The principle of proportionality is embedded in almost every national legal system and underlies the international legal order. Its function in domestic law is to relate means to ends. In armed conflict, the principle is used to judge first, the lawfulness in jus ad bellum of the strategic goals in the use of force for self-defense, and second, the lawfulness in jus in bello of any armed attack that causes civilian casualties. In the Gulf War, allied forces acted in individual and collective self-defense against Iraq under Article 51 of the UN Charter, but they disagreed whether the principle of proportionality permitted them to occupy Iraqi territory or oust Saddam Hussein. Many States felt that only the liberation of Kuwait was a permitted goal.

In the conduct of war, when a party commits a lawful attack against a military objective, the principle of proportionality also comes into play whenever there is collateral damage, that is, civilian casualties or damage to a nonmilitary objective. The U.S. attack on the Amiriyah bunker in Baghdad in 1991, which was aimed to destroy a military target but cost many civilian lives, is a case in point. If it was a military objective in which civilians were sheltering, an attack on the bunker would be lawful, subject to the principle of proportionality.

As formulated in Additional Protocol I of 1977, attacks are prohibited if they cause incidental loss of civilian life, injury to civilians, or damage to civilian objects that is excessive in relation to the anticipated concrete and direct military advantage of the attack. This creates a permanent obligation for military commanders to consider the results of the attack compared to the advantage anticipated. The target list has to be continuously updated as the conflict develops with special attention given to the safe movement of civilians. The attack on the Amiriyah bunker might have been illegal, if—which has never been proved—the United States did not follow carefully enough the movement of the civilians seeking shelter in Baghdad.

Some states ratifying Protocol I have stated that the concrete and direct military advantage anticipated from an attack can only be considered as a whole and not only from isolated or particular parts of the attack. Article 85 defines an indiscriminate attack undertaken in the knowledge that it will cause excessive damage to the civilian population is a grave breach and therefore a war crime. The principle is hard to apply in war, still harder after an attack has occurred. But grossly disproportionate results will be seen as criminal by all belligerent parties and the world community.

“Terror attacks” on the civilian population, or area bombardments that by their nature do not distinguish between military objectives and civilian targets, are prohibited, and the principle does not come into play. If deliberate, the bombing of the Sarajevo market square during shopping hours in 1994, which killed thirty-four civilians, would have been a war crime. The Rome Statute of the International Criminal Court reaffirms this by qualifying in Article 8 as a war crime intentionally directing attacks against the civilian population as such or against individual civilians not taking direct part in hostilities. The use of indiscriminate weapons such as cluster bombs in populated areas is a war crime as well.

(See civilian immunity; legitimate military targets; military necessity.)