From “ISIS After Paris” by Steve Coll in the 11/30/15 issue of The New Yorker:

If President Obama ordered the Marines into urgent action, they could be waving flags of liberation in Raqqa by New Year's. But, after taking the region, killing scores of ISIS commanders as well as Syrian civilians, and flushing surviving fighters and international recruits into the broken, ungoverned cities of Syria and Iraq's Sunni heartland, then what? ...

Syrian rebels overwhelmingly regard Assad's regime as their main enemy, and for good reason: his forces have killed more Syrians than anyone else has. In the absence of a political agreement with Assad or his removal from office, it is impossible to conceive of a Muslim-majority occupation force that would be able and willing to keep the peace after the Marines departed. Some may argue that it would be worthwhile, nonetheless, to wipe out the Islamic State on the ground and deal with the fallout later. After Paris, such an approach may hold emotional appeal. After Afghanistan and Iraq, however, it is not a responsible course of action.