How has ISIS managed to maintain its military survival in the face of much stronger international, regional and local forces? Seeking to explain this phenomenon, Omar Ashour gives a historical overview of the birth and rise of ISIS and analyses a series of battles that have taken place between 2013 and 2017. Based on fieldwork in the region and on in-depth personal interviews with military and paramilitary commanders, militiamen and soldiers who fought against ISIS, he also draws on interviews with defectors, former insurgency leaders and affiliates, and former and active security, military and state officials. He concludes with a set of observations on when, how, and why ISIS have won and lost, including their capacity to mix conventional military tactics and strategies with guerrilla warfare and urban terrorism to pull off military upsets. He also assesses the resulting policy implications. Going against the tide of conventional wisdom regarding how such groups undertake armed revolt, this book will transform our understanding of insurgencies and counterinsurgencies.