WHEN Donald Trump’s nominee for secretary of state, Rex Tillerson, said during his confirmation hearings that America should deny China access to the bases it had built on disputed reefs and islands in the South China Sea, many assumed that he was speaking off the top of his head, perhaps trying to impress the senators by sounding tough. But when, at a press briefing on January 23rd, the new president’s spokesman said something similar, it was not just jumpy Chinese who began wondering whether Mr Trump might deliberately and dramatically escalate military tensions with China.

At the briefing Sean Spicer, Mr Trump’s press secretary, was asked if he agreed with Mr Tillerson’s remarks. He replied, “It’s a question of if those islands are in fact in international waters and not part of China proper, then, yeah, we’re going to make sure that we defend international territories from being taken over by one country.”

Certainly, there are strong grounds for objecting to China’s ejection of neighbours’ forces from islands and reefs, to its naval build-up and, above all, to its island-building. Last July an international tribunal produced a damning verdict on China’s “historic claims” in the South China Sea, declaring them invalid. It said China’s tongue-shaped “nine-dash line”, which descends over 1,500km from the Chinese coast to encompass nearly all the sea (see map), had no legal standing under the UN Convention on the Law of the Sea, to which China is a signatory. The court also dismissed China’s claim to territorial waters around certain rocks, originally visible only at low tide, on which it had built. And it lambasted China for violating the rights of the Philippines, whose 200-nautical-mile (370-km) exclusive economic zone covers some of the rocks in question, and whose vessels China had prevented from fishing and prospecting for oil. China said flatly that it would ignore the ruling. If anything, it has increased its presence in the sea since. For instance, it has installed hangars for fighter jets on some of the islands, in spite of a pledge not to “militarise” them. In December the Chinese navy briefly seized an underwater drone that had been deployed by an American naval research vessel about 50 nautical miles from Subic Bay in the Philippines. China has long resented America’s (perfectly legal) naval patrols and surveillance operations near its coasts.

There is a good case for standing up to creeping Chinese expansionism. But the Chinese media are surely right when they say that a blockade of the islands would be construed as an act of war. Nor do America’s friends in the region want an escalation. The Philippines has had a change of government since bringing the petition to the tribunal. Its new president, Rodrigo Duterte, has said he will set the ruling aside. Australia, America’s closest military ally in Asia, has distanced itself from the Trump administration’s stance. And, in an abrupt change of course, Vietnam, another once-vocal critic of China’s claims, recently said it would settle its maritime disputes with China bilaterally, as China prefers.

Decades of ideological inculcation have seared the nine-dash line across the hearts of Chinese nationalists. It is there on maps on the wall of nearly every classroom, and is reproduced in all Chinese passports. Facing a blockade, China would not climb down lightly.

It is not clear whether Mr Trump endorses the measures, vague as they are, that Messrs Tillerson and Spicer seem to be sketching out. But it is hard to pretend that there is no change in attitude towards China. Mr Trump has tilted notably towards Taiwan—he has broken the taboo of questioning the “one-China” policy—and he seems bent on picking a fight over trade. It is all starting to sound quite hostile, notwithstanding the deep interdependence of the two powers. Yet if the stern talk on the South China Sea is followed by inaction, America’s credibility will be damaged.

A charitable interpretation of the emerging line, floated by Bill Hayton, an expert on the South China Sea at Chatham House, a think-tank in London, is that the hawkish comments have a narrower aim, of keeping China from building on the Scarborough Shoal, a set of reefs near the Philippines from which the Chinese chased the Philippine navy in 2012. A base there, in addition to ones already built in the Paracel Islands to the west and the Spratly Islands to the south, would allow China to dominate the sea. Last year Barack Obama’s administration is thought to have warned China that America would block any attempt to build on the shoal. Mr Tillerson may therefore simply be restating existing policy more bluntly.

Will it work? Perhaps. Satellite imagery suggests that China’s island-building stopped months ago. China’s new courtship of the Philippines argues against any provocative building on Scarborough Shoal. Besides, Xi Jinping, China’s president, has declared 2017 to be a year of stability, so he can scarcely afford a crisis in the South China Sea. Still, Mr Trump’s emerging line gives China an excuse to do what it swore not to, and fully fortify the islands it has spent years creating.