Three weeks ago in the National Interest, I proposed a five-step plan to destroy the Islamic State (IS) in Iraq and Syria. The plan called for the United States to: mobilize a major humanitarian-relief effort; catalyze political settlements to unify anti-IS groups in Iraq and Syria; field robust, supporting military operations; internationalize the anti-IS effort; and prepare the American people for a potentially costly, long-term mission.

President Obama last Wednesday unveiled his strategy to “degrade and ultimately destroy” IS. I applaud his message. The president’s strategy covers nearly all of the necessary steps. Mr. Obama did not specify a timeline for achieving his objective, but senior defense planners expect anti-IS operations to last at least three years. The strategy involves an intensification of the ongoing air campaign (with possible expansion into Syria); greater efforts to train, advise and/or equip various Iraqi forces, especially “National Guard” units in Sunni provinces and the Kurdish peshmerga; ramped up support for nationalist Syrian insurgents; an international effort to stem the flow of fighters and funds to ISIS; and an expanded humanitarian effort to ease the suffering of civilians trapped in the conflict.

The president’s declared strategy left out one critical component for success against IS: catalyzing a political settlement in Syria. He may prefer to focus on a political settlement at a later stage, calculating that air strikes against IS, along with the buildup of nationalist opposition forces, will spur progress in Iraq and bolster U.S. leverage in Syria. The president may also reason that an endgame solution for Syria will require regional consensus, which would be difficult to garner were Washington to announce U.S. political objectives at the outset. These are legitimate considerations. But the administration’s strategy to destroy IS will not succeed without a plan for a political settlement in Syria. Syria needs a leader who can unify all ethnic and sectarian groups against IS through power-sharing at the center and decentralization of the country’s political system. In other words, a settlement in Syria would resemble the political solution Washington is promoting in Iraq to unify Iraqis and motivate Iraqi Sunnis to fight against the IS army.

Syria is not Iraq, however, and the calculus of internal and external players may be quite different. In Iraq, replacing former prime minister Nouri al-Maliki with Haider al-Abadi did not undermine Shia power in Iraq, given that Shia comprise a majority of the country’s population, and that the new prime minister is from the same Da’wa party as his predecessor. Iran’s supreme leader Ayatollah Khamenei supported the change, as did all other major regional players. Iran also has both helped and participated in the fight against IS in Iraq, including Kurdistan.

In Syria, however, where ruling Alawites are a minority, Iran has opposed the removal of Assad and helped the regime massively with weapons, personnel and economic support. The Iranian assistance has been decisive in sustaining the regime. Tehran has been complicit in the brutal tactics employed against the rebels and Sunni civilians by the Assad government. Eliciting Iranian support will entail a far-more-complicated solution that would involve not only a timeline for Assad’s departure and an agreement on a leader who can unify the Syrians and rally the Sunnis against IS, but would also reassure the Alawites. In essence, Washington and its allies need a roadmap for Syria that both Iran and Sunni states can endorse and accept.

Conducting airstrikes against IS in Syria without corresponding political steps could work to the advantage of the regime in Damascus. Assad might well devote more resources to attacking the nationalist opposition. This in turn could lead disaffected Sunnis to rally in support of IS against the Assad regime. He might calculate that if he can eliminate the nationalist opposition and the fight becomes sharply one between his regime and IS, the world will choose him and his regime.

A strategy sets the general direction for a campaign, but details will matter, too. The president’s strategy to destroy IS will encounter major, perhaps fatal, obstacles if it fails to deliver on the following items:

First, significant material support must be delivered urgently to the nationalist Syrian opposition to mitigate the risk of airstrikes inadvertently helping Assad or IS. This is also important for changing conditions on the ground to make them more favorable to a political settlement—undermining the regime’s hopes for victory and confronting Iran with increased costs of sustaining the regime.

The administration’s record on this front is very poor. The administration shortchanged the nationalist opposition earlier, when it still enjoyed broad internal support. More recently, the Obama administration has embraced the idea of supporting the nationalist opposition at a time when it has lost ground to the regime and a significant part of the broader anti-Assad opposition has turned to extremism and terrorism. Very little of the administration’s $500 million support program is believed to have made it to the nationalist opposition. Now the administration has vowed to do more to support the nationalist opposition, but significant resistance to the plan has emerged on the right and the left in Congress. Their doubts are understandable, but the fact remains that if the nationalist opposition is defeated, Washington would find itself in a terrible dilemma: For our own security, the United States would have to continue to bomb IS, but the costs of doing so would increase. The absence of a nationalist opposition would allow the Assad regime to take advantage of the bombing and move against Sunni areas outside regime control, leading more Sunnis to support IS.

Second, the United States must monitor and influence local forces. The administration’s strategy for defeating IS relies on various local anti-IS forces (Syrian rebels, the Iraqi army, the peshmerga, Sunni tribes and Shia militias). This approach is prudent, but it won’t be easy. Many of these local forces are sectarian and are not be driven by the same objectives as is the United States. They will take miscalculated steps that work to the Islamic State’s advantage. For example, there have been disturbing reports recently of Shia militiamen in Iraq entering the town of Suleiman Bek with the aid of U.S. airpower, and then proceeding to torch Sunni homes and behead suspects. These kinds of atrocities will undermine the anti-IS campaign by pushing more Sunnis into the arms of IS. One option to deal with this challenge, reportedly favored by Centcom commander Gen. Lloyd Austin, is to allow, selectively, Special Operations forces to operate in a front-line role. This option may become necessary if specific local forces prove unable to perform their tasks without the direct assistance of U.S. special forces on the ground. Another option, which would require greater international cooperation, is to send small teams of international peacekeepers behind advancing Iraqi forces to prevent acts of sectarian revenge in areas liberated from IS.

Third, a lasting solution must be found to the problem of Shia militias in Iraq. A plethora of Iran-backed Shia militias operate in the country. The fact that these militias have played a sizable role in halting IS advances since the fall of Mosul in June is not an argument for ignoring their current atrocities or their previous role in exacerbating sectarian tensions, which led to the current crisis in the first place.

One way to deal with Shia militias is to transform them into “National Guard” units limited to security roles within their respective provinces. Prime Minister Abadi endorsed a similar concept when he presented his new cabinet and government program. Abadi’s idea is to give Sunni Arabs the leading role in the security of their respective regions, thereby reducing tensions caused by the presence of predominantly-Shia government forces. Replicating this model at a later stage with Shia militias in their respective provinces would guarantee the Shia community security during the period when the federal army is undergoing restructuring to become a more professional, and perhaps much smaller, force that provides support (firepower, aviation, special forces, and so on) to regional/provincial guards. The United States can advise the Iraqis as they implement this ambitious program and help them find the right balance in force size, armaments and roles between federal and regional units to reduce the chance of escalating security dilemmas.

Fourth, special attention must be paid to strengthening the Kurdish peshmerga. Of all the local forces in the region, the peshmerga and the Kurds generally are the most reliable and friendly towards the United States. But under pressure from former prime minister Maliki—who insisted that supplies intended for Kurdish forces must be inspected and approved by Baghdad—the United States kept the peshmerga poorly armed, reversing course only after IS threats to the Kurdish capital of Erbil forced Washington to airlift weapons and munitions to the Kurds. Even now, in the face of the urgent IS crisis, weapons shipments to the peshmerga are reportedly being delayed in Baghdad. As the Obama administration assesses how to help the peshmerga become a more professional force, it should be prepared to push back against resistance from the Iraqi government.