A shifting attack strategy

In Iraq and Syria, even with its territory greatly diminished, the Islamic State has persisted. Months after Prime Minister Haider al-Abadi declared “final victory” over the group in 2017, three Iraqi provinces have witnessed an uptick in attacks.

Still, the violence there is less devastating than it once was. The group once routinely hit Baghdad with attacks that could kill more than 150 people at a time. Now it tends to carry out smaller suicide attacks, hit-and-runs, ambushes and targeted executions, especially of village chiefs, who are known as moktars.

Mr. Knights, who tracks these low-level assassinations, estimates that more than three moktars are killed or wounded every week in Iraq, undermining official declarations that the militants have been vanquished.

“That means that 14 times a month, the most important person in the village is killed or seriously injured by ISIS,” he said. “Under those circumstances, do those people feel like they have been liberated? Stopping this type of targeted violence is the real challenge, and it’s much harder than clearing cities of ISIS fighters.”

Throughout cleared areas, Islamic State members are believed to have melted back into the population. They move and hide in cells made up of a handful of fighters, and occupy a network of safe houses, analysts say. In Syria, some believe these fighters are awaiting the departure of American forces before attempting a rebound.

If they do, they will pose a different type of threat.

The forces that drove the Islamic State from its lands were equipped to liberate occupied cities, not fight a dispersed, clandestine force. Their vehicles and weapons were designed for engaging the enemy frontally in heavy combat, not for rooting out individual fighters in hiding.

“It’s evolved back into an insurgent movement far faster than security forces can evolve into a counterinsurgency,” Mr. Knights said.