About 100 militants are believed to remain in the coastal city of Sirte, which in 2015 became the most important Islamic State stronghold outside of Iraq and Syria. They are holed up in a small, densely packed residential area. For months, U.S.-backed local militia fighters have struggled against militant defenses and sniper attacks; last week, 14 fighters were killed on one day alone.

The elusiveness of victory in Sirte underscores the challenges that continue to face U.S. efforts to defeat extremists from North Africa to Afghanistan: the limitations of local fighting forces, including inadequate battlefield support and poor morale, and the corrosive effects of local political feuds.

The same elements are certain to test the U.S.-backed effort to recapture Mosul, the large Iraqi city where a multipronged operation is now unfolding against the Islamic State.

“It matters for the United States and other Western countries that an operation that was initially thought to last weeks could last months, and it’s unclear what happens after ISIS disappears from Sirte,” said Mattia Toaldo, a senior policy fellow at the European Council on Foreign Relations. ISIS is one of the acronyms for the Islamic State.