Reportedly Iran prevailed on the others to realize that for now they can't fight a 'two-front war' and have to concentrate on beating the rebels first
Since it begun in August the Turkish invasion of northern Syria has wrestled control of nearly 2,000 square kilometers of sovereign Syrian territory originally held by ISIS.
There was a lot of speculation whether Erdogan originally crossed the border with Russian assurances that they would not stand in his way or not, but virtually every analyst I have read agrees that if he got a Russian greenlight in August it was at most to take a shallow border area, not to move deep into Syria and lay claim on al-Bab, Manbij and even Raqqa as he has.
The Turkish military and the Islamist rebels in their tow are currently encircling al-Bab in northern Syria from three sides and showing every intention of taking the city if they can. If they are successful al-Bab would become Syria's first major population center in Turkish hands.
This has had many Syria watchers wondering if Syrians and their Russian and Iranian backers will move militarily to block the Turks, but if one veteran reporter is right the answer for now is no.
Elijah Magnier, the chief international correspondent for the Kuwait-based al-Rai paper and a respected figure among Middle East observers reports that initially Syrians and Russians wanted to do just that, but that ultimately Iranians prevailed on them and got the point across that right now the pro-government camp is not able to fight a two-front war against rebels in Idlib and Turks and/or ISIS in Aleppo province at the same time:
From a well informed source in Damascus, Russia supported the Syrian President Bashar al-Assad who wanted at all costs to stop his most hated opponent, the Turkish President Recep Tayyib Erdogan from accessing al-Bab city and continuing towards Deyr Hafer, Assad lake and al-Tabqah to meet the US forces and their allies at the gates of Raqqah, the heart of ISIS. Assad was happy to close the road to Erdogan’s plan to annex al-Bab, especially after his bombing of Turkish forces last November which killed four Turkish men – despite Damascus’s denial of its involvement in the attack. The Russian-Syrian message to Erdogan was clear: you are not allowed into al-Bab.
But what has changed since?
Iran – according to the source – contested the Russian-Syrian plans by offering an approach different from the Russian-Syrian proposal. There is no interest in sending forces towards the northeast where the US is well established with Special Forces and military bases in the Kurdish controlled area. Moreover, there is no strategic interest in fighting ISIS at the moment because Raqqah is not supplied by outside support. It doesn’t represent a strategic priority”.
Iran expressed its willingness to dispatch more new troops to Syria if necessary for the battle of Tel El-Eis, to free the encircled cities of Fua and Kfariya, and Jisr el-Shughur and reach the most important target: the city of Idlib.
A constructive discussion took place between the allies leading to the conclusion that “useful Syria” is formed of the main cities where the majority of civilians live, even if the rural areas are not under government ‘control.
“The approval of Assad to accept the state of Raqqah as it is, for now, and preferentially direct forces toward Idlib – even if he will publicly deny any deal and attack Turkey – is due to his knowledge of the Syrian dynamic and the principle the state is based upon: the Allawites control the security apparatus and the Sunni control the Syrian economy.
If true this is a sensible strategy indeed. We only need to look at the recent loss of Palmyra to ISIS appreciate that the Syrian army is stil woefully overstretched and in no position to advance to advance on two separate fronts.
The imminent victory in Aleppo city will free some troops but of the estimated 25,000 Syrian troops there the majority are locals who will be loathe to be moved to theaters far away from home. In all likelihood the conclusion of the three year Battle of Aleppo will merely free up the most elite units of the Syrian army and foreign volunteers.
Moreover if Syria is capable of offensive in one directed only it makes sense for it to go for Idlib and attempt to finish off the rebels who by the virtue of being the only remaining faction that seeks to topple the Syrian government wholesale rather than just chip away at parts of its territory are the most dangerous.
ISIS is internationally isolated and no threat long-term. The Kurdish-dominated SDF merely seeks autonomy and other concessions from Damascus. Erdogan would love to see Assad fall in principle but seeing how Turkey has ignored the fall of eastern Aleppo there is every reason to believe he has largely given up on that goal and is now content to frustrate Kurdish plans in Syria, and to elbow his way to the eventual negotiating table to win force limited concessions on behalf of the rebels in his tow.
All of this taken into account consolidation in the more densely populated "useful Syria" makes a lot more sense in the immediate than carving out exposed salients into eastern Syria which being sparsely populated is nowhere near as valuable and which may embroil the Syrian army in fighting the powerful Turkish military or the US-backed Kurds.
Instead it is a far better move for Damascus to wait and see what transpires between Turks and the Kurds. As they are arrayed against each other in Turkey itself and covet the same pieces of northern Syria the differences between them are basically irreconcilable. The Turkish attacks on the Syrian Kurdish YPG militia have already served to create some distance between Turkey and the US and to raise Kurdish skepticism of the US.
If the Turkish-YPG conflict intensifies this will only benefit Damascus which might then have the choice of whether to ally with the Turks to extinguish Kurdish hopes of autonomy in Syria (which would probably be the more dangerous and foolish option) or else ally the Kurdish-led SDF to force the withdrawal of foreign intruders and to extinguish the last remnants of the Islamist rebellion.