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During an address to the nation that he delivered from the White House in September, President Obama vowed to “degrade and ultimately destroy” ISIS. The only thing that has been degraded and destroyed in the intervening months, however, is the credibility of the U.S.

U.S.-led air strikes have killed more than 6,000 ISIS fighters. But those losses have been more than made good by the stream of 1,000 foreign fighters who are estimated to join ISIS every month. ISIS’s snuff films, like one showing a Jordanian pilot being burned alive, may trigger widespread repugnance, but they also have a sick appeal to a dismayingly large number of young Muslim men who thrill at the chance to establish a new caliphate.

ISIS is not going to run out of cannon fodder anytime soon, and the U.S. approach, limited to air strikes, has shown scant ability to dislodge ISIS from its strongholds, especially in Syria, where ISIS has expanded its zone of control over the past six months. For air strikes to work, they need to be launched in coordination with an effective ground force, but that has been mostly lacking.

The only real exceptions are the Kurdish peshmerga fighters and the Iranian-backed Shi’ite militias. But neither the Kurds nor the Shi’ites will be able to clear and hold Sunni areas stretching from Fallujah to Mosul. Indeed, the more that bloodthirsty Iranian-backed militias gain prominence in the anti-ISIS cause, the more Sunnis will rally to ISIS as defenders of their embattled community.

Back in 2007–08, when al-Qaeda in Iraq, ISIS’s precursor, was pushed out of the Sunni-dominated northwest of Iraq, it was by Sunni tribal fighters working in conjunction with American troops. To inflict serious setbacks on ISIS today will require resurrecting that successful coalition rather than flatly refusing, as Obama has done, to put any “boots on the ground.”

It is in America’s interest to send as few troops as possible into harm’s way and to get our allies to do as much of the fighting as possible. But sending only 3,000 troops and essentially prohibiting them from leaving base, as Obama has done, is a recipe for ineffectiveness. If we’re going to have any impact on the fight against ISIS, we need to take off our self-imposed shackles.

It’s hard to know now what commitment may be necessary, which is why it’s vital not to pass an Authorization for the Use of Military Force that would prohibit “enduring offensive ground combat operations.” It is folly to tell ISIS in advance that it has nothing to fear from the best ground troops on the planet.

Credible estimates of how many troops we should send range from 10,000 to 25,000. Just as important as the troop numbers are the rules of engagement under which they operate. It is imperative that U.S. advisers and joint tactical air controllers be able to operate on the front lines with the local troops they support. This was the formula that made possible the rapid overthrow of the Taliban in Afghanistan in the fall of 2001.

In addition to sending advisers along with support personnel to protect and sustain them, we should be sending joint Special Operations task forces–composed of Navy SEALs, Army Delta Force and other Tier 1 operators–to target ISIS as they once did so successfully with al-Qaeda in Iraq. While aircraft can drop bombs and kill people, only commandos can capture and interrogate high-level terrorists, gathering intelligence that has the potential to wipe out an entire enemy network.

With a slightly larger commitment of American forces, we might be able to galvanize more local opposition to ISIS in Syria and Iraq. But we need to be careful not to make the U.S. the enabler of Shi’ite death squads working at the behest of Iran’s General Qasem Soleimani, the commander of the country’s far-reaching, elite Quds Force. The entire Iraqi army may be so badly compromised by militia infiltration that it is better to focus American efforts on persuading the Sunni tribes of Syria and Iraq to join forces against ISIS. Baghdad–and Soleimani–might not approve, but the U.S. must ignore those concerns. Without the support of the Sunni tribes, the West will face an impossible task in the war against ISIS.

Boot, a senior fellow at the Council on Foreign Relations, is the author of Invisible Armies: An Epic History of Guerrilla Warfare From Ancient Times to the Present


This appears in the March 09, 2015 issue of TIME.

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