By doing nothing decisive, we’re ceding ground to be bad guys in the resistance. There is plenty of money going to the extremists, and their networks (not destroyed or ‘back on their heels’) of fighters and funders are working overtime. By not trying to find reliable partners to cooperate with among the rebels and giving them the tools to get the job done, we are ceding ground to al-Qaeda in whatever shape post-Assad Syria takes. - Walter Russell Mead
This is a common lament, both among pro-interventionist Western commentators and among Syrian rebel forces themselves. But how true is it? Let's presume the U.S. arms the rebels - but only the Good Ones Who Share Our Values - and they're able to fight more effectively against Assad's forces. Will the jihadists decide to quit the battlefield? Why would they do that? Are we supposed to assume that the Syrian forces fighting the Assad regime will instantly turn their guns on the jihadists in their midst if and when they succeed in overthrowing Assad? Won't they have bigger fish to fry at that point?
The possibility of jihadist enclaves in Syria is very real and it's reasonable to assume that the longer fighting drags on, the more opportunities there will be for safe havens and terror networks to take root. But these same networks are just as likely to feed on post-war instability and chaos - which is what is likely to attend the downfall of Assad. What's needed to prevent extremists from gaining traction inside Syria is not American weapons but some plan to actually achieve internal security inside the country. Short of that, arming the rebels may have some merit, but it's not likely to prevent al-Qaeda and its cohorts from taking root in the country.