Jihadist terrorism until recently had Europe on the defensive. Now the continent is getting tough and fighting the threat with measures that would have been unthinkable six or seven years ago.

The old notion that Europe is weak on terrorism gained traction in the mid-2010s. Between 2015 and 2017, some 350 people were killed by jihadists across Europe. Terrorism rose to the top of polls of public concerns, and criticism of European counterterrorism capabilities grew explicit. A PBS “Frontline” investigation into the 2016 Brussels...