U.S. President Donald Trump’s decision to withdraw American forces from Syria is a mistake. But if he insists on going ahead with it, the best option for the United States is to do what it can to ensure that Moscow—not Ankara or Tehran—ultimately replaces it and negotiates a political settlement that prevents a new conflict in eastern Syria. The two of us have long argued for greater U.S. engagement in Syria, so we find this approach unpleasant. But given Trump’s decision, it is the best way to avert further conflict, prevent the Islamic State (ISIS) from reemerging, and limit Iran’s influence in eastern Syria.

THE RISKS OF RETREAT

The withdrawal of U.S. forces from eastern Syria has the potential to create a security vacuum in which ISIS could regenerate, especially if fighting flares up again. The United States will be abandoning its best local partner in the campaign against ISIS: the Syrian Democratic Forces (SDF), which U.S. commanders expected would stabilize eastern Syria after ISIS’ defeat. Iran will likely take the opportunity to shore up its supply lines across the Levant and reinforce the missile stockpiles it is building in western Syria. By walking away from a region that makes up nearly one-third of the country, including some of its most important electricity-producing resources and its most significant water, oil, and wheat reserves, the United States will be giving up its biggest piece of leverage in any negotiation over the final disposition of the country.

Even if Trump changes his mind, or slows down the withdrawal, the damage to the U.S. position in Syria will have been done. The various regional actors, including the United States’ closest Arab partners, are already behaving as though the Americans have left. Given this reality, the only remaining question is how to best protect U.S. interests.

U.S. policymakers should focus on ensuring a peaceful transition in eastern Syria. If the withdrawal leads to renewed conflict between Turkey and the SDF’s Kurdish members in the northeast, or between the SDF’s Sunni Arab components and the forces of Syrian President Bashar al-Assad in the southeast, the results would be devastating. ISIS’ remaining soldiers in the Middle Euphrates River Valley in southeast Syria and in the neighboring areas of western Iraq would thrive. The conflict would cause further misery for civilians and push even more people into Jordan, Iraq, Turkey, and government-controlled Syria.

The best of the United States’ meager options is to encourage a political deal between Assad, the SDF, Iran, Russia, and Turkey. But U.S. policymakers should be realistic about the limits of their influence once the drawdown is in full swing.

Because Turkey is a member of NATO, it might seem the best partner to maintain stability in northern and eastern Syria, but that is an unrealistic prospect. Despite President Recep Tayyip Erdogan’s promise to Trump that he will fight ISIS, any effort by Turkey to retake parts of eastern Syria will be wholly motivated by its desire to limit Kurdish influence and will likely result in an ugly renewed conflict. What’s more, ISIS’ remaining forces are based primarily in the Middle Euphrates River Valley and western Iraq, far from the Turkish border. The Turks have neither the will nor the capacity to fight that far south.

Iran offers an even worse option as a partner. Its position in western Syria has given the Islamic Revolutionary Guard Corps and Hezbollah the ability to threaten Tel Aviv with missile strikes. Israel has responded with air strikes targeting Iranian facilities in Syria. In the wake of the U.S. withdrawal, Iran will seek to take over as much Syrian territory as possible and then use it to control the Syrian-Iraqi border region and transport weapons and fighters between Baghdad and Damascus. That will allow it to project power more effectively in the Levant. Iran will be particularly interested in seizing control of the al-Tanf region, which lies on the Syrian-Jordanian border, along the best road from Baghdad to Damascus, where U.S. forces have been deployed for the past few years.

As Iran expands its influence in the aftermath of a U.S. withdrawal, it will likely ratchet up tensions with Israel. So far, neither side has escalated beyond tit-for-tat strikes, but an accident or miscalculation could spark a larger conflict. The risk will increase if Israel sees Iran as the big winner of the United States’ retreat and seeks to compensate by escalating its air campaign and covert efforts against Iranian proxies and military infrastructure in Syria.

MAKING THE BEST OF IT

Turkey and Iran cannot and will not hold together a political settlement in eastern Syria. That leaves Russia as the best of a bad series of options. Russia does not want to see a new conflict that gives ISIS an opportunity to recover and delays international aid for Syrian reconstruction. It also wants to avoid a major war between Israel and Iran, which would undermine its efforts to consolidate its victory in Syria. Israel, Jordan, and the Gulf states are all relying on Russia to provide some measure of stability. Moscow does not want to throw away that status.

Russia, Turkey, Assad, and the SDF will need to come to some kind of agreement in northern and eastern Syria. The Kurdish components of the SDF are already negotiating with the Assad government and will likely be more willing to return under its umbrella than to fight Turkish-backed forces. Turkey will worry that any deal would create a new safe haven for the Kurdistan Workers’ Party (PKK) in eastern Syria. Russia is best positioned to prevent this, as the Turks put little trust in Assad. The United States should also use its relationship with Turkey to urge restraint.

In the Middle Euphrates River Valley, where Russia has deployed special forces and private military contractors, Russian and U.S. forces maintain a direct communications channel to prevent military accidents from escalating. The two countries should use this channel to coordinate the peaceful handover of territory. As part of this effort, the United States should share information with Russia on the remaining ISIS-held territory in this region and ask Moscow to prioritize retaking that territory and preventing ISIS’ reemergence.

The United States will be giving up much of the leverage it might use to contain Iranian influence if it withdraws, but there are still things it can do. U.S. diplomats should support Israel’s efforts to counter Iran through limited strikes in Syria. That should motivate Russia to find a solution acceptable to Israel, in order to avoid a major new conflagration, which could undermine Russia’s position in Syria. The United States should leave the withdrawal from the Middle Euphrates River Valley and al-Tanf—the most important part of eastern Syria to Iran—until last. It should press Russia to commit to keeping Hezbollah fighters away from Jordan’s border and encourage the Israelis and Jordanians to echo this point with the Russians. Jordan should be particularly receptive, as al-Tanf is so close to Jordan’s vulnerable eastern border.

By announcing a precipitous U.S. withdrawal and then sending confusing and contradictory signals, which suggest the president and his national security adviser are not on the same page, Trump has significantly reduced the United States’ leverage, so the approach we recommend will have a limited impact at best. Russia does not have the same motivation or ability to fight ISIS as the United States does. Turkey’s desire to eliminate the Kurdish stronghold on its border may be so strong, and Russia may so lack commitment to the SDF, that Moscow will simply cut a deal with Ankara, allowing Turkey to conquer large areas of northern and eastern Syria. Russia does not want to own all of Syria and has only limited leverage over Iran, which has deployed thousands of Hezbollah fighters to Syria from Afghanistan, Iraq, Lebanon, and Pakistan and is actively building local Syrian security forces that are loyal to Assad and the Islamic Revolutionary Guard Corps. Iran will inevitably seize territory as the United States withdraws, and the Russians may not be able or willing to stop it.

The bottom line is that the U.S. withdrawal from Syria is a bad idea. Keeping a small U.S. presence to hold eastern Syria has protected U.S. interests—ensuring the defeat of ISIS and limiting Iran’s influence in Syria—at a reasonable cost. But if Trump is intent on leaving, the best his advisers can do is to limit the damage by using the meager tools at their disposal to encourage Russia to keep ISIS down and contain Iran.