Source: https://ihl-databases.icrc.org/customary-ihl/eng/docs/v2_cou_us_rule145_SectionA
Timestamp: 2019-04-24 18:18:41+00:00

Document:
(a) Definition. Reprisals are acts of retaliation in the form of conduct which would otherwise be unlawful, resorted to by one belligerent against enemy personnel or property for acts of warfare committed by the other belligerent in violation of the law of war, for the purpose of enforcing future compliance with the recognized rules of civilized warfare. For example, the employment by a belligerent of a weapon the use of which is normally precluded by the law of war would constitute a lawful reprisal for intentional mistreatment of prisoners of war held by the enemy.
(c) Against Whom Permitted. Reprisals against the persons or property of prisoners of war, including the wounded and sick, and protected civilians are forbidden. Collective penalties and punishment of prisoners of war and protected civilians are likewise prohibited. However, reprisals may still be visited on enemy troops who have not yet fallen into the hands of the forces making the reprisals.
(d) When and how employed. Reprisals are never adopted merely for revenge, but only as an unavoidable last resort to induce the enemy to desist from unlawful practices. They should never be employed by individual soldiers except by direct orders of a commander, and the latter should give such orders only after careful inquiry into the alleged offense. The highest accessible military authority should be consulted unless immediate action is demanded, in which event a subordinate commander may order appropriate reprisals upon his own initiative. Ill-considered action may subsequently be found to have been wholly unjustified and will subject the responsible officer himself to punishment for a violation of the law of war. On the other hand, commanding officers must assume responsibility for retaliative measures when an unscrupulous enemy leaves no other recourse against the repetition of unlawful acts.
(f) Procedure. The rule requiring careful inquiry into the real occurrence will always be followed unless the safety of the troops requires immediate drastic action and the persons who actually committed the offence cannot be ascertained.
(g) Hostages. The taking of hostages is forbidden. The taking of prisoners by way of reprisal for acts previously committed (so-called “reprisal prisoners”) is likewise forbidden.
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 497(a), (d), (f) and (g).
(1) It must respond to grave and manifestly unlawful acts, committed by an adversary government, its military commanders, or combatants for whom the adversary is responsible.
(2) It must be for the purpose of compelling the adversary to observe the law of armed conflict. Reprisals cannot be undertaken for revenge, spite or punishment. Rather, they are directed against an adversary in order to induce him to refrain from further violations of the law of armed conflict. Thus, reprisals serve as an ultimate legal sanction or law enforcement mechanism. Above all, they are justifiable only to force an adversary to stop its extra-legal activity. If, for example, one party to an armed conflict commits a breach of law but follows that violation with an expression of regret and promise that it will not be repeated, and even takes steps to punish those immediately responsible, then any action taken by another party to “right” the situation cannot be justified as a lawful reprisal.
(3) There must be reasonable notice that reprisals will be taken. What degree of notice is required will depend upon the particular circumstances of each case. Notice is normally given after the violation but may, in appropriate circumstances, predate the violation. An example of notice is an appeal to the transgressor to cease its offending conduct and punish those responsible. Thus, such an appeal may serve both as a plea for compliance and a notice to the adversary that reprisals will be undertaken.
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 10-7(c)(1), (2) and (3).
If an act is a lawful reprisal, then as a legal measure it cannot lawfully be the excuse for a counter-reprisal. Under international law, as under domestic law, there can be no reprisal against a lawful reprisal. In fact, reprisals have frequently led to counter-reprisals, and the escalation of the conflicts through reprisals and counter-reprisals is one of the reasons for decline in the use of reprisals.
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 10-7(a).
[The reprisal] must be publicized. Since reprisals are undertaken to induce an adversary’s compliance with the recognized rules of armed conflict, any action taken as a reprisal must be announced as a reprisal and publicized so that the adversary is aware of its obligation to abide by the law.
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 10-7(c)(7).
(1) While it is both lawful and proper to plan reprisal actions, as a practical matter, reprisals are often subject to abuse and merely result in escalation of the conflict.
(3) In most twentieth century conflicts, the United States has, as a matter of national policy, chosen not to carry out reprisals against the enemy, both because of the potential for escalation and because it is generally in our national interest to follow the law even if the enemy does not.
(4) The term “reprisal” is sometimes used to refer to any act of retaliation between the parties to a conflict. In law, however, the term should be limited to otherwise illegal acts done in reply to prior illegal acts of the enemy, as described in this paragraph.
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 8-4(b).
A reprisal is an enforcement measure under the law of armed conflict consisting of an act which would otherwise be unlawful but which is justified as a response to the unlawful acts of an enemy. The sole purpose of a reprisal is to induce the enemy to cease its illegal activity and to comply with the law of armed conflict.
2. It must respond to illegal acts of warfare committed by an adversary government, its military commanders, or combatants for which the adversary is responsible. Anticipatory reprisal is not authorized.
4. Its purpose must be to cause the enemy to cease its unlawful activity. Therefore, acts taken in reprisal should be brought to the attention of the enemy in order to achieve maximum effectiveness. Reprisal must never be taken for revenge.
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 6.2.3.1.
Although reprisal is lawful when the foregoing requirements are met, there is always the risk that it will trigger retaliatory escalation (counter-reprisals) by the enemy. The United States has historically been reluctant to resort to reprisal for just this reason.
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 6.2.3.3.
A careful inquiry by the injured belligerent into the alleged violating conduct should precede the authorization of any reprisal measure. This is subject to the important qualification that, in certain circumstances, an offended belligerent is justified in taking immediate reprisals against illegal acts of warfare, particularly in those situations where the safety of his armed forces would clearly be endangered by a continuance of the enemy’s illegal acts.
United States, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 6.2.3.1, footnote 38.
Acts taken in reprisal may also be brought to the attention of neutrals if necessary to achieve maximum effectiveness. Since reprisals are undertaken to induce an adversary’s compliance with the recognized rules of armed conflict, any action taken as a reprisal must be announced as a reprisal and publicized so that the adversary is aware of its obligation to abide by the law and to ensure that the reprisal action is not, itself, viewed as an unlawful act.
United States, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 6.2.3.1, footnote 40.
United States, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 11.7, footnote 45.
If one nation violates the law, it may expect that others will reciprocate. Consequently, failure to comply with international law ordinarily involves greater political and economic costs than does observance. In short, nations comply with international law because it is in their interest to do so.
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, p. 20.
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, §§ 6.2.4 and 6.2.4.1.
United States, Military Tribunal at Nuremberg, List (The Hostages Trial) case, 8 July 1947–19 February 1948.
Where innocent individuals are seized and punished for a violation of the laws of war which has already occurred, no question of hostages is involved. It is nothing more than the infliction of a reprisal. Throughout the evidence in the present case, we find the term hostage applied where a reprisal only was involved.
Where legality of action is absent, the shooting of innocent members of the population as a measure of reprisal is not only criminal but it has the effect of destroying the basic relationship between the occupant and the population. Such a condition can progressively degenerate into a reign of terror. Unlawful reprisals may bring on counter reprisals and create an endless cycle productive of chaos and crime. To prevent a distortion of the right into a barbarous method of repression, International Law provides a protective mantle against the abuse of the right.
In 1980, in a footnote to a memorandum of law on the “Reported Use of Chemical Agents in Afghanistan, Laos, and Kampuchea”, a legal adviser of the US Department of State noted that “reprisals are permitted under the laws of war only for the limited purpose of compelling the other belligerent to observe the laws of war”.
For the purpose of the law of armed conflict, reprisals are lawful acts of retaliation in the form of conduct that would otherwise be unlawful, resorted to by one belligerent in response to violations of the law of war by another belligerent. Such reprisals would be lawful if conducted in accordance with the applicable principles governing belligerent reprisals. Specifically, the reprisals must be taken with the intent to cause the enemy to cease violations of the law of armed conflict … As in the case of other requirements of the law of armed conflict, a judgment about compliance of any use of nuclear weapons with these requirements would have to be made on the basis of the actual circumstances in each case, and could not be made in advance or in the abstract.
United States, Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 30.

References: § 497
 § 10
 § 10
 § 10
 § 8
 § 6
 § 6
 § 6
 § 6
 § 11