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United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 145. Reprisals
Section A. Definition and purpose of reprisals
The UK Military Manual (1958) states: “Reprisals between belligerents are acts of retaliation for illegitimate acts of warfare. One of their objects is to cause the enemy to comply in future with the recognised laws of war. Reprisals are by custom admissible as a means of securing legitimate warfare.” It further states: “The illegitimate acts in respect of which reprisals are admissible may be committed by a government, by its military commanders, or by some person or persons whom it is impossible, for the time being, to apprehend, try and punish.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 642 and 643.
The manual also notes:
Reprisals are an extreme measure of coercion, because in most cases they inflict suffering upon innocent individuals. Nevertheless, in the circumstances of war, they often provide the only remedy as a punishment, as a deterrent and as a means of inducing the enemy to desist from his unlawful conduct. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 644.
In addition, in a footnote on a provision regarding the procedure before recourse to reprisals is made, the manual states: “A certain caution should be exercised before deciding to institute reprisals, as in some cases counter-reprisals may follow, thus defeating the purpose of the original reprisals.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 646, footnote 2.
The UK LOAC Pamphlet (1981) states: “Under customary law reprisals are permitted to counter unlawful acts of warfare. They can only be taken if: a. they are intended to secure legitimate warfare.” It also states: “Reprisals are an unsatisfactory way of enforcing the law. They tend to be used as an excuse for illegal methods of warfare with a danger of escalation through repeated reprisals and counter-reprisals.” However, the manual also states:
The United Kingdom reserves the right to take proportionate reprisals against an enemy’s civilian population or civilian objects where the enemy has attacked our own civilians or civilian objects in violation of [the 1977 Additional Protocol I]. 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 4, p. 17, §§ 14, 15 and 17.
The UK LOAC Manual (2004) states:
Enforcement of the law of armed conflict can involve a wide variety of measures. “Enforcement” is taken here to mean action to ensure observance of the law and also action that may be taken following alleged or actual violations. Action aimed at effective enforcement of the law can include, but is not limited to:
k. Reprisals by an aggrieved state. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.1.
The manual further states:
16.16. Reprisals are extreme measures to enforce compliance with the law of armed conflict by the adverse party. They can involve acts which would normally be illegal, resorted to after the adverse party has itself carried out illegal acts and refused to desist when called upon to do so. They are not retaliatory acts or simple acts of vengeance. Reprisals are, however, an extreme measure of coercion, because in most cases they inflict suffering upon innocent individuals. Nevertheless, in the circumstances of armed conflict, reprisals, or the threat of reprisals, may sometimes provide the only practical means of inducing the adverse party to desist from its unlawful conduct.
Conditions for reprisal action
16.17. In order to qualify as a legitimate reprisal, an act must comply with the following conditions when employed:
a. It must be in response to serious and manifestly unlawful acts, committed by an adverse government, its military commanders or combatants for whom the adversary is responsible;
b. It must be for the purpose of compelling the adversary to observe the law of armed conflict. Reprisals serve as an ultimate legal sanction or law enforcement mechanism. Thus, if one party to an armed conflict breaches the law but then expresses regret, declares that it will not be repeated and takes measures to punish those immediately responsible, then any action taken by another party in response to the original unlawful act cannot be justified as a reprisal;
e. A reprisal must be directed against the personnel or property of an adversary;
g. It must be publicized. Since reprisals are undertaken to induce an adversary’s compliance with the laws of armed conflict, any action taken as a reprisal must be announced as such and publicized so that the adversary is aware of the reason for the otherwise unlawful act and of its own obligation to abide by the law. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 16.16–16.17.
The manual also restates the interpretative declaration made by the UK upon ratification of the 1977 Additional Protocol I (see infra). 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.19.1.
During discussions on reprisals in Committee I of the CDDH, the United Kingdom stated that it was “not true that at the present time [in 1976] the old system of lawful counter-measures was excluded; it still existed under customary law, and any exclusion must be expressed, as in the Geneva Conventions”. It also stated that it would “support the principle embodied in [the French proposal on a prohibition of reprisals]”. 
United Kingdom, Statement at the CDDH, Official Records, Vol. IX, CDDH/I/SR.47, 29 April 1976, p. 74, § 38.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
A belligerent reprisal is an action, taken by a party to an armed conflict, which would normally constitute a violation of the laws of armed conflict but which is lawful because it is taken in response to a prior violation of that law by an adversary … To be lawful, a belligerent reprisal must meet two conditions … It must meet the criteria for the regulation of reprisals, namely that it is taken in response to a prior wrong … [and] is undertaken for the purpose of putting an end to the enemy’s unlawful conduct and for preventing further illegalities. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, p. 58.
Upon ratification of the 1977 Additional Protocol I, the United Kingdom stated that in the event of violations of Articles 51–55 of the 1977 Additional Protocol I by the adversary, the United Kingdom would regard itself entitled to take measures otherwise prohibited by these Articles, noting, however, that this would be true “to the extent that it considers such measures necessary for the sole purpose of compelling the adverse party to cease committing violations under those Articles”. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § (m).