If history is any guide, ISIS will encounter opposition from other elements within Islam and, despite its troops and its wealth will find itself under attack from those elements that want to remain the leading powers, both nationally and from non-state factions. The best course of action should be to let the Sunnis and the Shiites sort it out as they have for more than a millennium of savagery against one another. …That is not what history shows as a guide. There would have been no Crusades if any of that were true, and certainly no Caliphates.
The time when Obama might have been able to do something effective was when ISIS's Toyota convoys were forming in Syria and initially invading Iraq, and the first week or two in Iraq. Air strikes could have inflicted significant damage during those days, but once several Iraqi cities had fallen and the defenders fled, it was "too late" as McArthur said.
Now it will take major commitments of troops and planes to dislodge ISIS, and I don't see anyone who might do that.