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.
Alawites are actually crypto-Christians in origin. They celebrate a form of mass, believe in a Trinity and have other beliefs that indicate they must have originated as Christians who tried to hide their beliefs under the Fatimids and took on the exterior look of Muslims. Later they associated with Shias as Shia allow a lot of flexibility in beliefs — REALLY! Shia Iran is a lot more flexible than Sunni Saudia or Pakistan or other Sunni places