Qatari gas pipeline to Europe. This may be why many of the euros were for the strikes originally and also why Russia is vociferous in support of Assad. Qatari gas would compete directly with Russian gas which currently enjoys a near-monopoly in Europe.
Which gives Russia a national economic interest in saving Assad which we do not have and thus a reason to fight against us if we stick our noses in. If Europe wants that gas pipeline then they need to do the dirty work themselves we have no national interest in helping al Quaeda over Russia's historic dictator partner just so Europe can outmaneuver Russia on oil.
We definetly do not have a national interest in bombing Assad and leaving those WMDs in al Qaeda's hands.