A reprisal is a limited and deliberate violation of international law to punish another sovereign state that has already broken them.^[1][2] Since the 1977 Additional Protocol I to the Geneva Conventions (AP 1), reprisals in the laws of war are extremely limited,^[3] as they commonly breach the rights of non-combatants.

The word came from French, where it originally meant "act of taking back", for example, raiding back the equivalent of cattle lost to an enemy raid.^[4]

Reprisals refer to acts which are illegal if taken alone, but become legal when adopted by one state in retaliation for the commission of an earlier illegal act by another state.^{[5][failed verification]} ICRC’s Database of Customary International Humanitarian Law states in Rule 145: "Where not prohibited by international law, belligerent reprisals are subject to stringent conditions."^[5] "Counter-reprisals" are generally not allowed.^[5]

An example of reprisal is the Naulila dispute between Portugal and Germany in October 1914, when they were on opposite sides of the World War I chasm. After three Germans were mistakenly killed in Naulila on the border of the then-Portuguese colony of Angola (in a manner that did not violate international law),^[6] Germany carried out a military raid on Naulila, destroying property in retaliation. A claim for compensation was brought by Portugal. The tribunal emphasized that before reprisals could be legally undertaken, a number of conditions had to be satisfied:

• There had to be a previous act by the other party that violated international law.
• Reprisals had to be preceded by an unsatisfied demand for reparation or compliance with the violated international law.
• There must be proportionality between the offence and reprisal.

The German claim that it had acted lawfully was rejected on all three grounds.^[7]

During the Irish War of Independence reprisals were authorized by British authorities in areas of Ireland that were under martial law. From December 1920 until June 1921 approximately 150 "official" reprisals were carried out. In December 1920 the General Officer Commanding-in-Chief (Nevil Macready) informed the British Cabinet that Military Governors in martial law areas had been authorized to conduct reprisals in response to attacks on the military and police, under these conditions:^[8]

Punishments will only be carried out on the authority of the Infantry Brigadier, who before taking action will satisfy himself that the people concerned were, owing to their proximity to the outrage or their known political tendencies, implicated in the outrage, and will give specific instructions in writing, or by telegram to the officer detailed to carry out the operation.

Bennett writes that the events of World War II can be seen through either the prism of negative reciprocity or the prism of reprisal. If the latter, "the rules also required that reprisals be used ‘only as an unavoidable last resort to induce the enemy to desist from illegitimate practices’".^[9]

The official 1940 American Rules of Land Warfare stated that "commanding officers must assume responsibility for retaliative measures when an unscrupulous enemy leaves no other recourse against the repetition of barbarous outrages."^[9]

Both Rogers and Bennett write that "[s]tate practice in the Second World War was characterised by, among other factors, the doctrine of belligerent reprisal."^[10][11]

After 1945, as a result of the general prohibition on use of force imposed by Article 2(4) of the United Nations Charter, armed reprisals in time of peace are no longer legal, but the possibility remains of non-armed reprisals (also known as countermeasures)^[12] as well as belligerent reprisals during hostilities when the law of international armed conflict (LOIAC) is violated.^[13]

In the case of belligerent reprisals, apart from the three factors in the Naulila case:^[13][9]

• a warning must also be issued beforehand;
• once the other party has stopped violation of LOIAC, belligerent reprisals must also be terminated;
• and the decision to engage in belligerent reprisals must be taken by a competent authority.

All four Geneva Conventions prohibit reprisals against, respectively, battlefield casualties, shipwreck survivors, prisoners of war, and protected persons (civilian or military),^[14][15] as well as certain buildings and property. The 1977 AP 1 defines what is an "indiscriminate attack".^[16][17]

• Collective punishment
• Letter of marque (license to hunt enemy ships and retake lost ships from the enemy)

• Anderson, Kenneth (1999). "Reprisal killings". In Gutman, Roy; Rieff, David; Anderson, Kenneth (eds.). Crimes of War: What the Public Should Know . New York: W. W. Norton & Company. ISBN 9780393319149.
• Bennett, John (2019). "Reaping the Whirlwind: The Norm of Reciprocity and the Law of Aerial Bombardment During World War II" (PDF). Melbourne Journal of International Law. 20: 1–44.
• Brownlie, Ian (2008). Principles of Public International Law (7 ed.). New York: Oxford University Press. ISBN 978-0-19-921770-0.
• Darcy, Shane (March 2003). "The Evolution of the Law of Belligerent Reprisals". Military Law Review. 175: 184–251.
• Dinstein, Yoram (2004). The Conduct of Hostilities under the Law of International Armed Conflict. Cambridge: Cambridge University Press. ISBN 0-521-54227-8.
• Hanke, Heinz Marcus (1993). "The 1923 Hague Rules of Air Warfare: A Contribution to the Development of International Law Protecting Civilians from Air Attack". International Review of the Red Cross. 33 (292): 12–44. doi:10.1017/S0020860400071370.
• Rogers, A.P.V. (2012). Law on the Battlefield (3rd ed.). Manchester: Manchester University Press.
• Shaw, Malcolm (2008). International Law (6th edn). Cambridge: Cambridge University Press. ISBN 978-0-521-72814-0.