Reprisal

A reprisal is a limited and deliberate violation of international law to punish another sovereign state that has already broken them. Reprisals in the laws of war are extremely limited, as they commonly breached the rights of non-combatants, an action outlawed by the Geneva Conventions. It is not to be confused with retorsions, as these constitute unfriendly acts generally permitted by international law.

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.

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. Counter-reprisals are generally not allowed.

An example of reprisal is the Naulila dispute between Portugal and Germany in October 1914. 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), 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:

The German claim that it had acted lawfully was rejected on all three grounds.

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) as well as belligerent reprisals during hostilities when the law of international armed conflict (LOIAC) is violated.

In the case of belligerent reprisals, apart from the three factors in the Naulila case, 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), as well as certain buildings and property. An additional 1977 protocol extends this to cover historic monuments, works of art, and places of worship.

...
Wikipedia