Strikes against Isis
Syria's president Assad (and Russia and Iran) condemns (2) that the US will try to stop Isis. For me, this is an indication the group may be on Assad's side instead of fighting Assad or he would have welcomed that one of his opponents gets destroyed so he needs to fight one powerful group less.
In addition, Assad's army seems not to have attacked the city Raqqa (3), Isis's main centre, while the army attacked other groups closer to Aleppo (a main centre of opposition against Assad). For me, another indication it may be part of Assad's strategy to regain power as Isis may weaken the strength of other opposition groups such as the FSA (Free Syrian Army) as it seems they oppose each other.
Further, it even seems the Syrian military has been buying crude oil from Isis (3) so they seem to support instead of fighting each others. Thus, Isis or Syria reclaiming the fields results in oil for the Syrian army and thus this should be stopped, something that may be welcomed by other groups.
Finally, it seems Isis rose in only a few years from being an unimportant group to one that controls large areas in Syria and Iraq, huge amounts of money as well as oilfields (4). And although of course they may have done this by themselves, how could they control part of Syria's oilfields when they were still small? But maybe I underestimate the powers of certain groups.
In opposition of support between Assad and Isis is the fact that when Isis conquers military bases, the militants seem to kill the soldiers, suggestion indeed they don't like each other (5). Nevertheless, ... .
Still, many opposition fighters seem to wonder why the US turns its attention towards Isis and not Assad (6) and may even consider changing sides in favour of Isis to fight against the US, certainly when innocent Syrians die. Indeed, after years of fighting, they are angry that the West didn't help them fighting Assad, even when he used chemical weapons against them (although we achieved that those weapons were removed (we think) (7)) and thus they may help a group that seems to oppose Assad and is opposed by the West. Indeed, when innocent people seem to be killed by the West, this may become dangerous. Supplying food and medicines for the opposition fighters may already increase morale and increase thrust in the West as it seems this is in short supply although this will probably not be enough.
Therefore, if indeed there is evidence that Isis is part of Assad's strategy to fight the opposition, then this should be clearly explained before people may decide to fight together with Isis (although many may not believe the West because, whatever the West does, we can never do good: leave a dictator and people are angry, fight him or a dangerous group and people are angry). If the reasons for fighting Isis are well explained, then the opposition groups may even help giving information about the whereabouts of the Isis fighters although we should not hope for this. Of course, Assad may change tactics to demonstrate they are not together; still this would mean he has to fight them and thus this would weaken his position as then the Isis fighters will have to defend themselves against him while his army may be angry that he sacrificed his own soldiers (if he did).
But be sure about the above because lying about our reasons for attacking Isis will result in angry voices and thus people turning against the West. Therefore, if indeed there is no evidence Isis is part of Assad's strategy then the West should focus on destroying Isis in Iraq as that government and its people requested help while we should wait until Isis is bad enough in Syria so also people in that country will request our help because if people don't believe we want to help they will not welcome us. In addition, it may be easier to first focus on one region. But that is for the experts to determine.
And if the only reason for attacking Isis is the beheading of two journalists, it will probably not bring back peace, certainly not when innocent people are killed.