From terrorist disputes to splinter offshoots, an inside look at how armed groups break apart. Terrorist, rebel, and insurgent groups are highly unstable. Amid fears of defeat and even death, intense disagreements have torn many organizations apart, from Syria to Iraq, Ireland to Spain. And while some of these divisions have preceded a group's decline and eventual defeat, others have launched some of the most notorious and deadly organizations in recent history. In Divided Not Conquered, Evan Perkoski analyzes how armed groups fracture and how breakaway splinter groups behave. Perkoski takes an unprecedented look inside these organizations to understand the specific disagreements that cause groups to break apart, like those over ideology, leadership, and strategy. Drawing on research from organizational studies to social psychology, and leveraging analogies from business firms to religious sects, Perkoski shows how these disputes uniquely shape the behavior and survivability of emerging splinters. When motivated by single, shared disagreements, splinters exhibit higher cohesion, clearer objectives, and greater survivability. When motivated by strategy, splinters attract hardline operatives who steer the group towards increasingly lethal tactics and strategies. Including case studies of republican militants in Northern Ireland, Basque militants in Spain, and the Islamic State in Syria and Iraq, Divided Not Conquered demystifies a complex yet common phenomenon with ramifications for counterinsurgency, counterterrorism, and our understanding of increasingly fragmented conflicts around the globe.