-ADIZ as a signaling device. An ADIZ can be used to signal something important about the state that issues it. The audience of the signaling may be domestic or international or both. Declaring an ADIZ in the face of foreign opposition may signal resolve. It may also signal anger, and thus indirectly, formidability, when responding to a preceding event that hurts the ADIZ-declaring state. The enforcement of an ADIZ may signal capability. Resolve, anger and capability may act to deter foreign states from hurting the ADIZ-declaring state. Can an ADIZ be employed to reassure others of the declaring state’s cooperative intention? One observer argues that China tried to use its East China Sea ADIZ as an “instrument of engagement, not aggression.” However, the international opposition to both the East China Sea and the South China Sea ADIZ suggests that only a fool would use it to signal cooperation.

-ADIZ as a deterrent. Not only signals sent by the announcement of an ADIZ, but the serious possibility of an ADIZ can also act to deter others from doing things that the ADIZ-declaring state does not want. While the former function can only work after the ADIZ is declared, the latter will cease to work at that same moment. For an unborn and hypothetical ADIZ to serve as a deterrent, it has to be designed in ways that are highly detrimental to the interests of the deterred. With its possible functions as an exclusion zone, a sovereignty marker and a position booster, China’s ADIZ can serve as a deterrent. In fact, China has developed a consistent narrative on the South China Sea ADIZ, saying whether it will declare an ADIZ in the South China Sea depends on the threat level it faces.

When Will China Set Up an ADIZ in the South China Sea?

China’s decision to declare an ADIZ will most likely be the result of its cost-benefit calculation. If China has plans to declare an ADIZ in the South China Sea, it will likely make the announcement when the anticipated benefits exceed the anticipated costs. The benefits derive mainly from the utility of an ADIZ; the costs depend largely on foreign reaction. Circumstances such as a North Korea crisis that makes the United States depend on China’s support or an international court ruling on the South China Sea dispute could also influence this cost-benefit analysis. These international events may either tie the hands of China’s rivals or allow them to be more assertive in the South China Sea. They may also change the military, political, legal and diplomatic value that China can extract from a South China Sea ADIZ.

While the benefits are arguably more or less clear to China, the costs add a thick fog of uncertainty to the matter. It is generally hard to know how a foreign state will react, and China may well misread foreign signals. For outside observers, predicting whether and when China will announce a South China Sea ADIZ is a business that is bound to fail. Nevertheless, the uncertainty associated with this business is not unlimited—several factors set the parameters of Beijing’s decision.

If China wants to use a South China Sea ADIZ for military purposes (early warning and anti-access area denial), effective enforcement is a key requirement. The building of necessary infrastructure and the deployment of necessary assets are key indications of when China will set up its ADIZ. China may formally declare an ADIZ once its capabilities for enforcement are in place, but if the circumstances are not favorable, it may impose its early warning systems and exclusion zones under names different than ADIZ, or it may enforce them undeclared on a de facto basis.

If China employs its ADIZ as a “sovereignty marker” (to register sovereignty over the South China Sea territories and get international recognition or acquiescence), a bargaining chip, or a signaling device, a declaration is more important than de facto enforcement. The benefits of an ADIZ in these respects are potentially large, but the risks associated with a declaration are also significant. These risks are much higher than those associated with the declaration of China’s East China Sea ADIZ. As the South China Sea dispute involves more parties, a South China Sea ADIZ will turn more states against China. While the East China Sea ADIZ caught the world by surprise, a South China Sea ADIZ will not. This means that China’s rivals have time not only to think about their best reaction, but also to put pressure on China to prevent a South China Sea ADIZ. Still, these risks may plunge temporarily when a number of China’s rivals need its cooperation for a different issue of higher priority. Also, some circumstances may temporarily catapult the value of a South China Sea ADIZ as a bargaining chip or a signal of resolve. These moments will provide the best chance for China to declare a South China Sea ADIZ.

Size and Scope

If China imposes an ADIZ in the South China Sea, the maximum scope of this ADIZ will likely be roughly that of the nine-dash line. A larger scope will cause much additional opposition while bringing little additional utility. The cost-benefit ratio of an ADIZ varies with its scope. Essentially, China has six choices with the scope of a South China Sea ADIZ.

The smallest ADIZ would cover the Paracel Islands. This ADIZ can also include parts or whole of the Hainan Island. Such an ADIZ will have the smallest number of opponents. It will likely be opposed by Vietnam, which claims the Paracels and the United States, but it may avoid strong reaction from most other states.

In a second scenario, China can declare an ADIZ along its South China Sea coast, encompassing not just the Paracel Islands but also the Pratas Islands, which lie 180 nautical miles southeast of Hong Kong. As the Pratas are administered by Taiwan, an ADIZ that covers this territory will not only increase the number of opponents but also inadvertently create an opportunity for Taiwan to step up in the international arena.

A third version of a South China Sea ADIZ would stretch out from China’s southern coast and involve the Pratas, the Paracels, and Scarborough Shoal, which lies 100 nautical miles from the Philippine coast. This ADIZ will cause strong opposition from at least the United States, Vietnam, Taiwan and the Philippines.

In a fourth version, an ADIZ that covers only the Paracels and Scarborough Shoal can avoid provoking Taiwan but still turn the United States, Vietnam and the Philippines against Beijing.

A fifth choice is a full ADIZ that contains roughly all the area China claims in the South China Sea, including the Pratas, the Paracels, Scarborough Shoal, the Spratly Islands and the waters between and surrounding them. This ADIZ will have the largest number of opponents but also the largest benefits among the different versions of a South China Sea ADIZ. China can leave the Pratas Islands outside the full ADIZ, but this smaller version may not significantly reduce Taiwan’s opposition as it still involves the Taiwan-held Itu Aba Island in the Spratlys.

Finally, a sixth choice for China is to declare an ADIZ over the Spratly Islands with the possible coverage of Scarborough Shoal, but not the Paracel Islands. As all claimants of the South China Sea disputes are involved in the Spratlys, this ADIZ will still have the maximum number of opponents. However, it could significantly reduce the risks by avoiding a Vietnamese ADIZ over the Paracels. A Vietnamese ADIZ that encompasses the Paracel Islands can act as a Vietnamese sovereignty marker, allowing Hanoi to register some exercise of sovereignty and administration over an area where China is denying there is a dispute.

A “Red Line” or Just an “Orange Line”?

China’s possible ADIZ would not be the first ADIZ in the South China Sea. During the Cold War, a Philippine ADIZ was established in 1953, and a South Vietnamese ADIZ also existed until the end of the Vietnam War in 1975. However, the Philippine ADIZ is inoperative, while the South Vietnamese ADIZ is nonexistent even for the Vietnamese military of today.

On the contrary, a Chinese ADIZ in the South China Sea is perceived as highly threatening and intolerable. Vietnam’s top defense diplomat Nguyen Chi Vinh noted in a January 2014 interview that an ADIZ “would be more dangerous than even the nine-dash line” because it would come with more excessive regulations than the latter. Suggesting that China’s South China Sea ADIZ might be unacceptable to Hanoi, Vinh said in the same interview that it would “kill” Vietnam. Similarly but more bluntly, then Philippine Foreign Secretary Alberto del Rosario said during a joint press conference with his British counterpart in January 2016, “whether this [China’s South China Sea ADIZ] is done in terms of a de facto basis or it is official, this will be deemed unacceptable to us.” At the same press conference, British Foreign Secretary Philip Hammond stated, “Freedom of navigation and overflight are non-negotiable. They are red lines for us.” The United States’ official position is that an “ADIZ over portions of the South China Sea” would be “provocative and destabilizing.” U.S. Secretary of State John Kerry has repeatedly called for China not to declare an ADIZ there. Australia’s Foreign Minister Julie Bishop also urged China not to create the South China Sea ADIZ. In June 2016, Taiwan’s Defense Minister Feng Shih-kuan told lawmakers, “We will not recognize any ADIZ by China.”