Europe in 2017 is racked with uncertainty—the eurozone crises, the endless challenges of the European Union, national elections that resemble endless rounds of bullet-dodging. Yet even these events are insignificant compared with the deep tectonic shifts beneath the Continent’s politics, shifts that Europeans—and their allies—ignore at our peril.

Throughout the migration crisis of recent years I traveled across the Continent, from the reception islands into which migrants arrive to the suburbs in which they end up and the...