A rebel fighter in Idlib takes down a picture of Assad after rebels captured the area in 2015.Ammar Abdullah / Reuters

A war that seems to be ending could still witness its most deadly assault yet. On Tuesday, President Donald Trump warned on Twitter that “President Bashar al-Assad of Syria must not recklessly attack Idlib Province,” adding it would be “a grave humanitarian mistake” for Russia and Iran to “take part in this potential human tragedy.” “Hundreds of thousands of people could be killed,” he concluded. “Don’t let that happen!”

Yet U.S. policy toward a seemingly imminent Syrian military offensive on the country’s rebel-held northwestern Idlib province seems as if it will indeed let that happen. The U.S. has threatened military action if Damascus again uses chemical weapons, but, with some muddled exceptions, has otherwise only emphasized that an attack on Idlib would be a “reckless escalation.” The U.S. threat of force, and Russia’s rhetorical counterpunch, have sucked up much of the media oxygen around the possible offensive. But Idlib seems not to be America’s fight.

Idlib’s fate hangs instead on neighboring Turkey, and on Syria’s ally Russia.

Idlib is the last real bastion of Syria’s opposition. Along with adjacent rebel-held areas, it holds almost 3 million people, mostly civilians. That includes nearly 1.3 million displaced from elsewhere in the country, fleeing a resurgent Syrian government.

Idlib is also “the largest al-Qaeda safe haven since 9/11,” according to Brett McGurk, the U.S. envoy to the coalition against the Islamic State. Its rebels include a major contingent of jihadists, among them Hayat Tahrir al-Sham, the latest incarnation of the former Syrian al-Qaeda affiliate Jabhat al-Nusrah.

For the Assad regime, capturing Idlib would mark its effective victory over Syria’s opposition. The Syrian military is now massing along Idlib’s perimeter, preparing to attack. Russian aircraft bombed areas of western Idlib on Tuesday.

The main protection Idlib has is a dozen observation points along its edge manned by Turkish troops. Turkey began sending in observer forces late last year to reinforce a “de-escalation” cease-fire covering the province, agreed to jointly with Iran and Russia. As part of the de-escalation, Turkey committed to halting violations of the ceasefire by opposition rebels and dealing with designated terrorist organizations operating inside the zone. Turkey’s positions around Idlib seem dubiously useful as a defensive line against a full-scale attack. Instead, they are a visible manifestation of Turkey, Iran, and Russia’s trilateral agreement, and a demonstration of Turkish commitment. Turkey has already sacrificed Turkish lives setting up these posts.

Turkey wants to forestall a Syrian offensive that sweeps Idlib and sends a wave of refugees toward Turkey’s border or Turkish-controlled sections of the neighboring Aleppo province. Turkey already hosts 3.5 million Syrians, according to government statistics. It can hardly handle more, much less a significant proportion of the population of Idlib. And any new refugee flow would necessarily include some of Idlib’s militants, among them jihadists who could threaten Turkey’s domestic security or, as Turkish officials are quick to warn, travel on to Europe or elsewhere.

Idlib’s jihadists, who have enmeshed themselves in the Syrian opposition and the area’s civilian population, have proved an intractable challenge for Turkey, the U.S., and the many countries that consider them a menace to global security. Those jihadists give the Syrian government and Russia their rhetorical justification for the threatened offensive. Militants inside Idlib have also violated the de-escalation agreement, including by repeatedly attacking Russia’s main air base in Syria with makeshift drones. But while Idlib’s jihadists are real and dangerous, “counterterrorism” is also a useful pretext for Syrian military action against opposition-held areas generally, and one that Damascus and its allies have used previously to crush “de-escalation zones” with a relatively minor jihadist presence. In Syria’s southwest, Russia in fact claimed that military action was not a violation of that area’s de-escalation agreement, but in fact the fulfillment of the agreement’s counterterrorism provisions.

For the Syrian government and its allies, an Idlib offensive would let them destroy their insurgent enemies, but it would also in their view be a chance to definitively reassert Syria’s sovereignty and territorial unity.

Still, there are reasons for restraint—for Russia, in particular, whose posture will be critical. It is Russia’s choice to provide air support to a Syrian ground offensive that will effectively decide whether an attack is viable. But Russia’s intervention in Syria has twinned a military campaign with a political one. Russia seeks to deliver victory to a Syrian ally that is, at war’s end, rehabilitated diplomatically and on its way to internationally sponsored reconstruction.

Russia cannot do that alone. It needs European donors, whom it has been trying to enlist in a new push for the organized return of Syrian refugees, which Russia is trying to leverage into diplomatic normalization and reconstruction funds for Damascus. And Russia needs Turkey, which has made itself an integral part of Moscow’s political efforts to manage the conflict and retool Syria’s political process to better suit Russian ends. By supporting a full-scale Idlib offensive, Russia risks complicating all of that.

Russia has also helped broker the negotiated surrender of de-escalation areas in Syria’s center and south, facilitated by overwhelming Russian air power. Russia is attempting something similar in Idlib, but it seems likely to fail. Idlib’s fall cannot be neatly choreographed. As Damascus has retaken the rest of the country, it has bused irreconcilable militants and activists to Syria’s north, mainly to Idlib. People now in Idlib are there for a reason. Many cannot survive under Assad’s reconstituted rule and will instead fight to the death. Local rebels have made a show of detaining individuals suspected of being in contact with Russia or the Syrian regime. One town erected a gallows as a message to would-be “collaborators.”

A Syrian military victory in Idlib will necessarily mean death on a huge scale, and mass displacement toward either toward Turkey or to Turkish-held areas whose humanitarian capacities, already strained, will be totally overwhelmed. Idlib’s population is multiples larger than Aleppo’s rebel-held eastern neighborhoods or Damascus’s East Ghouta suburbs, which fell bloodily to government forces in 2016 and earlier this year, respectively. And this time, there is no other Idlib to which residents can flee.

Carnage in Idlib, at this late stage of Syria’s war, would make a farce of Russia’s claims that Syria is stabilizing and that the time has come for large-scale, organized refugee return and reconstruction. It would deeply injure Turkey, whose top officials have warned that an attack on Idlib will collapse the political scheme Turkey and Russia have co-sponsored. That multilateral arrangement has been the basis for Russia’s multiplying, parallel political initiatives, aimed at re-centering Syria’s political process from political transition—which would mean remaking or removing Syria’s current leadership—to issues such as constitutional reform that are less threatening to the status quo. Many in Syria’s opposition who have participated in these processes, with Turkey’s encouragement, would have little reason to go on. They have no obvious future in an Assad-ruled Syria themselves, but they can at least engage on constitutional reform in order to buy time and space for Idlib’s residents. But if Idlib ends in violence, why would they pursue this path? Russia will have trouble organizing a political process with even the minimum level of inclusiveness and legitimacy that prospective Western donors demand.

There is little prospect that the U.S. will intervene militarily for Idlib’s sake and risk a confrontation with Russia. But Turkey and Europe nonetheless possess real leverage. In a briefing for International Crisis Group, my colleagues and I have argued that Turkey and European donors need to make clear to Russia that a Syrian military victory in Idlib will have a substantial political cost, in addition to the toll in Syrian lives. They need to convince Russia that its own aims in Syria are better served by restraint and a return to negotiations with Turkey and Iran on a compromise solution, not by strong-arming Turkey and killing a large proportion of Idlib’s population.

Idlib’s jihadists and the persistent violations of the de-escalation ceasefire are genuine problems that require a solution. But that solution can still be one that is consensual and agreed between Turkey, Russia, and Iran, and one that spares innocent human life.

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