Prior to 2016, debates about the global order mostly revolved around its structure and the question of whether the United States should actively lead it or should retrench, pulling back from its alliances and other commitments. But during the past year or two, it became clear that those debates had missed a key point: today’s crucial foreign policy challenges arise less from problems between countries than from domestic politics within them. That is one lesson of the sudden and surprising return of populism to Western countries, a trend that found its most powerful expression last year in the United Kingdom’