The roots of Sunni Islam's ailments it must be noted are not entirely to do with religion, as most journalists, politicians and "experts" in Europe and across the Atlantic never tire of repeating. Rather than scripture and theology, it is in politics and economics, in power balances, foreign interventions and the scramble for influence and resources that the causes of its ills reside.
Without an inchoate caliphate, ISIS becomes another bunch of terrorists roaming in the area, competing with several others.ISIS can be defeated, and quite readily. It set itself up as an easier target than other terrorist groups when it defined itself as seeking to found a state, governed in line with its particular interpretation of Islam.
If America doesn't have the stomach for such an open-ended commitment -- and honestly, it's hard to imagine a successful candidacy for the White House in 2016 built around the theme, "Let's Re-invade Iraq" -- the options get much more limited. But there are three things that would make a difference.