Соответствующая норма
Practice Relating to Rule 145. Reprisals
Section B. Reasonable notice and measure of last resort
Belgium’s Law of War Manual (1983) states that, when recourse is made to reprisals, the following conditions must be fulfilled: “2) attempts must first be made to stop [the violation of the LOAC by the adversary] or to prevent its repetition by peaceful means”. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 35.