THIS MEANS WAR: After three jihadist attacks in as many months, the United Kingdom is facing a protracted insurgency—not mere terrorism.

Britons trying to remain optimistic note that they survived and eventually defeated Irish nationalist terrorism not all that long ago. But this is a flawed analogy. In the first place, at any given time during the Troubles, the number of active Provisional Irish Republican Army terrorists seldom exceeded a hundred. Moreover, the PIRA was a “normal” terrorist group with rational political motives, not a religiously-motivated death cult, and it generally eschewed killing civilians for its own sake. Indeed, atrocities like the 1987 Enniskillen attack, which murdered 10 innocents, proved a black mark for the group, even among staunch republicans. Therefore, comparing the PIRA to ISIS and its murderous Western wannabes isn’t much help to practical counterterrorism.

That said, if Britain doesn’t soon devise tough countermeasures to its vast domestic jihadism problem, many of its cities may come to resemble Northern Ireland a generation ago, with armed soldiers in battle gear patrolling the streets as “aid to civil power” while enforcing frequent security checks on average citizens with the aim of stopping terrorists.

Even then, Britain didn’t win against the IRA until much of its foreign funding dried up after 9/11.