Throughout most of our species' long and bloody slog, warfare has primarily been carried out by bands of loosely organized, ill-disciplined, and lightly armed volunteers who disdained open battle in favor of stealthy raids and ambushes: the strategies of both tribal warriors and modern guerrillas and terrorists. In fact, conventional warfare is the relatively recent invention. It was first made possible after 10,000 BC by the development of agricultural societies, which produced enough surplus wealth and population to allow for the creation of specially designed fortifications and weapons (and the professionals to operate them).Nonetheless, since at least the days of the Greeks and the Romans, observers have belittled irregular warfare. Western soldiers and scholars have tended to view it as unmanly, even barbaric. It's not hard to see why: guerillas, in the words of the British historian John Keegan, are "cruel to the weak and cowardly in the face of the brave" -- precisely the opposite of what professional soldiers are taught to be.The long history of low-intensity conflict reveals not only how ubiquitous guerrilla warfare has been but also how often its importance has been ignored, thus setting the stage for future humiliations at the hands of determined irregulars. The U.S. Army has a particularly dismaying record of failing to adapt to "small wars," despite its considerable experience fighting Native Americans, Philippine insurrectos, the Vietcong, al Qaeda, the Taliban, and numerous other irregulars. To avoid similar calamities in the future, today's soldiers and policymakers need to accurately appraise the strengths and weaknesses of insurgents.