The international right to self-defense was subject to legal and political controversies long before September 11, 2001. On several occasions, states resorted to defensive military force against different internationally operative armed groups. This work seeks to show the precise conditions for self-defense against non-state actors as indicated by state practice since 1945. Based upon a detailed evaluation of almost 20 relevant conflicts, it is argued that state practice does not warrant limiting Article 51 of the U.N. Charter to armed attacks by states and ignoring non-state attackers. In fact, states almost never contended that already the formal legal nature of a non-state attacker excludes the possibility of military self-defense. The current view of the International Court of Justice is insofar not consonant with state practice.