It was the anti-Assad forces that were recruiting foreign fighters.
In any case it made no sense to overthrow Assad, and then turn around and help Iran get a nuke. It makes some sense to remove Assad as part of an overall anti-Iran strategy but under Obama the whole thing was incoherent. And half a million Syrians have died for no particular reason.
Anti-Assad and Pro Assad were both importing foreign fighters. Most of them were Muslim.
Allawites are a very private group. Little is really know about what they are exactly. It is an odd paring with Iran. Both are considered Shia. I think there is a side of Allawite beliefs that must touch on Shia concepts. (I’m not well read on Muslim beliefs, and have no direct interest in that. But I suspect it is connected.)
Mattis has said he felt this is the most complex place in the world to try to fight a war. He is probably right.