PBS Frontline's Evan Wexler and Sarah Childress put together this map showing which of the four major groups in Syria control which territory, as part of Frontline's new hour-long program on fighting there.
These groups are four different colors for good reason: with the quasi-exception of the Kurds, who mostly just want Kurdish autonomy or independence, the other three groups all hate each other, all want to control Syria and all have very different visions for the country. And, as you can see, each controls a lot of territory.
This is a big part of what makes the conflict so persistent. Negotiating between the rebels and the government was hard enough before the rise of ISIS over the past year. A three-way stand-off is much tougher to resolve than a two-way stand-off: a potential peace deal that might satisfy two of those groups is probably going to infuriate the third. Getting representative from all of those factions to simply come together or recognize one another's negotiating authority is, in itself, a major hurdle. Finding something they could all agree on would be herculean.
There's another reason this map and its divisions show the intractability of the Syrian conflict: no one side is anywhere close to a military victory. In order for the mainline opposition to win the war, for example, it would have to simultaneously defeat the Syrian government and ISIS. Successfully taking on two major adversaries at once is probably not something that ISIS, the government or the mainline opposition is currently capable of doing.
Outright military victories are even tougher in three-way conflicts; as one side nears victory, the two others have every incentive to temporarily work together to avoid defeat.
There are two ways that wars end: either all sides negotiate peace, or one side simply vanquishes the other. That Syria is so deeply divided between these factions makes both of those outcomes very difficult and, as a result, not terribly likely.
In October, before the Syrian conflict had so obviously become a three-way war, I looked at the political science around contemporary civil wars and found that it generally predicts they last about a decade, sometimes much longer. The development of another front, against ISIS, is one of many worrying signs that Syria's conflict could be on the longer end.