An American-led coalition is closing in on the last major Islamic State strongholds — Mosul in Iraq and Raqqa in Syria — and officials say military and counterterrorism leaders face an urgent challenge in devising plans to deal with the consequences of that success.

“Hundreds of hardened killers who are not going to die on the battlefield” will flow out, James B. Comey Jr., the F.B.I. director, said this month, adding that the fallout from “crushing the caliphate” would dominate the bureau’s attention for the next five years. The F.B.I. has agents in Europe and elsewhere working with foreign counterparts to track and combat the global threat.
Some Islamic State fighters in Iraq and Syria have filtered back into Europe since early 2014. But Western officials fear the squeeze on the group’s territory could greatly accelerate that flow.

Even top Islamic State leaders acknowledge the inevitable collapse of their declared caliphate, and they appear to be shifting to a new strategy that threatens Europe on multiple fronts: with cells developed in Europe over the past two years, with returning fighters, and with inspired followers who heed the jihadist group’s recent call to carry out attacks in their home countries.