It used to be common knowledge – not just among policymakers and economists but also high school students with a grasp of history – that tariffs are a terrible idea. The phrase “beggar thy neighbour” meant something to regular people, as did the names of former senator Reed Smoot and representative Willis Hawley. Americans broadly understood how much their 1930 tariff, along with other protectionist and isolationist measures, did to turn a global economic crisis into another world war. Thirteen successive presidents all but vowed never to repeat those mistakes.

Until Donald Trump. Until him, no US president has been so ignorant of the lessons of history. Until him, no US president has been so incompetent in putting his own ideas into practice.