Air defence identification zones (ADIZs) are in the spotlight again, after China started its latest round of aerial incursions into Taiwan's air defence zone last Friday, the country's 72nd national day.
At least 150 Chinese military planes, including bombers, fighter jets and anti-submarine aircraft, have entered Taiwan's ADIZ in the past week.
What is an ADIZ?
It is a zone unilaterally declared by a country or a territory for various reasons, including safeguarding of security.
The Chicago Convention on International Civil Aviation in 1944 defined an ADIZ as a "special designated airspace" and aircraft are required to seek permission before entering the zone.
It is different from a state's territorial airspace. The American Journal of International Law cites that a state's territorial waters, and hence its sovereign territorial airspace, ends 12 nautical miles from its coastline. An ADIZ can cover a much larger space and sometimes even overlap the ADIZs of neighbouring states.
Because of its larger size, an ADIZ gives a state or territory some time to scramble its jets to intercept intruding aircraft.
The United States declared the world's first ADIZ during the early Cold War era in 1950 to protect against surprise attacks by Soviet Union bombers. Other places with ADIZs include China, Japan, South Korea and Taiwan.
Why it is useful
If ADIZ boundaries are respected, the zone will help lower the risk of accidental collisions and the need to send warplanes to inspect intrusions. In 1960, for example, the Soviet Union did not have an ADIZ and the resulting confusion led to a US spy plane being shot down over international waters.
Why it can be contentious
Demarcating zones can become contentious, particularly when one country's unilateral declaration overlaps with that of another nation's.
For example, in 2013, Japan and China separately declared ADIZs in the East China Sea that covered the disputed Senkaku or Diaoyu islands.
Taiwan's ADIZ, established in 1953, extends into mainland China. But defence analyst Gerald Brown said Taiwan does not record flights over the mainland as aerial incursions, only doing so when a Chinese aircraft leaves its airspace and enters Taiwan's ADIZ, often by crossing the median line of the Taiwan Strait.
Why it matters to China
China does not recognise the median line. Last year, it claimed Taiwan as an "inalienable part" of the mainland, flatly rejecting the median line.
China's interests in ADIZs appear to be multi-fold, according to Ms Mercedes Trent, a research associate for the Federation of American Scientists. She said in a report that besides displaying its air supremacy, unauthorised incursions could help the People's Liberation Army (PLA) gather intelligence on other militaries as the crossings will trigger procedural responses.
Regular incursions could also provide long-range flight training for its pilots that include interaction with foreign air forces.
According to the US Defence Intelligence Agency, the PLA, by maintaining a regular presence in Taiwan's ADIZ, is preparing for contingencies in the Taiwan Strait "to deter and compel Taiwan to abandon moves towards independence".
There are fears that China may declare an ADIZ over the disputed South China Sea. It has laid claim to nearly all of the waters, angering others with competing claims - Taiwan, Vietnam, the Philippines, Malaysia and Brunei.
Why it matters to Taiwan
Taiwan's Defence Minister Chiu Kuo-cheng has predicted that China could be ready to mount an invasion of the island by 2025.
Taiwan has labelled China's repeated flybys as "grey zone" warfare. Taiwanese military expert Lu Li-shih said the PLA's planes could have been deployed to wear out Taiwan's defence forces by forcing them to scramble and intercept the planes, and to gather intelligence on its response procedures.
According to its Ministry of National Defence, the island has seen over 600 incursions by PLA planes so far this year, up from 380 last year. Its defence forces had to scramble 2,972 times to intercept Chinese aircraft last year at a hefty cost of NT$25.5 billion (S$1.24 billion).
Taiwan International Strategic Study Society chairman Wang Kung-yi said PLA planes not intercepted could take just 200 seconds to reach Hsinchu, the island's closest point to the Taiwan Strait's median line. According to the Institute of Peace and Conflict Studies, China's actions may lead to miscalculations, or worse, aerial clashes with the "potential to trigger wider conflict".
What may happen
Analysts expect Beijing to mount another bout of aerial drills into Taiwan's ADIZ to coincide with the island's national day on Sunday.
Threats of a Chinese attack could further unite Taiwanese public opinion behind its current government, experts say. Last year, President Tsai Ing-wen sealed a second term with a record haul of votes after an election clouded by Chinese President Xi Jinping signalling his intent on reuniting Taiwan with the mainland, with force if necessary.
