The more I read of the War Nerd, the more convinced I become that ISIL's real strength lies in manipulation of their image in the media-- PR, basically. Other than that, as soon as they encounter a real opposing force, like, say, the Kurds, or a hypothetical Baghdadi Shia militia, they're out.
ISIS's strength is illusory for the most part. Consider this: they have a strength of force roughly equal to the number of police officers in NYC, depending whose estimates you believe. I can't find concrete numbers about how many civilians live in the area they control, but it seems probably lower than the population of NYC. On the other hand, NYC is 470 sq mi. ISIS's territory is 35,000 sq mi (again, depending whose estimates you're going off), or roughly one fighter per sq mi. That's why they need the PR. They need people's cost/benefit analysis of what it would mean to cross them to be so heavily weighted toward cost that everyone agrees not to fuck with them. Contrast ISIS with the American fighting force in Iraq, which at its peak was at least 150,000, with the best weapons ever created, and we couldn't hold the country. The "troop surge" was bullshit. They way we got them to stop fighting was to "lose" a few palettes of hundred dollar bills off the back of some cargo planes. That's right, we paid them to stop fighting. ISIS can't do that. Eric Shinseki was one of Bush's top military commanders back in '02. He was fired for saying that to stabilize Iraq, our invading fore would need to be a minimum of 500,000, and anything else would be a gigantic waste of money, lives, and materiel. Nothing has changed since then. There is no Islamic State. They don't pose a threat to anyone outside their tiny sphere of influence. Obama's fight against them is nothing more than a gift to ExxonMobile.