F ollowing the Brexit referendum in 2016, Marine Le Pen was in a predictably celebratory mood. Britain’s vote to leave the European Union, she told one interviewer, was the first breach in the “Brussels Wall”. Ms Le Pen’s Front National party was at that point committed to taking France down the same path, given the chance. But as the UK’s self-inflicted economic wounds became a cautionary tale for the rest of Europe, the notion of Frexit was put on ice.

Instead, Ms Le Pen now hopes to lead far-right allies in hollowing out the EU from within. This month, both she and the leader of Alternative für Deutschland, Alice Weidel, outlined their visions of a “Europe of nations” in which EU institutions would be stripped of both cash and power. Ms Weidel’s “new deal for Europe” would abolish elections to the European parliament, and seek German opt-outs in areas such as migration, defence and common borrowing.…