Although particularly complex, the Syrian civil war is an example of a general trend: civil wars are lasting longer and are increasingly likely to end with a one-sided victory rather than a negotiated settlement. 

Civil wars tend to end the way that external forces think they ought to end. In other words, norms—ideas of appropriate behavior—play a direct role in civil war outcomes.

Historically, most civil wars have ended with the military defeat of the losing side. In the 1990s, by contrast, civil wars usually ended with a negotiated settlement. What accounts for this anomaly?