It came out of a clear blue sky. On Nov. 23, China declared a huge swath of mainly international airspace above the East China Sea to be part of a new Chinese “Air Defence Identification Zone,” or ADIZ: All aircraft intending to enter the zone had to file flight plans with Chinese authorities, maintain radio communications and follow the instructions of Chinese controllers — or face “defensive emergency measures.”

Japan’s two main airlines rushed to comply with the new rules. But then it was America’s turn to surprise, with two B-52 bombers sent over the Senkaku islands inside the zone on Nov. 26, on what was claimed to be a regular exercise. They had, the Pentagon said, followed “normal procedures,” that is, “not filing flight plans, not radioing ahead and not registering our [radio] frequencies.” Suddenly a standoff loomed between the world’s superpower and Asia’s emerging great power.

China insists that its air-defense zone is nothing unusual. The United States and Japan, among others, have had them since the early days of the Cold War, when countries were anxious about Soviet incursions. Yet America insists that aircraft identify themselves only if meaning to enter its airspace; planes simply passing through the zone (which extends well beyond territorial limits) do not have to. China is insisting that all aircraft in its ADIZ abide by the new rules.

More provocatively, China’s ADIZ covers the uninhabited Senkaku islands, which China calls the Diaoyu. Japan has held these since the late 19th century, but since the 1970s China also has claimed them. The Japanese government last year bought from their private owner three of the five islands it did not own. China saw this as a provocation and set out to undermine Japan’s control of the islands through incursions of surveillance vessels and, later, patrol aircraft — to which Japan responded by scrambling fighter jets.

The latest move represents a significant ratcheting-up of China’s challenge to Japanese control of the Senkakus. The new zone increases the risk of military escalation, accidental or otherwise. In the future, China may think that the zone forms grounds on which to take action against Japanese aircraft operating within the perimeter. Not only does the ADIZ cover Japanese-held territory, it also overlaps significantly with Japan’s own zone.

Meanwhile, by running close to both Taiwan (which also claims the Senkakus) and South Korea, it has alarmed those neighbors, too. An adviser to Japanese Prime Minister Shinzo Abe calls it a “whole new game” and the biggest challenge in recent memory to freedom of movement in or above the East China Sea. Japan lodged a strong protest, which was rebuffed. Australia, South Korea and Taiwan have also expressed concerns. U.S. Defense Secretary Chuck Hagel said China’s move was a “destabilizing attempt to alter the status quo in the region.”

The decision to respond with an overflight of the B-52s, the Japanese insist, was a joint understanding. China, meanwhile, says that it monitored the (unarmed) bombers, but claimed that they flew only along the eastern edge of its zone. That claim may be a way of saving face at home. And on Nov. 27 a spokesman appeared to backpedal by claiming China would respond in the zone according to the perceived threat.

China’s move throws into question the depth of President Xi Jinping’s desire for a “new type of great power relationship” with America. President Obama seemed to get on well with Xi when they met for a two-day personal summit in June; he cannot have envisaged having to dispatch bombers as a warning six months later.

Shi admits that the zone has raised tensions but predicts that Washington and Tokyo will want to avoid “too much risk of conflict” — i.e., America and Japan will back off and accept the new situation. But that seems wishful thinking, as does a growing view among Chinese policymakers that fear of conflict with China will push America to weaken its long-held commitment to underwrite Japan’s security.