IT IS getting hard to sail across the South China Sea without bumping into a warship. On September 30th an American destroyer passed within 50 metres of a Chinese naval vessel which was conducting “unsafe and unprofessional” manoeuvres, according to the Americans. Earlier in the month Japan sent a submarine to conduct drills in the sea for the first time. In August a British warship was confronted there by Chinese ships and jets. And this month ships from Australia, Singapore, Malaysia, New Zealand and Britain will take part in more than two weeks of joint naval drills in the same crowded waters.

The maritime hubbub is an attempt to push back against China’s claim to the entire South China Sea, which other littoral states dispute and which a UN tribunal has rubbished. China wants military vessels and aircraft to notify it before passing through the sea, something America and others would view as an infringement of international norms even if China’s claims had been upheld. To make matters even more fraught, China has reclaimed land around a series of reefs and rocks in the sea to build bases teeming with guns, missiles and radar. Should these constructions be deemed rocks or islands under international law, and rightful Chinese territory, then certain restrictions would apply to military vessels passing within 12 nautical miles. But America and the UN tribunal, among others, consider several of them “low-tide elevations”—shoals, in effect—that do not enjoy the same rights. America and its allies keep sending warships to sail around the sea in ways that demonstrate that they do not accept China’s position.

Since 2015 America has conducted 12 of these “freedom of navigation operations” (FONOPs, in Pentagon jargon). These flout China’s claims in several different ways. By sailing within 12 nautical miles of genuine islands, for example, America’s navy demonstrates that it does not need and will not seek permission to exercise its right of “innocent passage”. By conducting military manoeuvres within 12 nautical miles of other fortified specks it shows that it considers them mere elevations around which no restrictions are warranted. And by entering the sea at all, it rejects China’s stance that it has any say in military activity in open waters within the area it claims.

FONOPs have grown “more regular and strident” under the Trump administration, says Alessio Patalano of King’s College London. America’s European and regional allies are not quite as confrontational. They tend to keep a greater distance from China’s bristling baselets. But simply by showing up, they help to demonstrate a united front. Australian, Japanese, British and French vessels have all sailed across the sea together, in various pairings. The hitch is that there are a lot more warships ploughing around, and so a lot more scope for dangerously heated encounters.