Since 2011, civil wars and state failure have beset the Arab world, underlying the perceived misalignment between national borders and identity in the region. This book is about the separatist movements that aim to remake those borders—the Southern Movement in Yemen, the federalists in eastern Libya, Kurdish nationalists in Syria and Iraq, and the Islamic State (IS). These movements took advantage of state breakdown to seize territory and set up states-within-states. They ran schools, hospitals, and court systems. Their militias provided security to those whom the state had failed. Separatists drew inspiration from the ideals of self-determination that emerged after World War I during the brief “Wilsonian moment.” They built off the historical legacies of prior state-building projects that had failed to gain recognition. New international norms, such as responsibility to protect, offered them hope to correct mistakes of the past. Separatists reached out to the international community for acknowledgement and support. Some served as crucial allies in the campaign against terrorism. Yet the United States and the rest of the international community refused to grant them the recognition they sought. This book shows how understanding the separatist movements’ efforts to break borders in their own terms can help illuminate avenues toward a more stable regional order in the Arab world.