The Islamic State’s self-proclaimed caliphate, the brutal “state” the extremist group ruled for three years in Syria and Iraq, is rapidly collapsing.
After months of grueling combat, Iraqi troops have finally retaken Mosul, their country’s second-largest city, at the cost of thousands of lives and the destruction of its ancient center. In neighboring Syria, Kurdish and Arab fighters with American advisers are closing in on Islamic State’s capital of Raqqa.
The United States and its allies are winning the major battles. But they are still in danger of losing the war.
The fall of Mosul and Raqqa won’t solve the problem that led to Islamic State’s rise: the misrule of Sunni...