MIKE POMPEO, America’s secretary of state, raised Canadian hackles earlier this month when he said during a speech in Finland that Canada’s claim to the Northwest Passage was “illegitimate”. He was there to attend the Arctic Council, a body set up by the eight countries around the Arctic Ocean to resolve differences and disputes relating to the polar region. But if Canada does not own the Northwest Passage (see map), who does?

For centuries, finding a route between the 36,000 islands that form Canada’s Arctic archipelago was a dream for ambitious adventurers seeking to sail from the Atlantic to the Pacific. Sir John Franklin, a British naval officer, famously perished in 1847 seeking an ice-free passage (an illustration of the scene is pictured above). Roald Amundsen, a Norwegian explorer, completed the first successful crossing of the passage in 1906. Since then, global warming has reduced ice coverage in the late summer and early autumn by more than 30% since 1979, allowing more vessels to operate in the north. Only a few Arctic transits are made each year, mostly on the Russian side of the Arctic Ocean where there is less ice. In the Northwest Passage, 289 transits have taken place since 1906, 32 of them in 2017.

America has long maintained that the Northwest Passage, which has up to seven different routes, is an international strait through which its commercial and military vessels have the right to pass without seeking Canada’s permission. It bases its claim on the case of the Corfu Channel, separating Albania’s coast from the Greek island of Corfu, that was brought before the International Court of Justice in 1947. The court ruled that Albania could not claim the channel as territorial waters because it was an international route for ships between two parts of the high seas over which no country had a claim. Similar disputes exist about the Strait of Hormuz between Iran and Oman; the Bab al-Mandab strait between Yemen and Djibouti; and in parts of the South China Sea. Canada, which officially acquired the archipelago along with Britain’s remaining possessions in North America in 1880, claims sovereignty over the passage because all of its routes run between islands that are Canadian territory. But it has never sought to settle the question in court.

Canada and the United States finessed their disagreement in 1988 with a political rather than legal fix, called the Canada-US Arctic Co-operation Agreement. America said it would seek Canada’s consent for any transit, but maintained that this did not mean it agreed with Canada’s position. Mr Pompeo’s recent comment suggests that, with the Arctic opening up to more shipping, America is no longer happy with the deal. Cargo ships can chop days if not weeks off certain journeys by using the Northwest Passage. In 2014 the Nunavik took 26 days to carry nickel from Quebec to China, compared with a previous trip through the Panama Canal lasting 41 days. But the dispute, like Arctic ice, could be transitory. Scientists now speculate that the entire Arctic Ocean will be ice-free in summer in the next few decades. That means ship owners will not have to ask anyone’s permission if they choose a route through the unclaimed high seas at the top of the world.