Sir Keir Starmer recently tried to take credit for his role in Donald Trump’s partial climbdown on tariffs relating to Greenland. Whether he played any meaningful part is debatable. What is beyond doubt, however, is that Donald Trump will happily reach for the tariff weapon again whenever it suits him. Even against long-standing allies, and especially against those he believes are acting against US interests. Just look at Canada and the increasingly heated rivalry between Trump and the country’s prime minister, Mark Carney. The president last week threatened eye-watering tariffs of 100pc on all goods and services entering the US....