The only surprise in this is that it did not take place sooner.
The Syrian Kurds fought bravely and won the fight against ISIS. They were the only American financed and controlled force to do so. They were expecting a reward. Instead, as confirmed in Manbij, the US made a gift of their territory to Erdogan, helping him to win the elections. Erdogan has implied that he will approach again the West once elected; given the choice, the US prefer to have better relations with Ankara than the Kurds.
The Syrian Kurds have also witnessed, recently, that the Russians allowed the Turkish military intervention in Afrin. Hence, getting closer with Russia, is not an option for them, for now.
Getting closer to Assad is their only short-term option.
The Russians have no reason to oppose this, as it detaches the Kurds from the very close relationship that they had up to now with the Americans.
Assad is welcoming them with open arms.
Having the Kurds inside a Syrian State, even a federal one, is better than a rump Western Syria State. The major Syrian Oil and Gas fields are located in the areas under Kurdish control. The Kurds can sell Hydrocarbons through Assad, much easier versus any other way. It is a win-win for both.
Military cooperation between them is also important.
To this point, Aldar Khalil, who is the co-chair of the Movement for a Democratic Society (Tev-Dem), the multi-ethnic governing coalition of Syrian Kurdistan, has stated that solutions must be forged among Syrians and not outside forces, which: “have their own plans and goals. Why can’t we look for a solution between ourselves? We told the regime delegation we are ready.” Assad, Muallem and Assad’s adviser Bouthaina Shaaban, all reacted positively.
Erdogan has indicated to the US that, if he was to win the elections, he was willing to do a turn to the West. The US delivered with the Pompeo-Cavusoglu agreement, allowing Manbij to Turkey.
A Syrian-Kurdish Union is going to contest the Turkish occupation of Syrian areas, particularly now that Assad, through the good offices of Putin, is being accepted again by Israel as a safer neighbor than the forces that opposed him.
Iran, with its economic and financial problems accumulating, is not going to fight along Turkey against the Syrian Kurds. In particular, it does not wish to pour oil in the fire of its own Kurdish problem; in addition, its alliance with Assad, although recently weakened, is of primary importance. Iran has no goodwill for Turkey. Theirs is a relation built on, temporary, present need.
If Erdogan keeps his promise and tilts to the West, a displeased Putin would encourage the Assad-Syrian Kurds alliance to violently reclaim Manbij and other Syrian areas from Turkey.
Presently, Iran is trying to reach a new deal with the US, if it can save face by doing so. Iran knows that when the sanctions start to bite, the position of the regime will not be stable anymore.
The power shifts are increasingly visible.
Iran is weakened but will remain an important power in the region.
The US have still considerable influence in the Syrian Kurdish army which depends a lot on them for financial and logistics support. If the US were to stop this to help Turkey, it would eliminate their influence and even presence in all Kurdish areas.
Saving Turkey is not a US priority, particularly since Turkey is not necessarily going to obey, the US directed, Iranian embargo. The US are obliged to use, stick and carrot to bring Turkey back to the West, yet Iran remains their primary target.
The Russians are in the Middle East region for good building both, on their long-standing relationship with the family of Syria’s dictator, as well as, on the significant increase in Status, established with all the Arab nations, Iran, Turkey and even Israel.
Turkey considers itself a rising regional power and acts increasingly independent of the preferences of the U.S., its NATO ally. Whether it plays its own hand in the regional power game or returns to the West, friction with Syria is guaranteed to continue.
The whole frontier strip amongst Turkey, Iraq and Syria, starting in Iran and going all the way to Lebanon is going to remain a grey, contested and unstable zone for a long time, dominated by the Kurdish issue.
The Kurds have a saying that states “The only friends of the Kurds are the mountains”.
The Syrian-Kurds live in the Plains.
And the Plains go all the way to Damascus.