In This Article Expand or collapse the "in this article" section Self-Defense

  • Introduction
  • General Overviews
  • Textbooks
  • Judicial Decisions

International Law Self-Defense
Mary Ellen O’Connell
  • LAST REVIEWED: 12 April 2019
  • LAST MODIFIED: 26 November 2019
  • DOI: 10.1093/obo/9780199796953-0028


Humanity has always recognized that individuals should have the right to defend themselves from violence. In international law this basic normative intuition is codified for states in the Charter of the United Nations, Article 51 (see Randelzhofer 2002, cited under Conditions in Article 51). Article 51 is an exception to the Charter’s general prohibition on the use of force found in Article 2(4). The prohibition on the use of force is at the heart of the Charter, given that the most fundamental aim of the Charter and the UN organization created by the Charter is to “save succeeding generations from the scourge of war” (Preamble). It stands to reason that any right to use force as an exception to the general prohibition on resort to force would be narrow. Article 51 permits a state to act in unilateral or collective self-defense only “if an armed attack occurs.” This article concerns the international law exception to the prohibition on force for self-defense. The commentary on Article 51 is extensive and generally falls into one of two categories: first, scholarship, judicial decisions, and government policies that support Article 51’s plain terms; second, scholarship and government policies that advocate expanding the right to use force beyond Article 51’s provisions. The writers in these two categories have various labels but are most commonly referred to as the “strict” interpreters versus the “broad” interpreters. One author refers to the groups as the “restrictivists” versus the “antirestrictivists.” The divergence of views can be explained to some extent by the differing assessments writers make about the utility of resort to military force. The UN Charter was drafted at the end of World War II, when confidence in military force was certainly low and commitment to ending the use of force was high. Fifty years later, perhaps frustrated by the lack of success with other means, writers (especially in a few militarily powerful states) urged relaxing the rules against force to respond to terrorism, weapons programs, and computer network attacks. Some try to justify force under the principles of necessity and proportionality, rules beyond the UN Charter but equally important in the long history of normative thinking on killing in self-defense.

General Overviews

The right of self-defense was codified in the Charter of the United Nations. With most states in the world as parties, the Charter became the authoritative document on self-defense. Scholars and government officials almost immediately began to parse its terms for exceptions and arguments expanding the right to use force. Bowett 1958 launches the trend of searching for broader rights than contemplated in the UN Charter. Brownlie 1963 responds in a defense of the UN Charter. Some governments, especially in militarily strong states, followed Derek Bowett’s position, but the International Court of Justice (ICJ) in 1986, in a case between the United States and Nicaragua, pronounced that the Charter rules on self-defense had entered into customary international law (see International Court of Justice 1986). The court even pointed to references by the United States characterizing the prohibition on the use of force as a peremptory norm of international law (jus cogens). The ICJ emphasized the limits on self-defense found in Article 51 and in general international law beyond the Charter, especially in the form of the principles of necessity and proportionality. Some government officials and scholars have chafed at the Nicaragua v. United States of America decision (International Court of Justice 1986). They continue to revisit the terms of Article 51, searching for alternatives to using force, such as the Anglo-American correspondence of 1841 over the sinking of a ship called the Caroline. Despite these efforts, the Nicaragua case has generally maintained its authority. Alexandrov 1996 focuses on Nicaragua but also returns to Bowett’s argument of a right to attack under preexisting customary international law. The Charter was challenged again in the aftermath of the 9/11 terrorist attacks in the United States, when that country declared a global war in self-defense against terrorism. The United States announced in its National Security Strategy of 2002 a right of “preemptive” self-defense against terrorist threats, threats posed by nuclear weapons programs, and the like. Again, as Ian Brownlie did in the 1960s, other scholars have defended the Charter against these post-9/11 challenges. In 2005 the UN completed a two-year review of the Charter and UN operations. The final document, World Summit Outcome 2005, recommitted the members to strict adherence to Charter terms. The document adds no additional support for a right to attack in self-defense in situations other than an armed attack. Corten 2014 reflects the same consensus as the outcome document, providing thorough analysis of ICJ jurisprudence on self-defense from 1948 to 2007.

  • Alexandrov, Stanimir. Self-Defense against the Use of Force in International Law. The Hague: Kluwer Law International, 1996.

    Alexandrov returns to Bowett’s arguments that force in self-defense may be exercised in more situations than armed attack and that this reading is justified by the Security Council’s failure to take effective action to ensure international peace and security.

  • Bowett, Derek. Self-Defence in International Law. Manchester, UK: Manchester University Press, 1958.

    Bowett argues against the need for an actual armed conflict to occur, stressing that the “inherent” right of self-defense refers to customary international law as restated in the 1841 correspondence over the sinking of the Caroline. Bowett urges that, in the absence of “any centralized machinery for the enforcement of the law,” the need for greater self-help is “obvious” (p. 3).

  • Brownlie, Ian. International Law and the Use of Force by States. Oxford: Clarendon Press, 1963.

    DOI: 10.1093/acprof:oso/9780198251583.001.0001

    Brownlie provides a point-by-point response to Bowett, including a history of Articles 2(4) and 51 to demonstrate that only a strict interpretation of the Charter rules is reasonable. Brownlie warns against the tendency of writers to claim that justifications for the use of force found in customary law prior to the 1920s remained valid after 1945.

  • Charter of the United Nations. (26 June 1945) 59 Stat. 1031, TS No. 993, 3 Bevans 1153.

    UN Charter Article 2(4) is a general prohibition on the resort to force, with only two exceptions: one allows UN Security Council authorization of force, the other is the right of self-defense found in Article 51. Article 51 refers to an “inherent” right of self-defense but also notes that self-defense arises “if an armed attack occurs.”

  • Corten, Olivier. Le droit contre la guerre: L’interdiction du recours à la force en droit international. Paris: Éditions A. Pedone, 2014.

    Translated as: “The law against war: The prohibition on the use of force in contemporary international law.” Corten responds to the post-9/11 challenges to the UN Charter. He returns to Bowett’s “inherent right” argument but rejects reading out the armed attack requirement of Article 51. Corten also takes up the post-9/11 controversy over the right to attack nonstate actors in self-defense. Such a right must be based on a territorial state’s responsibility for a nonstate-actor attack.

  • International Court of Justice. “Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America).” ICJ Reports (1986): 14.

    The Nicaragua case is the locus classicus on self-defense. The ICJ found that Article 51 is part of customary international law and that force used in self-defense against an armed attack may be exercised only absent Security Council authorization if the attack is significant. The ICJ also specified that lawful defensive force must be necessary to accomplish a defensive purpose without a disproportionate cost.

  • National Security Strategy. George W. Bush White House Archive. September 2002.

    The document states that, “even if uncertainty remains as to the time and place of the enemy’s attack,” the United States will act preemptively to “forestall or prevent” hostile acts (p. 15). Later versions use less provocative terms, but the United States still seems to uphold the idea of using force in self-defense even before obtaining evidence of an armed attack.

  • World Summit Outcome (15 September 2005). UN GAOR, 60th Sess., at 22–23, UN Doc. A/60/L.1.

    After a three-year review of the UN Charter and the operation of the organization, the full UN membership agreed to reconfirm the Charter rules as written. On self-defense, the document states in paragraph 78, “We reiterate the importance of . . . addressing international challenges and problems by strictly abiding by the Charter and the principles of international law.”

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