Given the fact that two-thirds of all intrastate wars since 1945 have included foreign interventions, what drives sovereign states to support non-state conflict parties? In order to understand causes and calculations of this particular type of third party intervention, this book connects some of the most important contemporary debates in international relations, ranging from security cooperation between states and non-state actors to the effects of intervention on both local conflict dynamics and interstate relations. Presenting a new theoretical framework and a multidimensional concept of support (endorsement, hosting, as well as financial and military assistance), this book establishes a systematic path between international as well as domestic incentives and specific types of sponsorship policies. In a subsequent comparative analysis, the author examines conditions and dynamics of Syria's cooperation with Fatah, the Kurdistan Workers' Party, and Hizballah from 1964 to 2006.