President Donald Trump is taking his enthusiasm for tariffs to a new level. In his latest move, he has announced a policy of "reciprocal" tariffs that would tax imports from other countries at the same rate they tax imports from America.

If the US genuinely tries to close its overall deficit with big tariffs all round, it will cause a crunching recession. If Trump discriminates between trading partners, the heavily tariffed ones can nip through the side door of those who escaped more lightly.

Donald Trump’s tariffs are a headache for all his trade partners. But the U.S. president’s plan to impose ad hoc “reciprocal” levies on imports are a particular problem for the European Union. By enabling him to target whichever countries in the bloc he pleases,