As I write these words, American pundits and political junkies are struggling to come to terms with some curious, even alarming, developments. Donald Trump has claimed the Republican presidential nomination while advocating a ban on Muslim immigration to the United States and the construction of a wall along the Mexican border. While many would blame his rise on the particular dysfunction of the G.O.P. or of America’s political culture more generally, a larger context makes that view impossible. A populist and sometimes xenophobic campaign succeeded in persuading British voters to leave the European Union. Marion Maréchal-Le Pen has risen…