Then there is the terrorist without an ideology, such as Salih Khater. On Aug. 14, 2018, he drove his car into pedestrians outside the Houses of Parliament. Coming more than a year after Khalid Masood launched a similar attack near the same location, the attack set London on edge again.

In sentencing Mr. Khater, Justice Maura McGowan concluded that he had committed a terrorist act, but she couldn’t identify a clear ideology he was advancing: “You replicated the acts of others who undoubtedly have acted with terrorist motives. You deliberately copied those others. . . . You have never explained your actions and have not given any account, before or today, that is capable of dissuading me from drawing the conclusion that this offending had a terrorist connection.”

In a growing number of terrorism cases, ideology is at best an appendage to an act of planned, performative violence. These terrorists are people driven by personal demons or interactions on chat forums or online communities, people with social disorders or mental-health issues, or people with a desire to make statements in a world on which they have failed to make an impression.

Where was Baghdadi in Mr. Khater’s attempted car-ramming attack and others like it? In this new age of terror, ISIS provided only the background idea by popularizing the method of driving cars into pedestrians. The group generated the meme, or at least helped it become viral, making it easy for others to replicate.