This is the rather terrible but also very troubling question asked by Stephen M. Walt on Foreign Policy in a paper titled “What Should We Do if the Islamic State Wins?”
The author does no really answer the question he asks but I find it mind-opening to start from his postulate: Daesh is not automatically going to disappear because we want it to (or because this is a monstruous group). It may take much more than that to avoid the group which proclaims itself “Islamic State” or “Califate” really becoming an islamic state, stable and well poised between Irak and Syria, which may be what it is doing despite international efforts.
As Stephen M. Walt, I absolutely do not like this perpsective, but we’d better start exrecising our intellect on the worst hypothesis in front of us. And I hope that some other authors will be able to bring some fairly good/intersting (or even mind-blowing) answers.