http://www.theatlantic.com/magazine/archive/2015/03/what-isi...
It's a great, lengthy read...it's been posted a few times on HN but without much discussion. But relevant to the OP's opinion: the Atlantic writer asserts that the success of ISIS is heavily contingent on it having material victories, including territorial conquests:
> One way to un-cast the Islamic State’s spell over its adherents would be to overpower it militarily and occupy the parts of Syria and Iraq now under caliphate rule. Al‑Qaeda is ineradicable because it can survive, cockroach-like, by going underground. The Islamic State cannot. If it loses its grip on its territory in Syria and Iraq, it will cease to be a caliphate. Caliphates cannot exist as underground movements, because territorial authority is a requirement: take away its command of territory, and all those oaths of allegiance are no longer binding.
This could be an argument for or against intervention...for intervention, because U.S.-powered intervention would likely overwhelm the ISIS forces and take back territory. But OTOH, the writer asserts that besides unforeseen problems of U.S. intervention (as seen in Iraq and Afghanistan)...ISIS wants America to invade for prophetic reasons.
The Atlantic writer asserts:
> Given everything we know about the Islamic State, continuing to slowly bleed it, through air strikes and proxy warfare, appears the best of bad military options. Neither the Kurds nor the Shia will ever subdue and control the whole Sunni heartland of Syria and Iraq—they are hated there, and have no appetite for such an adventure anyway. But they can keep the Islamic State from fulfilling its duty to expand. And with every month that it fails to expand, it resembles less the conquering state of the Prophet Muhammad than yet another Middle Eastern government failing to bring prosperity to its people.