Insurgent movements around the world vary widely in their military efficacy, from little-known and short-lived organizations to the Taliban, which completed its takeover of Afghanistan in 2021. What accounts for this variation in insurgent military power and success on the battlefield?Rebels in the Field answers this question by developing the cadre theory: an insurgent organization's ability to conduct complex military activities, like guerrilla warfare, depends on its development of cadres that create strong links between the organization, the environment, and its members. More complex forms of military power, argues Alec Worsnop, require organizations to develop effective small-unit combat leaders: military cadres. Revising conventional wisdom, Worsnop finds that strong social and political ties do not result in meaningful combat power on their own--and can even detract from military potential. After building the cadre theory through the lens of the Taliban (2001-2021), the book tests the theory by evaluating 17 organizations in Vietnam (1940-1975) and Iraq (2003-2016), reshaping our understanding of civil wars and military organization.