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The last decade has witnessed increased recourse by states to military force to respond to terrorist attacks on their soil that have originated from abroad. A number of states -- including the United States -- have justified these military actions as lawful self-defense in response to an armed attack, as permitted under Article 51 of the United Nations Charter. These claims raise multiple interpretive questions about the meaning of "armed attack" under Article 51 and of the various options that are allowed in response to one. This essay explores the contemporary understanding of an "armed attack" in terms of an attack's origins (i.e., can an attack under the Charter originate from nonstate actors?), scale (i.e., must such attacks meet a threshold of intensity to trigger lawful self-defense?), and military nature. It also focuses on the the permissibility of attributing an attack by nonstate actors to a particular state, as well as the consequences of such an attribution. It then identifies a number of outstanding areas of disagreement in the law as well as practical issues for states contemplating the use of force in response to terrorist attacks. The essay concludes with a series of recommendations for resolving these disagreements in a way that respects the imperative of avoiding an escalation in force while deterring such attacks.


Reproduced with permission. Originally published as: Steven R. Ratner, "Self-Defense Against Terrorists: The Meaning of Armed Attack." In Counter-terrorism Strategies in a Fragmented International Legal Order: Meeting the Challenges, edited by N. Schrijver and L. van den Herik, 334-55. Cambridge: Cambridge University Press, 2013. DOI: