The Washington Post reports: 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.

On Monday, President Obama cited the recent gains in Syria as evidence of progress. “When we have an effective partner on the ground, ISIL can be pushed back,” he said in Washington after the Pentagon briefed him on the status of the war.

“ISIL’s strategic weaknesses are real,” he added, using an acronym for the Islamic State.

But the absence of reliable local forces to press the fight deeper into the Islamic State’s home turf has revealed the weakness of the U.S. strategy, analysts say. And rising tensions between Arabs in the area and their purported Kurdish liberators risk jeopardizing the gains.

The offensive is taking Kurdish forces far beyond traditionally Kurdish territory and into areas where Syrian Arabs are in the majority, drawing allegations from Syrians and also the Turkish government that the Kurds are taking advantage of the U.S.-led air war to carve out a Kurdish state.

The Syrian opposition has accused the Kurds of driving Arabs from their villages to consolidate their control. [Continue reading…]