Proportionality

Under jus ad bellum, a state is entitled to use military force in self-defence against an "armed attack" against it. But in accordance with customary international law the principle of proportionality holds that such force may only be used that is needed to repel an armed attack. Thus, for example, punitive or clearly excessive use of force will violate the condition of proportionality. In the Oil Platforms case between Iran and the United States of America (USA), the International Court of Justice recalledSee paragraph 76 of its 2003 judgment. that in its landmark 1986 judgment in the Nicaragua caseParagraph 176, Case concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. USA)., the Court referred to a specific rule "whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it" as "a rule well established in customary international law".

Under international human rights law and criminal justice standards, any use of force for law enforcement purposes must be proportionate to the threat and the lawful objective sought. Exessive or gratuitous use of force will therefore be unlawful. For example, Principle 5 of the 1990 Basic Principles on the Use of Force and Firearms by Law Enforcement Officials holds inter alia that:

Whenever the lawful use of force and firearms is unavoidable, law enforcement officials shall:

(a) Exercise restraint in such use and act in proportion to the seriousness of the offence and the legitimate objective to be achieved;

(b) Minimize damage and injury, and respect and preserve human life;

Under IHL, the rule of proportionality in attacks holds that in the conduct of hostilities during an armed conflict:

Launching an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated, is prohibited.

This rule applies as a matter of custom both in international and in non-international armed conflicts, according to the International Committee of the Red Cross (ICRC) 2005 study of customary IHL. The ICRC further affirms that "launching an attack in the knowledge that such attack will cause incidental loss of civilian life, injury to civilians or damage to civilian objects which would be clearly excessive in relation to the concrete and direct military advantage anticipated" constitutes a war crime (our emphasis). 

Last updated on: 30 November 2013