Splintered Belgium had lost control of Molenbeek. A heavily Muslim district of Brussels had in effect seceded. If this were the extent of the problem, it would be grave. But Molenbeek is just the most acute manifestation of a European failure.

The large-scale immigration from Turkey and North Africa that began a half-century ago at a time of economic boom has — at a time of economic stagnation — led to near-ghettos in or around many European cities where the jobless descendants of those migrants are sometimes radicalized by Wahhabi clerics. As the French prime minister, Manuel Valls, warned recently, an extremist minority is “winning the ideological and cultural battle” within French Islam.

The fact that the jihadis, often Syrian-trained, are a minority, and that many Muslims who immigrate to Europe are leading successful and integrated lives, is little consolation. After the carnage in Paris and Brussels, the laissez-faire approach that had allowed those clerics to proselytize, private Muslim schools to multiply in France, prisons to serve as incubators of jihadism, youths to drift to ISIS land in Syria and back, and districts like Molenbeek or Schaerbeek to drift into a void of negligence, has to cease. Improved intelligence is not enough. There is an ideological battle going on; it has to be waged on that level, where it has been lost up to now. The moderate Muslim communities of Europe need to do much more.