Quite apart from the realpolitik aspects of the Syrian civil war, the issue should be viewed through a far larger prism, viz., what we are witnessing is the millenium+-old conflict between Shi'a and Sunni Islam. This is nothing less than the ummah's Thirty Years' War that we are watching play out.

The best option for the (khufr) West is to contain the conflagaration beyond its borders and, to the extent reasonably possible, secure innocent non-combatants inside safe zones- whether they be in Syria or KSA or Jordan or.... But under no circumstances attempt to intermeddle in a pan-regional theological war between co-religionists.