I'm really not sure what, if anything, the US will do beyond arming the rebels. Taking Al Qaeda's side in this fight is a hard bargain, but then letting Assad massacre civilians, conventionally or chemically, is also strongly undesirable. I also don't think the US can really convince the various rebel factions to reject extremism and Al Qaeda either. If we had intervened earlier on, maybe we could have, but at this point it's too late. Even if there had been an early intervention, I don't think we could've stopped Al Qaeda-types from getting involved as the major ideologies of the region strongly favors it.