Look at any map of Europe from the Middle Ages or the early modern era, before the Industrial Revolution, and you will be overwhelmed by its dizzying incoherence—all of those empires, kingdoms, confederations, minor states, “upper” this and “lower” that. It is a picture of a radically fractured world. Today’s Europe is, in effect, returning to such a map.

The decades of peace and prosperity, from the 1950s to 2009, when the European Union’s debt crisis began, made the political and economic contours of the continent look...