Barbarians are back. These small, highly mobile, and stateless groups are no longer confined to the pages of history; they are a contemporary reality in groups such as the Taliban, Al-Qaeda, and ISIL. Return of the Barbarians re-examines the threat of violent non-state actors throughout history, revealing key lessons that are applicable today. From the Roman empire and its barbarian challenge on the Danube and Rhine, Russia and the steppes to the nineteenth-century Comanches, Jakub J. Grygiel shows how these groups have presented peculiar, long-term problems that could rarely be solved with a finite war or clearly demarcated diplomacy. To succeed and survive, states were often forced to alter their own internal structure, giving greater power and responsibility to the communities most directly affected by the barbarian menace. Understanding the barbarian challenge, and strategies employed to confront it, offers new insights into the contemporary security threats facing the Western world.