What kinds of acts should be resorted to as reprisals is a matter for consideration by the injured party. Acts done by way of reprisals must not, however, be excessive. They must bear a reasonable relation to the degree of violation committed by the enemy. 

Acts of reprisal that are grossly excessive against non-protected persons … constitute a war crime. During the Second World War German forces applied a “hundred to one” order in occupied territories, whereby one hundred civilians would be seized at random and shot as a reprisal for the killing of one German. On occasions civilians already held as prisoners were shot in the same proportion. 

A reprisal must be in proportion to the original violation. Whilst a reprisal need not conform in kind to the act complained of, it may not significantly exceed the adverse party’s violation either in degree or effect. Effective but disproportionate acts cannot be justified as reprisals on the basis that only an excessive response will forestall further violations. 

To be lawful, a belligerent reprisal must meet two conditions … It must meet the criteria for the regulation of reprisals, namely that it is … proportionate … It has been argued that the use of nuclear weapons could never satisfy the requirements of proportionality … This argument, however, suffers from the same flaws as the argument that the use of nuclear weapons could never satisfy the requirements of self-defence. Whether the use of nuclear weapons would meet the requirements of proportionality cannot be answered in the abstract: it would depend upon the nature and circumstances of the wrong which prompted the taking of reprisal action.