Vice President Biden claimed late last month that the Islamic State in Iraq and the Levant (ISIL) "can be routed by local forces without U.S. boots on the ground." He cites as evidence the Kurdish peshmerga and Iraqi army recovery of the Mosul Dam from ISIL.

But a military operation to recover a big, fixed installation in an unpopulated area is far easier than retaking Mosul, a city of about 2 million (the second largest in Iraq, after Baghdad). ISIL has deep roots in Mosul, which it had ruled at night for some time before it frightened off the Iraqi police and army in the daytime. ISIL has held at least parts of Fallujah and other towns in Anbar province since the beginning of the year, eradicating local resistance and established governing structures that are arguably doing better than the Iraqi government in delivering services. ISIL captured Tikrit more recently, but it is proving difficult for the Iraqi security forces to retake it.

To roll back ISIL, someone will have to be prepared to chase its mostly Iraqi cadres from the Sunni-majority cities of the Ninewa, Anbar and Salah Din provinces block by block. If that is attempted without the support of the local populations — by the Iraqi army or the Kurdish peshmerga — it could look like sectarian warfare. While some Sunni tribal leaders have announced their opposition to ISIL, Sunnis in general have so far shown little inclination to challenge it. They certainly won't want to do so if they think it will lead to rule by Kurds or Shia.

The administration, to its credit, recognizes this and is insisting on formation of an inclusive government in Baghdad that Sunnis will have reason to accept as legitimate, before expanding the use of military force. But even an inclusive government will have a hard time if the forces it employs are overwhelmingly Kurdish and Shia, as the peshmerga and the Iraqi army are. Baghdad and Washington need to give some serious thought to building Sunni-staffed forces, and perhaps inviting in international police from Sunni Arab countries, to provide law and order as cities are taken back from ISIL. Otherwise, Baghdad's victories may prove temporary and reversible.

If rollback of ISIL in Iraq is challenging, its defeat will be even more so, because it will require at least the containment, if not the defeat, of its safe havens in eastern Syria. Without that, ISIL will be able to retreat, refresh and return to Iraq at will. Containment of ISIL in eastern Syria will require destruction of the Syrian air defense system, so that it can be attacked from the air if it seeks to return to Iraq. Containment will also require some means of ensuring that any territory ISIL loses falls to opposition rather than regime control. The Syrian Opposition Coalition that Washington recognizes as the political representative of the Syrian people is on the ropes in Aleppo and the east. It will need significant strengthening if it is to be relied on to govern in eastern Syria and prevent ISIL's return there.

Wherever rollback or defeat of ISIL occurs, there will be a real and urgent need to establish law and order, begin governing and reconstructing as well as providing services to the "liberated" populations. Iraq has the oil resources to pay its own bills. But Syria does not. Nor will the Iraqi government and the Syrian opposition necessarily be able to do these things quickly and effectively, without arousing sectarian passions. Some sort of international intervention force without U.S. participation — Arab League? UN? — is conceivable, but we should be planning for it now.

Can all this be done without American boots on the ground?

Even the limited successes against ISIL so far — the Mosul Dam, evacuation of the Yezidis from Mount Sinjar and the retaking of a few towns in northern Iraq — have required both U.S. civilians and troops, who have allegedly not engaged directly in combat. The Americans staff command centers in Erbil and Baghdad as well as the humanitarian assessment team on Mount Sinjar and other aid operations. Even this limited activity has required a uniformed U.S. military presence of about 800, plus an unknown number of both government and nongovernmental civilians to work on humanitarian relief for hundreds of thousands of displaced Iraqis as well as Syrian refugees.

A much larger uniformed and civilian U.S. presence will likely be needed to defeat ISIL in Iraq and keep it contained in Syria. The slippery slope is not more than a step or two from where we stand.

Serwer is a professor at the Johns Hopkins School of Advanced International Studies and a scholar at the Middle East Institute. He blogs at and tweets @DanielSerwer. He is the author of Righting the Balance (Potomac, 2013).