(CNN) The Syrian regime has clawed back control of much of the country over the past few years: the city of Homs, the great prize of Aleppo, the countryside around Damascus and most recently the birthplace of the uprising, Daraa .

The regime, supported by its Russian and Iranian allies, is now preparing to retake the province, an area crammed with as many as 70,000 rebel fighters and nearly 3 million civilians, double its pre-war population.

Unless some deal is done to forestall such an assault, Idlib will be the "perfect storm," as the United Nations envoy to Syria Staffan de Mistura put it on Thursday.

Half the civilians trapped in Idlib are displaced from other parts of Syria. It's the world's largest open prison, one whose infrastructure -- hospitals, schools, utilities -- has been targeted for years by regime and Russian attacks , according to multiple accounts by human rights groups.

If it occurs, the battle for Idlib would be the last and bloodiest act of a horrific conflict that has reduced much of Syria to ruins and killed an estimated 400,000 people in the process

Russia talking tough...

The Kremlin is talking tough, about both eliminating the "terrorists" in Idlib and "provocations" being planned by the West . Foreign Minister Sergey Lavrov described Idlib as "the last major hotbed of terrorists" -- who were holding civilians hostage and stifling groups that wanted to negotiate.

"So from all points of view, this hotbed must be eliminated," he said Wednesday -- also describing Idlib as a "festering abscess."

Russian officials have also claimed there's a plan by rebels, with western connivance, to stage a chemical weapons attack in Idlib and then blame it on the regime. The Syrian Foreign Minister went further, saying that civil defense volunteers known as the White Helmets had kidnapped 44 children to stage such an attack. (An opposition activist in Idlib told CNN Thursday that the rebels had neither storage facilities nor the expertise to use chemical weapons.)

Lavrov claimed that "the Americans are trying to ratchet up the tensions around Idlib" and that Russia had "clearly and firmly warned our Western partners: don't play with fire." Not coincidentally the Russian defense ministry has announced a large naval exercise off the Syrian coast to begin Saturday. Anatoly Antonov, Russia's ambassador to the United States, said Moscow was concerned by signs that the United States was preparing new strikes on Syria.

...but also talking Turkey

There's no doubt that Idlib is dominated by radical Islamist factions such as the former al Qaeda affiliate Jabhat al Nusra (reinvented as Hayat Tahrir al Sham.) They are well organized and ideologically committed, defending territory they know well. De Mistura said Thursday there are about 10,000 fighters loyal to al Qaeda or al Nusra in Idlib (perhaps inadvertently providing the regime and Russia with the pretext to attack.)

An offensive to take most or all of Idlib would be costly, even more so than the immensely destructive battle for Aleppo. It would also damage or destroy Russia's carefully nurtured relationship with Turkey , which has troops in Idlib as part of an agreement to make the region a "deconfliction zone."

The Moscow/Ankara relationship is critical for the future of Idlib and indeed all of northern Syria. There have been almost daily contacts between the two governments this month. Turkish Foreign Minister Mevlut Cavusoglu said after an August 14 meeting with Lavrov that Turkey would not permit "a massacre" of civilians at the price of efforts to eradicate terrorist groups, according to the state-run news agency Anadolu.

President Recep Tayyip Erdogan says Turkey is working with Russia and Iran "to prevent an Aleppo-like disaster" in Idlib. As the Turkish military reinforces its dozen or so outposts in Idlib, Erdogan noted pointedly: "We are a nation that wins on the field not at the table."

To prevent disaster, Turkey is trying to persuade radical Islamist factions in Idlib such as Hayat Tahrir al-Sham, to announce they are dissolving, so as to make the terrorist problem "disappear." So far there are no takers, although one activist noted that militant groups were being offered a stark choice: "merge or die."

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Alexey Khlebnikov, a Middle East expert at the Russian International Affairs Council, told CNN it was "highly unlikely that there's going to be direct confrontation between the Syrian and Turkish military. If that happens it will push Ankara back into US hands, which goes contrary to Moscow's interests."

Relations between Moscow and Ankara have been painstakingly restored since a crisis in 2015 sparked by Turkey's shooting down of a Russian jet over its border with Syria.

'Beyond Conflict'

Somehow Russia has to keep both Iran and Turkey on board if it wants to get "beyond conflict" in Syria.

Critical to this is the return of refugees, of whom some 3 million still live in Turkey

An assault on Idlib would likely drive hundreds of thousands more civilians towards and over the Turkish border. UN officials envisage as many as 800,000 people trying to escape Idlib.

"Russia recently announced its main priority is return of the refugees -- not the opposite," says Khlebnikov. "Moscow is looking for a delicate solution which will take into account the interests and concerns of all parties."

So Russia may consider the establishment of humanitarian "corridors" as suggested by UN envoy de Mistura, in an effort to separate fighters from civilians. Whether many people from Idlib would want to enter regime-held territory is another matter. And ultimately where would these people go? 1.4 million of them have already been displaced once. "There is no other Idlib. Where can they go? Where can anyone go?" asks de Mistura.

War of attrition

For all these reasons, Moscow may prefer a more limited operation to eat away at the fringes of Idlib, starting in the south where the situation is not complicated by the presence of Turkish troops.

For sure, the Syrian regime has promised a bolder approach , with Foreign Minister Walid al-Moualem insisting after meeting Lavrov that the army would "go all the way" in Idlib. But it would need intensive Russian air support to succeed.

An opposition activist in Idlib told CNN that the regime was massing troops to the south of Idlib (in northern Hama province), to reinforce its existing military positions rather than initiate an imminent offensive.

US declines to engage

Where is the West now, as the denouement of the Syrian conflict beckons? With few cards to play, somewhere close to resignation.

The US State Department says the Trump administration has expressed its concern about an Idlib offensive, though it shares the goal of eliminating groups like al Nusra

Spokeswoman Heather Nauert said this week: "We have shared the concerns that we have about any potential offensive taking place, we have shared those concerns with the Russian government at many levels," including conversations between Secretary of State Mike Pompeo and Russian Foreign Minister Sergey Lavrov.

There is no real US engagement with Russia on Syrian reconstruction while Assad is in power; nor is there much engagement with the remnants of the moderate Syrian opposition. The US has sent mixed messages about its military support of the Kurdish-led Syrian Democratic Forces in the last enclave beyond regime control.