The New Yorker’s Steve Coll reviews some of political-science literature on civil wars for clues about how to defeat ISIS, and how long it will take:
Rereading these works in light of the infuriating problem of the Islamic State, two discouraging findings stand out. In 1945, many civil wars were concluded after about two years. By 1999, they lasted, on average, about sixteen years. And conflicts in which a guerrilla group could finance itself—by selling contraband drug crops, or by smuggling oil—might go on for thirty or forty years. The Revolutionary Armed Forces of Colombia, or FARC, has been around since 1964, sustained in no small part by American cocaine consumption.
Coll also highlights a point that is, in my view, underappreciated.