The unexpected rout of Islamic State forces across a wide arc of territory in their northeastern Syria heartland has exposed vulnerabilities in the ranks of the militants — and also the limits of the U.S.-led strategy devised to confront them.

Islamic State fighters have been driven out of a third of their flagship province of Raqqa in recent weeks by a Kurdish-led force that has emerged as one of the most effective American partners in the war. The offensive, backed by U.S. airstrikes, has deprived the militants of control of their most important border crossing with Turkey and forced them onto the defensive in their self-proclaimed capital of Raqqa city, something that would have been unthinkable as recently as a month ago.

The advance has shifted the focus of the fight from Iraq to Syria for the first time in months. A blitz of 18 coalition airstrikes against Raqqa over the weekend took out bridges and roads used by the Islamic State to move supplies to battlefronts elsewhere. The air attack was one of the most intense in Syria, according to a Pentagon statement and activists in Raqqa. […]