Multiple Syrias

Suriya al Kubra!

What's better than one Syria?

Easy! Multiple Syrias!

In one sense, a partitioned Syria is already visible, its contours drawn by the front lines of the civil war. President Bashar al-Assad has retreated from territory that was too difficult for his overextended forces to hold, giving up the attempt to reimpose nationwide control. (That doesn't mean he's on the run. Iran and Russia have made it clear they won't let that happen.)

Kurds hold the area near the Turkish border, having driven out Islamic State.

The competing factions in areas held by Sunni Arab rebels make for a more complicated picture, but a map of how the front lines looked this summer shows the outlines of a potential partition of Syria into three parts. The red designates regime control. The yellow is Kurdish. The green and black are Sunni Arab, including the area now controlled by Islamic State. (The white is sparsely populated desert.)

The competing factions in areas held by Sunni Arab rebels make for a more complicated picture, but a map of how the front lines looked this summer shows the outlines of a potential partition of Syria into three parts. The red designates regime control. The yellow is Kurdish. The green and black are Sunni Arab, including the area now controlled by Islamic State. (The white is sparsely populated desert.)

For example, the rebels are entrenched in the suburbs of the capital, Damascus, yet the regime would insist on holding the city. Similarly, Assad would want to hold onto Aleppo, Syria's largest and (before the war) wealthiest city; it's now mostly under rebel control and cut off from the regime's heartland. Either there would have to be a trade, or neutral zones established and secured by a heavily armed international peacekeeping force of the kind successfully deployed in eastern Croatia at the end of the Yugoslav war.  Similarly, rebels hold some pockets surrounded by regime-controlled territory along Syria's border with Lebanon. The regime holds the seacoast north of Lebanon, part of which is mainly Sunni, a fact that would be unacceptable to rebels who would want their entity to have access to the sea.   

The war is slowly resolving these issues as each side focuses its military resources on what it wants most, but it could take years.  An agreement that assures each of the outside powers that their clients would retain control in their designated territories could also go a long way to allowing Iran, Saudi Arabia and others to compromise, because the war would no longer be zero-sum.
It's impossible to say exactly what each country would demand, but here are some probable minimums.

Iran would want a friendly regime dominated by Alawites (a Muslim religious minority with connections to Shiite Islam) to control Damascus and a secure corridor from the capital's airport to Lebanon. Russia would want to know that its naval base at Tartus was secure, and that Syria as a whole would become neither a Sunni Islamist state nor a U.S. protectorate. Saudi Arabia would have to see Hezbollah leave Syria and Iran's influence squeezed. Israel would need to be sure the Syrian side of the Golan Heights wouldn't become a new playground for Hezbollah. Turkey would want a Sunni entity to control Aleppo and the north, and Kurdish autonomy limited.