From the article
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. Former pledges could of course continue to attack the West and behead their enemies, as freelancers. But the propaganda value of the caliphate would disappear, and with it the supposed religious duty to immigrate and serve it. If the United States were to invade, the Islamic State’s obsession with battle at Dabiq suggests that it might send vast resources there, as if in a conventional battle. If the state musters at Dabiq in full force, only to be routed, it might never recover.
And yet the risks of escalation are enormous. The biggest proponent of an American invasion is the Islamic State itself. The provocative videos, in which a black-hooded executioner addresses President Obama by name, are clearly made to draw America into the fight. An invasion would be a huge propaganda victory for jihadists worldwide: irrespective of whether they have given baya’a to the caliph, they all believe that the United States wants to embark on a modern-day Crusade and kill Muslims. Yet another invasion and occupation would confirm that suspicion, and bolster recruitment. Add the incompetence of our previous efforts as occupiers, and we have reason for reluctance. The rise of ISIS, after all, happened only because our previous occupation created space for Zarqawi and his followers. Who knows the consequences of another botched job?
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.