Related Rule
Practice Relating to Rule 140. The Principle of Reciprocity
Belgium’s Law of War Manual (1983) states: “The conventional law of war remains, in principle, obligatory between signatory parties, even if one of them violates it.” 
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. 18.
In a report on a symposium on IHL held in Belgium in 1974, a representative of the Belgian Ministry of Justice noted:
The notion of reciprocity, which has recently again been rejected by the Committee of Experts on Human Rights of the Council of Europe, has several times been mentioned, which appears to be somewhat shocking. In fact, it is difficult to imagine how one could justify “inhumane treatments” under the pretext that the adversary has recourse to them. 
Belgium, Ministry of Justice, Note for the Minister of Justice, 18 December 1974, Report on the Practice of Belgium, 1997, Chapter 5.7.