another state’s ADIZ can be used as an active greyzone tactic – again often as part of a salami-slicing campaign – to challenge authority and apply pressure without crossing any clear legal or military thresholds. Flying through an ADIZ without complying with its procedures is a way of signalling non-recognition of the declaring state’s claimed administrative role, reinforcing previous norms like neutrality and freedom of navigation.

Conducting repeated incursions begins to normalise presence in an area. What begins as an exceptional event can, through routine repetition, become accepted or expected behaviour – reducing the political impact of each flight and gradually weakening the deterrent value of the ADIZ.

Incursions also test the credibility of a state’s ADIZ enforcement. Each flight probes how consistently and robustly the declaring state responds, exposing any shortfalls in its readiness, capacity, or political will and allowing the incursionary force to collect information on operational response capabilities such as radar coverage and interceptor timing. By adjusting variables such as aircraft type, altitude, proximity, and duration, states can shape and map escalation ladders without triggering open confrontation.

Additionally, each interception imposed by an ADIZ incursion places cumulative strain on the defending force, contributing to pilot fatigue, accelerating wear and tear on aircraft, consuming fuel and flight hours, and diverting attention and resources from training, readiness, and other operational priorities.

For analysts, ADIZ developments are most useful when tracked as patterns and process rather than isolated incidents. Key indicators include: