Defence and security sectors in the Arab states have been undergoing significant, and sometimes radical, transformation as a result of local rebellions and civil wars, state crisis and fracturing, and external intervention since the start of the 21st century. In Lebanon, Syria, Iraq and Yemen, unfamiliar and fluid coalitions of national armed forces (or their remnants) and armed non-state actors are increasingly engaged in complex patterns of de-confliction, coexistence, and cooperation embedded within a wider context of persistent competition among them and of geopolitical rivalry between an array of external backers. A joint effort of the Italian Institute for International Political Studies (ISPI) and the Program on Civil-Military Relations in Arab States (CMRAS) of the Carnegie Middle East Center, this dossier explores how the resulting hybridization of security governance in these countries leaves them with forms of sovereignty that are both constrained and constantly contested.