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An Air Defense Identification Zone (ADIZ) is airspace over land or water in which the identification, location, and control of civil aircraft is required in the interest of national security. They extend beyond a country's airspace to give the country more time to respond to foreign and possibly hostile aircraft. The authority to establish an ADIZ is not given by any international treaty nor prohibited by international law and is not regulated by any international body.
The first ADIZ was established by the United States soon after World War II. Following the incident of September 11, 2001 (September 11 attacks) when civilian commercial aircraft was abused for mass destruction, ADIZ became prominent as a method to prepare or control a foreign aircraft from entering their territory. About 20 countries and regions now have such zones including Canada, India, Japan, Pakistan, Norway and the United Kingdom, People's Republic of China, South Korea, Taiwan, United States, Sweden, Iceland and more. Russia and North Korea have unofficial ADIZ for themselves as well.  Usually such zones only cover undisputed territory, do not apply to foreign aircraft not intending to enter territorial airspace, and do not overlap.
Air defense zones should not be confused with Flight Information Regions (FIRs), which are used to manage air traffic. FIR is different by the fact that they are areas established for the facilitation of air traffic and airspace management. 
The United States maintains four zones: The Contiguous US ADIZ; Alaska ADIZ; Guam ADIZ; and Hawaii ADIZ. Under U.S. law and policy, the zone applies only to commercial aircraft intending to enter U.S. airspace. An air defense command and control structure was developed in 1950, creating five Air Defense Identification Zones around North America. If radio interrogation failed to identify an aircraft in the ADIZ, the Air Force launched interceptor aircraft to identify the intruder visually. The air defense system reached its peak in 1962, however with the deployment of the SS-6 ICBM in the USSR, strategic threats shifted overwhelmingly to ICBM attacks, and bomber intrusions were considered to be less of a threat. It does apply to aircraft passing through the zone to other countries.
Canada also operates a section of the North American ADIZ.
Japan has an ADIZ that covers most of its Exclusive Economic Zone. Japan's ADIZ was created by the United States Armed Forces after World War II, with the western border at 123° degrees east. This resulted in only the eastern half of Yonaguni Island being part of Japan's ADIZ and the western half being part of Taiwan's ADIZ. On 25 June 2010, Japan extended its ADIZ around Yonaguni 22 km westwards. This led to an overlapping with sections of Taiwan's ADIZ. However, Taiwanese foreign affairs officials said that it does not make any difference, as an understanding has been reached between the two parties on how to handle it.
South Korea operates a zone that covers most but not all of its claimed airspace. It does not cover some remote spots. The zone was established in 1951, during the Korean War, by the United States Air Force to block communist forces. In 2013, in response to the establishment of a Chinese zone that covers disputed territory, the South Korean government considered expanding their zone to include islands of Marado and Hongdo, and Ieodo, a submerged rock within the overlapping exclusive economic zones of South Korea and China. On December 8, 2013, Defense Ministry of Republic of Korea announced the expansion of the Korea Air Defense Identification Zone (KADIZ).
When part of or the whole flight route of an aircraft enter the KADIZ area, it is required to send a specific flight plan 1 hour prior departure. Civilian aircraft with regular line should submit a repetitive flight plan to the air traffic control. Since there are no flight routes going across KADIZ that does not pass the territorial waters of the Republic of Korea, all flights are obligated to send a flight plan no matter what. There are no need for legal actions when an aircraft enter KADIZ as long as all aircraft follow to their flight plans reported to the Korean government. If there is a change in the flight passage or an approach without prior notification, the Korean air force have the right to immediately identify or track down the aircraft and be prepared for interception. However, military force such as shooting down the plane cannot be exercised.
After both South Korea expanded KADIZ and China established their ADIZ in 2013, certain areas overlap with each other. The KADIZ boundary area at northwest Jeju Island (about 2,300 km²) overlaps with the East Chinese Sea. Also, South Korea's domestic aircraft cannot avoid entering China's ADIZ near Ieodo even if the flight's purpose is for rescue or searching distressed/missing aircraft. More tension is increasing as the area of South Korea, China and Japan's FIR reiterate with each other. These sensitive issues are expected to bring military or diplomatic conflict between the three countries.
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The announcement of the zone drew attention and international criticism, including from most of China's East and Southeast Asian neighbors such as Japan, South Korea, Taiwan, and the Philippines, and as well as from the European Union and the United States. These responses focused on two related aspects: while hitherto zones had only covered territory that was undisputed at the time of their establishment, China's ADIZ in the East China Sea covers the Japanese-controlled Senkaku Islands (Diaoyu Islands in the PRC) and also includes Socotra Rock, which is claimed by South Korea. Secondly, China's zone overlaps with other countries' ADIZ and imposes requirements on both civilian and military aircraft regardless of destination.
It has been claimed that the Senkaku Islands dispute predates Japan's 1969 establishment of its ADIZ over the islands, although there is no record of the People's Republic of China announcing a claim to the Senkaku islands in 1969 or before.
The Chinese government has noted that any established international limits for defining and enforcing an ADIZ are not clearly defined. Chuck Hagel, the American Secretary of Defense, while acknowledging there is nothing new or unique in establishing an ADIZ, criticized the manner in which China had acted as "unilateral", "immediate" and "without consultation". American Vice-President Joe Biden made no public mention of the Chinese ADIZ following his meeting with Chinese President Xi, although elsewhere expressed concern that the Chinese move had escalated tensions in the region.
Tokyo brought the matter to the International Civil Aviation Organization, a U.N. agency to promote the safe and orderly development of international civil aviation throughout the world. Australia, Britain and the United States supported the Japan's proposal, but China reacted sharply against the proposal. 
As of Dec. 4, 2013 some 55 airlines in 19 countries and 3 regions have complied with China's ADIZ rules. South Korea's Ministry of Transport said South Korean airlines would not recognize the Chinese ADIZ. Japan said its airlines would also not recognize the zone.