The endgame of the war in Syria is likely to come down to the northwestern province of Idlib, on the Turkish border, where some 2.3 million people are now trapped. As Russian-Syrian forces now finish retaking the smaller southwestern province of Daraa, Idlib will be the last significant enclave in anti-government hands. If Russian-Syrian forces resume pummeling the city and surrounding area from the air, its civilians could face the horrible choice of bunkering in place or desperately trying to cross the Turkish border, which has been effectively closed since 2015.

Recently, however, there is some evidence that Russia might be willing to act more constructively. Russian officials have been seeking reconstruction aid for Syria from Western donors. According to sources close to United Nations-brokered negotiations among the parties to the Syrian conflict, Russia has floated the idea of stopping the military advance on Idlib, and perhaps handing over to Turkey a degree of control similar to that now exercised by Turkey over the neighboring region of Afrin, in return for a major Western commitment to help reconstruct Syria’s devastated cities and infrastructure. That may give the West new leverage to stop the atrocities taking place in Syria. The question is how to use it.

The Syrian war has been so extraordinarily ugly because Russian-Syrian air forces have been attacking civilians indiscriminately—and in some cases directly targeting them along with schools and hospitals. Syrian forces—with Russian backing—have also regularly used prohibited weapons such as cluster munitions, incendiary devices, and chemical weapons. There is compelling evidence that Russian forces themselves have used incendiary bombs.

The laws of war flatly prohibit these attacks, declaring them war crimes, but Presidents Bashar al-Assad of Syria and Vladimir Putin of Russia have, by their actions, ripped those laws up. This military conduct is a major reason why an estimated half a million people have been killed and more than 50 percent of Syria’s pre-war population has been displaced.

The Russian air force, in particular, has been an indispensable partner in creating this carnage, fighting alongside Syrian aircraft since 2015 and significantly bolstering the effectiveness of pro-government forces. It is a crucial reason why Assad, whose battlefield position had been tenuous, now looks likely to prevail.

Russia had an important part, for example, in the Syrian government’s aerial bombing campaigns, which led to the recapture, in 2016 and 2018 respectively, of Eastern Aleppo and Eastern Ghouta—two of the most populous enclaves once held by anti-government forces. Airstrikes killed hundreds of civilians in each area. Meanwhile, pro-government forces on the ground used crippling sieges to keep humanitarian supplies and aid workers from reaching civilians. The suffering and death toll were sufficient to force both enclaves to fall.

Russia’s official arms exporter, Rosoboronexport, is the biggest weapons supplier to the Syrian military. Russian diplomats give Assad overt political support, vetoing efforts to refer Syria to the International Criminal Court and trying to block, ultimately unsuccessfully, official investigations to identify which forces are using chemical weapons. Russian state-affiliated media such as RT and Sputnik have been at the forefront of whitewashing atrocities committed by the Russian-Syrian military alliance.

Until now, Idlib has provided a refuge for some Syrians. As anti-government enclaves fell, Syrian forces gave survivors the choice of the indignity of boarding the government’s notorious green buses to be dumped in Idlib or living in government-controlled areas where they would face the risk of reprisals—detention, torture, and execution—if they were suspected of being government opponents. For obvious reasons, many chose Idlib.

Roughly half of Idlib’s civilian population today is displaced from elsewhere in Syria. They are joined by a collection of anti-government militias that are themselves often abusive—committing summary executions, mistreating detainees, restricting humanitarian aid, and kidnapping for ransom. Now that Idlib is squeezed by pro-government troops on the ground and bombed from the air by the Russian-Syrian military alliance, there are few places left in Syria to flee.

In the past, civilians seeking to escape Russian-Syrian attacks might have crossed Idlib province’s border with Turkey, where some 3.5 million Syrian refugees now live. Since October 2015, however, Turkish security forces have routinely intercepted hundreds, and at times thousands, of asylum-seekers at the border and summarily deported them to Idlib. Fences line the border, and Turkish security forces have been firing at asylum-seekers trying to cross it irregularly, killing many and wounding others.

Whether Turkey will continue to keep its border closed to newcomers if thousands of Syrians are being slaughtered on the other side remains to be seen. But if Turkey were to experience a large new influx of asylum-seekers, few of whom would be eager to return to life under the Assad government, Ankara could face pressure from inside the country, where anti-refugee sentiment is growing, to suspend the deal it made with the European Union to curtail the flow of asylum-seekers across the Aegean Sea to Greece. Preventing a massacre in Idlib to begin with is a far better option.

Still, the Russian reconstruction proposal is controversial for several reasons, even if European governments could be persuaded to pay to rebuild cities that Russian and Syrian forces were largely responsible for destroying. There are significant concerns that, rather than allocating reconstruction aid on the basis of need, the Syrian government will prioritize areas where it perceives the residents remained loyal to it during the war. It has also divulged little about how reconstruction and recovery funds are being spent—a problem compounded by its insistence on restricting access to areas it has retaken for independent private humanitarian organizations. Syrian military and intelligence forces have already diverted large sums of humanitarian aid to line their own pockets and fund their operations, so there is every reason to fear that they would similarly divert reconstruction assistance.

What’s more, Russia has been silent about restrictions imposed by the Syrian government on the return of displaced residents to certain neighborhoods, even those that were retaken several years ago. Nor has Russia publicly opposed urban-planning schemes such as Law 10 of 2018, which allows the Syrian government to confiscate and redevelop residents’ property without due process or compensation. And Russia has done far too little to end Syria’s lawless and deadly detention practices—an enormous obstacle to return for the millions of Syrians who have fled the fighting.

In any event, the lives of Syrian civilians shouldn’t depend on payoffs and backroom deals. The alternative is to call out Russian complicity in Syria’s criminal military strategy and to vigorously press the Kremlin to end these atrocities.

Russia clearly has the necessary leverage over the Assad government to avoid a bloodbath in Idlib. Its aircraft could refuse to participate in joint offensives that indiscriminately bomb civilians and civilian infrastructure. Russia’s arms exporter could stop supplying weapons until the atrocities are halted. Its diplomats could stop shielding Syrian officials from international prosecution for their war crimes.

The key is getting Russia to use that leverage. Assad’s reputation is beyond repair—his main aspiration is to stay in power and avoid prosecution—but Putin still aspires to be treated as a respected global leader. He must be persuaded that he will fail in that quest so long as he continues to underwrite Assad’s atrocities.

This is not something that the US government under Donald Trump has shown any inclination to do, as illustrated recently by President Trump’s courting of Putin’s favor at the summit in Helsinki. The European Union is in a better position to act. If Russia wants better relations with the EU—any prospect of easing sanctions and improving its economic outlook—it should show a real willingness to end the bloodshed in Syria. It could start by protecting the 2.3 million Syrians in Idlib.