FOR many Brexiteers, the issue at the heart of the case for leaving the European Union was sovereignty. Membership of the EU was incompatible with self-determination, they argued. Britain is one of the world’s great military and economic powers; it would do just fine on its own. Remainers responded that in a globalised world, sovereignty is pooled. As David Cameron, Britain’s ill-fated prime minister, put it, Brexit held out the “illusion” of sovereignty: Britain would gain independence at the cost of real power.

On June 22nd, in an early test of these arguments, the UN weighed in on a dispute between Britain and Mauritius over the Chagos islands, a tiny but strategically important archipelago in the Indian Ocean (see map). Ninety-four countries sided with Mauritius; just 15 backed Britain. The result, says Jagdish Koonjul, Mauritius’s representative to the UN, was “beyond my expectation”. Only four members of the EU voted with Britain; one, Cyprus, voted with Mauritius and 22 abstained, including usually reliable allies France, Germany, Italy, the Netherlands and Spain. “It was a complete haemorrhaging of support for Britain,” says Philippe Sands, a lawyer representing Mauritius. “This should be a real wake-up call.”

The roots of the dispute go back to 1965, when Britain lopped off the Chagos islands from Mauritius, at the time a British colony. It loaned the largest island, Diego Garcia, to America to use as a military base. Since then the atoll, which is within striking distance of east Africa, the Middle East and South-East Asia, has become indispensable for America’s armed forces, who nickname it “the footprint of freedom”. It gives them control over the Indian Ocean and has served as a base for long-range bombers to pummel Afghanistan and Iraq. The CIA used it as a “black site” in its rendition programme. But taking over Diego Garcia for military use meant deporting some 1,500 Chagossians, mostly to Mauritius and the Seychelles. They have never been allowed to return; many moved to Britain. (After landing at Gatwick airport, they were given temporary accommodation nearby in Crawley, where most of them still live.) Mauritius claims the islands are part of its territory and wants the dispute referred to the International Court of Justice (ICJ). Britain argued that it should be resolved bilaterally. America took its side. But with its State Department understaffed and its president widely distrusted, its lobbying effort failed. Britain’s diplomacy was no better; the name of Boris Johnson, the foreign secretary, “raised an eyebrow or a laugh each time it was mentioned”, according to one person present. In an inauspicious sign for “Empire 2.0”, the nickname British officials use for their plan to forge closer links with the Commonwealth, the vast majority of Britain’s former colonies backed Mauritius or abstained. The importance of the vote should not be exaggerated. It refers the case to the ICJ, whose opinion will be non-binding. For many countries, the vote was a chance to take a dig at America and to reiterate their support for decolonisation. In future votes on more central issues of national security, Britain may still be able to rely on strong support from the EU.

Still, “It is a little cheep from the canary in the coal mine,” says Richard Whitman, director of the Global Europe Centre at the University of Kent. Other far-flung British territories, such as the Falkland islands, might face new challenges from rival claimants. “If you’re an Argentinian diplomat, you may start recomputing how much international support Britain has,” says Mr Whitman.

The ICJ will probably offer an advisory opinion on the matter, but not before the spring of 2019. By then Britain is due to have left the EU. And Mauritius may even have a more sympathetic negotiating partner. Jeremy Corbyn, Labour’s leader, is—perhaps inevitably—a longtime advocate for the rights of the Chagossians.