The principle of proportionality is a core constraint on the conduct of hostilities under international humanitarian law (IHL). It does not prohibit civilian casualties outright; rather, it prohibits attacks that "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." That formulation appears in Article 51(5)(b) of Additional Protocol I (1977) to the Geneva Conventions and is repeated in Article 57 in the context of precautions in attack.
The rule applies alongside, but is distinct from, the principles of distinction (combatants vs. civilians) and military necessity. Even a lawful target may not be struck if the foreseeable collateral damage is disproportionate to the military gain. The International Committee of the Red Cross's 2005 customary IHL study treats proportionality as binding in both international and non-international armed conflicts (Rules 14 and 19).
Application is notoriously difficult because the comparison is between unlike quantities — civilian lives against military advantage — and must be made ex ante, based on information reasonably available to the commander at the time. Key interpretive questions include: how to count "reverberating" or long-term effects on civilians; what counts as a "concrete and direct" advantage (as opposed to a speculative one); and how to handle dual-use objects such as power grids or bridges.
Proportionality also appears in two related but separate legal regimes: in the jus ad bellum (self-defence under UN Charter Article 51 must be proportionate to the armed attack, as affirmed by the ICJ in Nicaragua (1986) and the Nuclear Weapons advisory opinion (1996)), and in international human rights law governing the use of force in law enforcement. Delegates should be careful to specify which proportionality they mean — jus ad bellum, jus in bello, or human-rights — because the tests differ.
Example
In its 2024 advisory proceedings and various UN reports on the Gaza conflict, investigators repeatedly assessed Israeli airstrikes against proportionality under Additional Protocol I, weighing civilian casualties against stated military objectives such as Hamas command nodes.
Frequently asked questions
No. Jus ad bellum proportionality asks whether the overall scale of force in self-defence matches the armed attack faced. Jus in bello proportionality applies attack-by-attack, weighing civilian harm against military advantage, regardless of who started the war.
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