Curiously, there seems to be an alliance of sorts between the Russian Imperialists of Putin, Assad, Hezbollah and the Iranians.
That is hell on earth to the neocons and even the traditional folks at State. And there's no real dispute that the shiaa and sunni hate each other, and the shiaa getting the upper hand is not really good news for anyone.
The Russians had the upper hand in Afghanistan for ten years, killed 1,000,000 Afghans and then left in defeat. Without Russia, the Shi'ite alliance was losing, so the way to end the war is for the US to agree to support Russia's continued use of its naval base at Tartus in exchange for dropping its support of Assad and helping to stabilize Syria.