Assad and his allies aren’t targeting ISIS because ISIS is serving the purpose of making the Assad regime look mild in comparison. ISIS is out in the strategically relatively useless, arid, sparsely populated east.
According to der Spiegel and various other sources, ISIS has been set up by Sunni Iraqi Hussein loyalists. The ISIS modus operandi reveals the cruelty of the Saddamite security apparatus. Indeed not much unlike that of Assad. ISIS is able to take large Iraqi Sunni cities but not large Syrian Sunni ones for a good reason: their network is largely Iraqi.
Assad is complicit in their genesis as well, the FT has investigated and exposed the close ties between ISIS and the Assad regime many times, especially the oil deals expose them. Trade between enemies is of all wars but if ISIS really is so hell bent on the defeat of Assad it would not sell it that which uniquely powers the latter’s army: oil.
Putin understands ISIS, he uses the fear for ruthless men in controlling Chechnya and Eastern Ukraine and indeed much of Russia (Nemtsov being an example of how opposition voices are dealt with). Putin may bomb ISIS to keep the commentariat in the free media confused but ISIS is meaningless to his alliance. In Syria ISIS would fold too quickly under concerted pressure (like it did in Palmyra) and that would mean that the other opposition parties including genuine Syrian ones (which ISIS is not) would gain definition. No, ISIS is very useful.
Putin isn’t going to help make peace in Syria. Obama isn’t going to help either. We Europeans should stop expecting ‘great powers’ to fix our problems for us. Many flee towards Europe and many more may follow, our response cannot be contingent upon Washington and Moscow working together for the European interest. With Washington pivoting towards Asia, secure in its own oil supply, and with Russia willing to act boldly in what it thinks is its self interest, the interests of the EU are not on the agenda.