International Law Military Necessity
David Turns
  • LAST REVIEWED: 13 June 2017
  • LAST MODIFIED: 23 March 2012
  • DOI: 10.1093/obo/9780199796953-0008


Military necessity is one of the most fundamental yet most misunderstood and misrepresented principles of the international law of armed conflict. It has been invoked by military operators to justify any violent measures deemed necessary to win a given conflict, and it has also been dismissed by human rights groups, nongovernmental organizations, and other critics of the armed forces as a typical military excuse to explain away shocking collateral damage in modern military operations. At one extreme it has been distorted by the Clausewitzian approach of Kriegsraison geht vor Kriegsmanier (roughly, necessity in war overrides the way of fighting a war) into a doctrine that jettisons all restraints on military action in war; thus, at one end of the spectrum of interpretation, it may be seen as a doctrinal justification for the notion of “total war.” However, the modern law of armed conflict, with its strongly humanitarian emphasis, requires careful distinction between military and civilian targets; it is based on a subtle and often-elusive nonarithmetical equation that seeks to balance the military advantage anticipated from a given operation against the likely civilian casualties or damage to civilian objects. If proportionality is the fulcrum of that equation and humanity is one side of the balance, then military necessity is the other. Fundamentally and in its broadest interpretation, military necessity means that armed forces can do whatever is necessary—provided always that it is not otherwise unlawful under humanitarian law—to achieve their legitimate military objectives in warfare. For example, enemy armed forces that have not surrendered or are not hors de combat are always legitimate military targets in themselves and may therefore lawfully be attacked at any time and in any place, irrespective of where they are located or what they are actually doing. A more restrictive approach to the doctrine, however, interprets it as always placing limitations on military action, in the sense that no such action may be undertaken (regardless of its legality otherwise under the law of armed conflict) unless it is actually necessary in military terms. An example would be an invading force deciding not to attack certain enemy military formations that do not impede its attack or would represent a diversion from its main objectives. It should be emphasized that the legal notion of military necessity is not the same as “military advantage” (which is a factual descriptor of the outcome upon which actions are predicated). In sum, “military necessity” is best defined as the requirement, in any given set of circumstances, for the application of armed force (in accordance with the other rules of the law of armed conflict) to achieve legitimate military objectives.

Classic Approaches

Early statements of what would today be called military necessity can be found in the work of several classic authors from the period when modern public international law was beginning to develop. These first approaches are often buried amid a much broader discussion of the utility of going to war at all. Even before the start of the early modern period in warfare during the 17th century, Sun Tzu (Sun Tzu 2010) articulated a sense of the restrictions that the concept could impose upon a sensible commander. This theme was also picked up by Hugo Grotius (Grotius 2005) and Emmerich de Vattel (Vattel 2005), who, on the other hand, linked the jus in bello (law of war) doctrine to the necessity of waging war in the first place, that is, the jus ad bellum. The first attempt at a modern codification—albeit not in the form of an international treaty—of the laws of war in the modern era, the Lieber Code (Lieber 1863) remains seminal in its lucid, detailed, and recognizably contemporary statement of what military necessity does and does not permit in war.

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