When a belligerent party is hurt by conduct on the part of its adversary that it regards as a grave breach or systematic encroachment of the laws of armed conflict, one possibility is to retaliate by means of an action that itself violates the same body of law. While recourse to such retaliatory action can be arbitrary and in total disregard of any constraints, rules of customary law have developed in the past that provide the limits within which retaliation could be regarded as a legitimate reprisal. The main elements of this customary "right of reprisal" are: subsidiarity (failure of all other available means), notice (formal warning of the planned action), proportionality (the damage and suffering inflicted on the adverse party not to exceed the level of damage and suffering resulting from its unlawful conduct), temporary character (termination of the reprisal when the adversary stops violating the law).

A reprisal may be "in kind" (violating the same or a narrowly related rule of the laws of armed conflict) or "not in kind" (violating a nonrelated rule). In either case, the reprisal need not and usually cannot be directed against those persons on the adverse side who are responsible for the unlawful conduct, and hence tends mainly to affect people who are "innocent" of that conduct. Also, the adversary often regards an alleged reprisal as a plainly unlawful act that in turn justifies reprisals, leading to a spiral of increasingly serious damage and suffering.

These features have led to a trend to ban reprisals wherever possible. As a result, all four Geneva Conventions of 1949 categorically prohibit reprisals against the persons and objects they are designed to protect. Likewise, Article 20, which concludes the part of Additional Protocol I of 1977 on the wounded, sick, and shipwrecked, prohibits reprisals against the persons and objects protected by that part.

While these bans are generally accepted as entirely justified, the provisions in Articles 51 through 55 of Additional Protocol I prohibiting reprisals against civilians and civilian objects are highly controversial, and some States have entered reservations to their treaty ratifications. A recent example of a reserving State is the United Kingdom, which has formulated its reservation in terms that would permit it to undertake customary “in kind” reprisals.

Additional Protocol II of 1977 is silent on the matter of reprisals. This should not however be interpreted as a right for parties to an internal armed conflict to resort to retaliatory action; the better view is that essential requirements of humanity accepted for international armed conflict apply by way of analogy in internal armed conflicts as well.

(See protected persons; reprisal killings.)