The world is watching as Russia and the Syrian regime prepare to unleash the biggest bloodbath yet in the Syrian saga of horrors. Yet no one seems to know how to prevent a massive attack on Idlib, the last Syrian rebel-held stronghold, where nearly a million civilians are trapped.

U.N. Secretary-General Antonio Guterres just warned that a full-scale Russian assault on Idlib “would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict.” Moscow rebuffed the pleas of Guterres’ top emissary who called for humanitarian corridors to allow the temporary evacuation of civilians.

Nikki Haley, the U.S. ambassador to the United Nations, warned Syrian dictator Bashar Assad along with his Russian and Iranian allies that “the consequences will be dire. The world will hold them responsible.”

But neither Assad nor Vladimir Putin believe the world (or Washington) will lift a finger. Why should they, given the unchecked carnage they’ve unleashed on Syrian civilians over the past seven years, including the unpunished use of chlorine gas against civilians?

If Assad and company are permitted to commit atrocities in plain sight in Idlib, it will reconfirm that autocrats and dictators can get away with mass murder in plain site in today’s world.

There is plenty of blame to go around for the lead up to Idlib. Let us start with President Obama and the Saudis. Eager to avoid another U.S. military involvement like Iraq, Obama subcontracted to Saudi Arabia to help Syrian rebels eager to oust their dictator. The Saudis and Gulf States preferred to funnel funds to Islamists rather than more secular rebels, who might have provided a potent force had they received aid in 2012.

Then come the major culprits, Assad, Putin and Iran’s ayatollahs. In 2015, when Assad was reeling, Moscow and Tehran came to his aid, the former with planes and the latter with tens of thousands of Shiite militia fighters. Their primary targets were not the Islamists, but rather the more moderate rebels who might have provided a serious alternative to Assad. With Russian aid, Assad dropped thousands of barrel bombs on rebel-held cities, deliberately targeting schools and hospitals, and using starvation as a weapon. Rebels who surrendered were often tortured and murdered.

And let us not forget Turkey. Idlib sits close to the Turkish border. With Turkish help, and some Turkish troops, the region has become the last stand for rebels and many civilians who fled other conquered cities. The rebels include tens of thousands of relative “moderates” but also around 10,000 jihadis with Al-Qaeda links. Turkey has failed to take on hardline Islamists, who are now preventing civilians from fleeing.

Last but not least we have the Trump administration. Placing vain hopes in Putin to resolve the Syrian mess, the president produced no Syria strategy until now. The State Department has finally put a good team in place, but it may be too late.

Noted Syria expert Josh Landis is correct to point out that many share the blame for the desperate crisis in Idlib. “The United States, the Gulf states, and Europe,” says Landis, “encouraged a lot of Syrian (rebels) to become involved and now they don’t want to take responsibility. Everyone is trying to shed crocodile tears but we’re all complicit.”

All true. But the urgent question is what can be done to prevent the slaughter of civilians in Idlib.

First, it’s necessary to be crystal clear about the nature of the problem. Russian officials delight in proclaiming that they, and Assad, only want to fight terrorists. History proves that claim untrue; Assad simply wants to consolidate power. Moreover, fighting terrorists does not justify the deliberate targeting of civilians, which is the M.O. of Assad and his allies. Remember Aleppo.

A couple of examples: Only recently, the United Nations passed on the coordinates of 235 schools and hospitals in Idlib to Russia in hopes to spare them from bombing, but already four of the hospitals have been targeted. Moreover, the Russians — who still deny that Assad ever used chemical weapons — are proclaiming a plot by the West to organize fake chemical attacks in Idlib. What better cover story for Assad terrify Idlib civilians into believing he will use gas on them, even as he plans to bomb more hospitals and schools.

Which brings us to the question of how to prevent such civilian carnage. I see little chance to avoid a regime attack on Idlib. So the best way to save civilians is to get as many of them as possible out of the region. The pressure must continue on Russia to permit humanitarian corridors. Over and over, Washington must publicized the message that the blame for new Idlib bloodshed would lay with Putin and Assad.

Turkey must be encouraged to let the refugees shelter in Syrian territory it controls near the Turkish border. But that situation is only temporary. Eventually many of those refugees must leave Syria or face Assad’s torture. Yet the Trump administration admits almost no refugees from Syria.

“If it’s a slaughter,” President Trump said of Idlib, “the United States is going to get very angry.” If he truly cares about the fate of Syrian civilians, he should let thousands of them find safety here.

Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.