Islam's Dysfunctional State: In Isis-controlled Syria and Iraq everyday life is falling apart - Middle East - World - The Independent
Okay...they take a lot of territory. But once they do, that territory turns to shit because you can't run a city on Islam. You need infrastructure, doctors, medicine, sanitation, etc.
And new territory gained becomes exponentially harder to keep -- especially when you have no air power. When you can be bombed constantly and can't reach out to your enemy's homeland with air power, you won't last. Just ask Germany and Japan.
They'll do themselves in by reaching too far too fast -- which is exactly what they're doing. The hubris of ISIS is our best ally.