Check out my new Foreign Policy piece co-authored with Barak Barfi: "Al Qaeda's Syrian Strategy"

Al Qaeda is storming across northern Syria. Last month, the al Qaeda affiliate the Islamic State of Iraq and al-Sham (ISIS) captured the city of al-Bab in the northern province of Aleppo from a rival rebel militia. The capture of the city, one of the largest in the region, gives ISIS control over a key transit point linking Aleppo to its strongholds to the east. And that’s just the latest in a long string of ISIS’s military successes: After brief clashes with outgunned rebel opponents, ISIS took the towns of Azaz and Jarablus, which straddle Syria’s border with Turkey.
To commemorate its victories, the first thing ISIS did in these places was hang its black flag from the top of the highest building. After that, it began to gradually impose its strict interpretation of Islamic law.
ISIS has embarked on al Qaeda’s most comprehensive campaign yet to win Arab hearts and minds by providing social services to a war-ravaged society. But though the organization’s star is ascendant, its abuses, coupled with an international strategy to limit its influence, could still torpedo its plan to transform northern Syria into an Islamic emirate under its command.
ISIS is thought to count 5,000 to 6,000 fighters within its ranks. That means it’s a lot smaller than other rebel groups, such as the hard-line Salafi Syrian Islamic Front, which boasts 15,000 to 20,000 fighters. But ISIS has one important advantage: Many of its members have previously fought in other jihads, including in Iraq, Afghanistan, Yemen, and Libya.
Nowhere is ISIS stronger than in the northern province of Raqqa. It controls the governorate’s capital, Raqqa city, whose prewar population of approximately 277,300 residents has mushroomed due to an influx of displaced persons from other regions. Meanwhile, the brigades affiliated with the Free Syrian Army (FSA) are focused on squabbling among themselves. As a result, no FSA unit is strong enough to challenge the group in Raqqa, making it the largest city al Qaeda has ever controlled in the Islamic world.
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