Doing nothing in Syria is now out of the question

In other words, the national-interest question has shifted from whether actively helping the FSA will do more good than remaining on the sidelines, to whether it’s the regime or the rebels who are most likely to blunt the advance of ISIS. Standing on the sidelines has ceased to be an option, just as allowing al Qaeda to flourish on the Afghanistan-Pakistan border in the aftermath of 9/11 was not an option. And even if you refuse to acknowledge the categorical moral difference between a regime waging war on its citizens and the rebels (who include terrible people who have done terrible things) fighting to bring that regime down, it’s clear that the rebels view ISIS as their mortal enemy — and the regime does not.

Having said that, it’s hardly clear that the rebels have the capacity to do what the United States would like them to do. The moderate rebels, a vague phrase that may or may not encompass Salafist brigades that would fit many people’s definition of “extremist,” have barely sustained a stalemate against the combination of Assad’s artillery and air attacks and ground forces led by Hezbollah and Iranian officers. And now, they are simultaneously locked in combat with the battle-hardened jihadists of ISIS, who have begun to stream back from Iraq armed with missiles and even American Humvees. The Iraqi army melted away before ISIS battalions, despite in many cases greatly outnumbering them.