“Things fall apart; the centre cannot hold.” In an age of political upheaval in liberal democracies, the poetic imagination of William Butler Yeats has haunted politicians worried about a lurch to extremes. Following last month’s European elections, which saw a surge in support for the far right, the European Commission president, Ursula von der Leyen, was the latest to invoke Yeats’ famous line in The Second Coming. But she did so with a twist, bullishly noting: “There remains a majority in the centre for a strong Europe and that is crucial for stability. In other words, the centre is holding.”...