This article explores the phenomenon of Islamist foreign fighters, more specifically the movement of European Muslims to participate in the insurgencies in Syria and Iraq connected to the Islamic State/Daesh as well as the anti-Assad forces in Syria and the implications for European state stability. Drawing on personal psychology, social psychology, and social movement theory the article offers an integrated theoretical framework to analyze the radicalization of Islamist foreign fighters. Building on Danish data of Islamist foreign fighters, the article provides a first test of the analytical usefulness of this framework. The article further considers what distinguishes the Islamists that go from those that under similar circumstances stay behind, and whether this is a differences of kind or a difference of degree. Finally, we discuss the question of how much of a threat foreign fighter returnees pose to European states.