IT CAME out of a clear blue sky. On November 23rd China declared a huge swathe 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 the 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 November 26th, 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 stand-off loomed between the world’s superpower and Asia’s emerging great power.

China insists its air-defence zone is nothing unusual. America and Japan, among others, have had them since the early days of the cold war, when countries were anxious about Soviet incursions. Yet America only insists that aircraft identify themselves if meaning to enter its airspace; planes simply passing through the zone (which extends well beyond territorial limits) do not have to. China insists 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 has also claimed them. Last year the Japanese government 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. Recently an unmanned Chinese drone flew over the islands. When Japan threatened to shoot down the next one, a Chinese general said that would be an act of war.

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 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 (see map). Meanwhile, by running close to both Taiwan (which also claims the Senkakus) and South Korea, it has alarmed those neighbours too. An adviser to Shinzo Abe, the Japanese prime minister, 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. Chuck Hagel, America’s defence secretary, said China’s move was a “destabilising 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 November 27th a spokesman appeared to back-pedal by claiming China would respond in the zone according to the perceived threat. Now South Korean and Japanese aircraft have defied its rules. Yet nothing suggests China is having second thoughts about its new ADIZ. Shi Yinhong of Renmin University in Beijing says it marks China’s first action since 1949 that “substantially expands its strategic space” on the western rim of the Pacific. Countries in South-East Asia with which China has maritime disputes wonder whether China will now impose an ADIZ over the South China Sea in a bid to turn it into a Chinese lake too (see Banyan). For now, China’s move throws into question the depth of President Xi Jinping’s desire for a “new type of great power relationship” with America. Barack Obama seemed to get on well with Mr Xi when they met for a two-day personal summit in June; he cannot have envisaged having to dispatch two bombers as a warning six months later. A better sense of how the two countries intend to handle matters will come with a visit to China in early December by the vice-president, Joe Biden.

Mr Shi admits that the zone has raised tensions, but predicts that Washington and Tokyo will want to avoid “too much risk of conflict”—ie, 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. In this view, America will urge Japan into an accommodation with China, first by acknowledging the existence of a territorial dispute.

Instead, it is now more likely that America and Japan will be more assertive. Bonnie Glaser of the Centre for Strategic and International Studies, a Washington think-tank, notes that flights such as the one taken by the B-52s this week are only the latest act in a long history of America’s proving the freedom of navigation in international skies and waters. (And it seems to have reassured Japan’s commercial airlines, which after lobbying by their government announced that they would not be following Chinese rules in future.) In Tokyo defence guidelines to be released at the end of the year appear likely to articulate a new policy of patrolling the seas and skies around the Senkaku islands constantly, rather than intermittently, as now. The stakes have just got higher.