The poorer the country from which migrants come, the higher the social cost of absorbing them. Consider the experience of Sweden, which on a per capita basis has one of Europe’s largest immigrant populations. More than 15 percent of Swedes are either foreign-born or were born in Sweden to two foreign-born parents. The country has extended a special welcome to refugees from some of the world’s most troubled places, including Somalia, Iraq, and Syria. But as Sweden’s intake from poor countries has grown, the economic performance of its immigrant population has lagged. The Economist reports that in 1991, the median income for non-European immigrant households was 21 percent below that of long-settled Swedish households. By 2013, the gap had widened to 36 percent.

Immigrants’ economic frustration and ensuing social isolation has in turn fostered political radicalization and violent extremism. Extremist views are held by a minority of immigrants, but that minority poses Europe’s severest internal security threat since World War II. In response to this growing threat—which is traceable to migration—European governments have imposed ever-tightening surveillance upon their societies. Thus, as Christopher Caldwell lamented several years ago in Reflections on the Revolution in Europe, his superb book on how migration has transformed Europe, the price of increased diversity has been diminished liberty.

All of this has produced a dismaying confluence: frustration among migrants and their children, resentment on the part of older citizens, rising extremism on one side, authoritarian xenophobia on the other, and an increasingly obtrusive (if ineffective) security state. Many people on both sides of the Atlantic find these facts uncomfortable to acknowledge. But if mainstream leaders won’t respond to the uncomfortable, demagogues will.