An air defense identification zone (ADIZ) is an area of airspace defined by a nation within which "the ready identification, the location, and the control of aircraft are required in the interest of national security"[1]. Typically, an aircraft entering an ADIZ is required to radio its planned course, destination, and any additional details about its trip through the ADIZ to a higher authority, typically an air traffic controller.
In North America, the United States and Canada are surrounded by an ADIZ, which is jointly administered by the civilian air traffic control authorities and the militaries of both nations, under the auspices of the North American Aerospace Defense Command or NORAD. (The Canadian ADIZ when discussed separately is known as the Canadian Air Defense Identification Zone or CADIZ.)
The joint US/Canadian ADIZ, which is almost exclusively over water, serves as a national defense boundary for aerial incursions. Any aircraft that wishes to fly in or through the boundary must file either a Defense Visual Flight Rules (DVFR) flight plan or an Instrument Flight Rules (IFR) flight plan before crossing the ADIZ. The aircraft must have an operational radar transponder and maintain two-way radio contact while approaching and crossing the North American ADIZ.
In the United States, the FAA handles the requests of international aircraft and Transport Canada handles Canadian requests. Any aircraft flying in these zones without authorization may be identified as a threat and treated as an enemy aircraft, potentially leading to interception by fighter aircraft.
The exact nature of the external ADIZ claim of the United States is unclear.
14 C.F.R. 99.11a states "No person may operate an aircraft into, within, or from a departure point within an ADIZ, unless the person files, activates, and closes a flight plan with the appropriate aeronautical facility, or is otherwise authorized by air traffic control", which appears to claim authority over all aircraft in the external U.S. ADIZ regardless of destination.
However, the U.S. Navy's Commander's Handbook on the Law of Naval Operations states the ADIZ applies only to commercial aircraft intending to enter U.S. sovereign airspace, with a basis in international law of "the right of a nation to establish reasonable conditions of entry into its territory"[2]. The manual specifically instructs U.S. military aircraft to ignore the ADIZ of other states when operating in coastal areas:
The United States does not recognize the right of a coastal nation to apply its ADIZ procedures to foreign aircraft not intending to enter national airspace nor does the United States apply its ADIZ procedures to foreign aircraft not intending to enter U.S. airspace. Accordingly, U.S. military aircraft not intending to enter national airspace should not identify themselves or otherwise comply with ADIZ procedures established by other nations, unless the United States has specifically agreed to do so.
Meanwhile in actual practice the U.S. does attempt to apply its external ADIZ to military aircraft which pass through its extended ADIZ without intending to enter U.S. sovereign territory [1]. A U.S. Air Force university dissertation states:
These regulations do not pertain to military aircraft, but to enter US airspace, without inducing the scrambling of fighter interceptors, these rules must be complied with and followed. The US does not claim sovereignty over these zones per se, but does closely monitor and request information of all objects entering the zone.[3]
The Washington Air Defense Identification Zone was created after the September 11 attacks. This ADIZ, however, shared none of the regulatory similarity with the border area ADIZes. This airspace was subsequently renamed the DC Special Flight Rules Area to avoid confusion.