“No one ever expected the Kurds to battle Iran.”
The YPG has repeatedly been engaged in combat with Iranian commanded militias, as well as Russian Mercenaries and Syrian Arab Army units. They compete of the the control of land. US air and artillery support have kept large scale offensives at bay.
“Countering Iran’s influence in Syria is Russia”
Russia and Iran are allies in Syria. Russia provides air support to Iranian commanded militias on the ground, in a combined combat team. They have done so for years. Russia has been militarily supporting the expansion of Iran’s influence in Syria.
“Syria is quite likely “a bridge too far” for Iran.”
Well, they are there now. They have captured and held the border crossing town of al Bukamal, and have held it for months now. They have been able to convoy military materiel directly from Iran to Lebanon and the Israeli border - a long sought strategic goal. They are now massing missiles near the Israeli border, and building missile production facilities in Lebanon. War with Israel seems likely this year.
Those are trivialities. Of course the Kurds rebellion against Syria was squelched by Syria and it’s allies.
The Kurds are no longer rebelling but coming to terms.
Also there are millions of Kurd hostages in Iran. They’re treated pretty well, though are second-class citizens. That would change if hostilities broke out.
This is a common means of “alliance” in the Mid East.
Russia and Iran have competing aims in Syria. Basically both want to be “China’s guy” there.
Iran has to consolidate it’s expanded influence. It’s currently just a drain on them.