With the US pulling out of Syria, Russia holds the cards to determine how far Turkey can go with a military option. Diplomatic and military factors may be causing Turkey to think twice.
Top Turkish and Russian officials met in Moscow on Saturday to coordinate Syria policy as the US prepares to withdraw from the country.
Russian Foreign Minister Sergey Lavrov said after the talks that they had focused on "new circumstances" that had arisen as a result of the impending US exit from Syria.
"An understanding was reached of how military representatives of Russia and Turkey will continue to coordinate their steps on the ground under new conditions with a view to finally rooting out terrorist threats in Syria," Lavrov said, without providing details.
Turkish Foreign Minister Mevlut Cavusoglu said that "we will continue close cooperation with Russia and Iran on Syria and regional issues," a reference to the three countries being the guarantors of the so-called Astana Process to end the Syrian war.
The talks also addressed the de-escalation zone in Idlib province.
Turkish Defense Minister Hulusi Akar, intelligence chief Hakan Fidan and presidential adviser Ibrahim Kalin were also in attendance, as were Russian Defense Minister Sergey Shoygu, chief of general staff Valery Gerasimov, presidential envoy for Syria Alexander Lavrentiev, and presidential aide Yuri Ushakov.
Syrian Kurds play with Assad regime
Saturday's meeting was one of a flurry planned in the coming weeks between world and regional powers as they scramble to secure their interests in advance of the US withdrawal of some 2,000 troops from northeast Syria.
The sudden planned US withdrawal has jolted the Syrian Kurds, who Washington backed against the "Islamic State" and who now face a possible Turkish offensive.
Ankara considers the Kurdish YPG militia, the main force of the US-backed Syrian Democratic Forces (SDF), the same as the Kurdistan Workers' Party (PKK) fighting a more than three-decade insurgency against the Turkish state.
The Turkish military and its Syrian rebel allies have reinforced troops and equipment around Manbij, a strategic town where US and French Special Forces are based alongside SDF forces. There have also been Turkish troop reinforcements along the border.
On Friday, the YPG and the Syrian regime announced that government forces had entered Manbij in an apparent bid to ward off a potential Turkish attack when US troops withdraw. Such a move in Manbij may partially appease Turkish concerns around Manbij and foreshadow a larger deal between the Kurds and the Assad regime.
Both Iran and Russia, the main backers of Syrian President Bashar Assad, welcomed the announcement as it would work toward their goal of the regime reasserting sovereignty over all of Syria.
US officials and reports from the ground indicated that regime forces had not yet entered Manbij, but appeared to be on the periphery.
Turkey in a quandary
The sudden US withdrawal announcement has caught Turkey off-guard after it delivered multiple threats in recent months that it would attack the Syrian Kurds, an event that would have brought it up against its US ally.
Even with the US withdrawal, a Turkish military offensive against the Syrian Kurds would be risky and difficult against tens of thousands of battle-hardened Kurdish fighters and their Arab allies in hostile territory. It is also questionable that the Turkish military could also fight the "Islamic State" in its last Syrian holdout around Hajin some 200 kilometers (125 miles) from the border.
The Turkish military would want to avoid directly engaging the Assad regime, which seeks to prevent Turkey from carving off more territory in Syria after taking Afrin and a section of northern Aleppo province. An attack on the Assad regime would be an act of war against Iran and Russia's ally.
France has said several hundred of its troops would remain to back the SDF against the "Islamic State." Even with a US withdrawal from Syria, a French presence may force Turkey to rethink a military offensive.
A Turkish invasion into Syria would also be militarily and diplomatically risky unless it had a green light from the United States and Russia, not least because it would need the US and Russia to open Syrian airspace to Turkish jets.
Turkish President Recep Tayyip Erdogan has hinted that he would accept the Assad regime returning to northeast Syria to replace the Syrian Kurds. However, such a scenario also poses a challenge for Turkey.
The YPG follows the leftist ideology of imprisoned PKK leader Abdullah Ocalan. The PKK says the YPG is operationally distinct and the two only share Ocalan's ideology.
Assad and the Syrian Kurds
Syria was the PKK's main external backer in the 1980s and 90s. In 1998, Turkey threatened a military invasion of Syria, which led to the expulsion of PKK leader Abdullah Ocalan from Syria and his eventual capture. Up until the Syrian civil war started in 2011, around one-third of PKK fighters were Syrian Kurds. The degree to which Syrian intelligence cut ties with the PKK or used it to control Kurdish nationalism in Syria has always been in question.
The Syrian Kurds under the YPG and its political wing, the PYD, took control of parts of northeast Syria in mid-2012 after the Assad regime tactically withdrew from the area. With US support since 2015, the YPG under the banner of the SDF pushed back IS, taking almost a third of Syrian territory including vital water resources, hydroelectric dams, oil fields and Syria's grain belt.
Rather than fighting, the regime and the Kurds have had a tacit agreement throughout the war, more concerned about Turkey and jihadi groups than each other.
The Assad regime has maintained some intelligence and other government capacity in the self-administered Kurdish areas, including the airport in the region's biggest city, Qamishli.
Representatives from the Syrian Kurdish administration and the Assad regime have been officially negotiating since this summer, but have failed reach agreement over Kurdish demands for a degree of autonomy. The talks have been encouraged by Russia, which has had open ties with the secular Syrian Kurds and would prefer a political deal especially now that its US rival in Syria is leaving.