By Martin Chulov Middle East correspondent, The Guardian–

Why does Idlib matter?

Pro-government forces are massing in Syria’s north-west for what could be a large-scale offensive on Idlib province, the last opposition stronghold in the country. The UN has warned of the “worst humanitarian catastrophe” of this century if the attack goes ahead, since there are no remaining opposition areas to which those fleeing Idlib could be evacuated.

The province is central to the outcome of the war and also the fate of the broader region. As the seven-year conflict draws to a close, each participant in the war, Syrian and non-Syrian, is trying to secure their interests, many of which run through Idlib. But few of those interests align. Most are mutually exclusive.

External powers trying to carve out influence in northern Syria are Russia and Iran, to whom Bashar al-Assad, the Syrian president, owes his successes so far, and Turkey, in whom rebel groups continue to pin their hopes. There is also what remains of a fractured anti-Assad opposition, jihadist groups and a regime desperate to claim it has won the war.

Above all there at least three million people cornered in north-west Syria with nowhere to go. Many live in frugal shelters in crammed towns and villages, and some have already fled to the fields in fear of the same sort of intensive Russian-led bombardment seen elsewhere in Syria.

NGOs are predicting that as many as 700,000 people could leave their homes or shelters if a major attack is launched. The vast majority are likely to be unwilling to cross back into regime-controlled territory, fearing the retribution of a government from which they have fled.

Who is living there?

Of Idlib’s three million citizens, at least 1.5 million are believed to be exiles from elsewhere in Syria, particularly areas taken in conflict by Russian, Iranian and Syrian government forces, such as Ghouta and Darraya near Damascus, Homs, east Aleppo and Deraa. After each battlefield victory, communities who have not come to terms with Syrian officials have been given safe passage to the province.

As the numbers of people have mounted, fears have risen that Idlib is being set up as a killbox – a place where jihadists are mixed in with civilians to reaffirm a Syrian government narrative that the 2011 anti-Assad uprising was all about jihad in the first place. Assad’s officials have branded all those who stood against him as terrorists, and years of dehumanising language has clouded the reality of who is living in Idlib and to whom they are aligned.

The vast majority of the province’s population are Syrian citizens, mostly Arabs but with significant numbers of Kurds. Civilians greatly outweigh fighters, by at least 1001 to one, according to UN estimates.

Among those who have taken up arms in the province are the latest incarnation of original anti-Assad groups, reinforced by militants who have arrived from elsewhere in Syria. These groups are backed by Turkey and are in a contest for influence with the conservative Islamist group Tahrir al-Sham, which remains the strongest organisation in the area.

Roughly half of the 10,000 fighters in the Islamists’ ranks are foreigners, with the al-Qaida-aligned Hurras al-Din part of the group. The dissolution of both is a key demand of Russia, which will call the shots in any offensive. Turkey has said it needs more time to force the militants to leave. Where they would go remains unclear and Ankara’s demands for a truce, made during a recent trilateral summit in Tehran, were rebuffed by Moscow.

Who is fighting whom?

Tahrir al Sham remains the strongest group on the anti-Assad side, and the Turkish-backed Northern Front is also prominent. Both groups are well armed but are no match for the Russian air force or ageing, though effective, Syrian cannons and artillery pieces.

Idlib is likely to be the last stand for those who first fought against Assad and others who have since joined the battle. The original uprising, beginning in the 2011 Arab Spring, was fed partly by a class struggle that gave rise to a protest movement, joined by nationalists, conservatives, Islamists, jihadists – who used Syria as a platform for broader ambitions – and opportunists in search of power at any cost.

On the pro-Assad side are what remains of the Syrian army, among them nationalists committed to saving the country and others loyal to the Assad regime. There are also militias raised as a home guard who played increasingly prominent roles when state structures were tested as the conflict went on.

Key are Russian forces, led by the air force, which saved Assad from defeat in September 2015 and have asserted control over the war ever since. While the Russians have controlled the skies, Iranian-backed militias have dominated the ground war. They were the first into east Aleppo and have been active on most battlefields. Lebanon’s Hezbollah, the most important Iranian proxy, has lost more than 1,700 fighters in Syria. Afghani, Iraqi and Yemeni fighters, as well as Iranians, have also died in large numbers.

Whether Iranian-backed forces join the fight for Idlib will be key to whether the offensive is launched and how long it lasts. The Syrian opposition political bloc believes Iran is reluctant to commit the numbers it deployed earlier in the war. The Syrian army, meanwhile, is struggling to muster a large-scale fighting force that could support its heavy weapons and helicopters.

What will happen if an offensive starts?

The UN believes that as many as 700,000 newly displaced people would head towards the Turkish border or areas east of Idlib within 48 hours. While supplies of food and water into Idlib remain stable for now, an exodus of that scale would severely test an aid response. Turkey has said it will not let large numbers of new refugees across its now fortified border. That leaves a swathe of northern Syria it controls, known as the Euphrates Shield zone. This area is likely to be a destination for rebels looking for the last possible safe haven on Syrian soil.

Ankara plans to use the Euphrates Shield area as leverage in Syria in the post-Idlib phase, but it would come under intense pressure to allow it to be used as a final refuge if and when an attack is launched. Meanwhile, it has said it will not allow Idlib province to be turned into a “lake of blood”.

Owning a humanitarian catastrophe of a scale that dwarfs anything else in Syria would be a big price for Russia to pay. Syria, too, would be likely to viewed as culpable in an offensive that leads to a larger-scale loss of life than in earlier attacks. While international opinion has mostly mattered little to protagonists on both sides of the war, the scale of the potential bloodshed is galvanising outrage.

It may not be enough to stop the war but it is likely to mean that fears of an imminent full-scale blitz could be misplaced. Instead, a slowly increasing rate of attacks that leads people to steadily leave rather than flee in panic could well a plan of action.