Speculation is rapidly growing that China will soon declare an Air Defense Identification Zone (ADIZ) over parts or all of the South China Sea. It is the most likely response to an unfavorable ruling in the case challenging China’s claims in the South China Sea.

In anticipation of the verdict in the next few weeks, many are already speaking up to try and stop Beijing from making such a declaration. U.S. Secretary of State John Kerry warned Beijing that such a move would be “provocative and destabilizing.”

For its part China dropped hints it is thinking long those lines: “If the U.S. military makes provocative moves to challenge Chinese sovereignty in the region, it would give Beijing a good opportunity to declare and ADIZ,” said a source quoted in the South China Morning Post.

China’s first ADIZ, which it declared in November 2013 over a part of the East China Sea, was generally considered to be a response to Tokyo’s decision to nationalize the disputed Senkaku (Diaoyu) Islands.

So what would a new Chinese ADIZ mean for Asia?

Boundaries. Much would depend on the boundaries that Beijing chooses to declare. One could be a large circle around the Spratlys; another circle around the Paracels, or both, or possibly even one that approximates the nine-dash line that Manila is challenging in court.

Beijing could also declare an ADIZ along its southern coast and Hainan Island. That would make good defensive sense as China has many navy bases and assets in this region. However, it would not serve to protest the tribunal’s expected verdict.

In an ADIZ civilian airlines are supposed to file a flight plan with the claiming authority, maintain an open radio contact and respond to inquiries. If they don’t answer, fighter aircraft are scrambled to investigate and conceivably force the aircraft to land.

A glance at the map would show how an ADIZ would impact airline traffic in the South China Sea. A Spratly’s ADIZ would sit directly in the path of flights from Manila to Jakarta or Singapore, for example.

The East China Seas ADIZ that Beijing announced in 2013 covers only a part of the off-shore waters, though a portion of it covered the Senkaku. Much of mainland China was untouched.

Historically, countries that have declared air defense zones, such as the United States or Japan, have done so for the entire coastline, not just pieces of it. That brings into question exactly what was its purpose?

Washington famously flew a pair of bombers through the zone after it was declared (and likely to do so again if China declares a new one). On the other hand, it advised American civilian airlines to comply with the rules. According to the Chinese, 55 airlines from 19 countries traverse the zone.

China does not seem to be strictly enforcing the current East China Sea ADIZ. By some accounts it has actually stopped enforcing its zone, although it is difficult to tell, as China does not publish figures on fighter scrambles.

To date no Japanese airline has been harassed, even though, at the urging of the government, they do not file flight plans with China.

Only one incident occurred in 2015 when Chinese authorities turned back a Lao Airlines Flight. But it was determined that it had not received permission to overfly the mainland on its way to Vientiane. “It had nothing to do with the ADIZ”, Beijing said.

Enforcement. Whether China will enforce any of its new ADIZs or essentially do nothing will depend a lot on the given situation, especially the impending decision of the tribunal, plus the number of fighter assets that can be deployed to the new Spratly island bases.

The Chinese already have fighters stationed on islands in the Paracels in the northern part of the South China Sea that could be scrambled to engage with any suspicious flights. They can also depend on fighters stationed on Hainan Island.

Of course, China has already been building bases on several of the marine features in the Spratlys it claims as Chinese territorial waters. A few now have airstrips long enough to handle high performance aircraft, potentially rotated from the mainland.

For the present, this contingency remains theoretical, as China has not yet dispatched any jet fighters to the Spratly bases. It claims the features, including a large lighthouse, are for protection of civilian marine commerce and uses as typhoon shelters.

However, fully developed military airfields, would “create a mechanism in which China would have de facto control over the South China Sea in any scenario short of war,” says the outspoken commander of the U.S. Pacific Command Adm. Harry Harris.

Legalities. If China should declare an ADIZ in the South China Sea, there isn’t much that anyone can do about it except to sputter in impotence. Secretary Kerry can complain all he wants that the move is “provocative and destabilizing,” to no avail.

There is no arbitration tribunal, no Security Council to appeal to. ADIZ’s are not seriously governed by any international agreements. No law requires advanced notice for declaring such a zone. All declared ADIZs, including the United States, have been unilateral.

There is no law that says countries must obey them, though most countries do. Russia is an important exception. It doesn’t recognize anybody’s ADIZ and flies through them without a by-your-leave. That is why, until recently, most Japanese fighter scrambles have been against Russian intruders rather than Chinese.

Taiwan’s new defense minister recently announced that Taipei will not recognize any new ADIZ declared in the in the South China Sea (where Taipei has its own claims.)

Air defense zones are basically relics of the early Cold War when long-range Russian bombers were the main threat. They are obsolete in the age of intercontinental missiles. The U.S. imposed its own ADIZ as far back as 1950 and has not lifted it.

It is worth remembering that the first Chinese ADIZ covers only a portion of the country’ coastline, which brings into question whether they are meant serve any real defensive purpose, but rather they are mainly counters in the ongoing struggle over who controls the East and South China Seas.