Related Rule
Belgium
Practice Relating to Rule 46. Orders or Threats That No Quarter Will Be Given
Belgium’s Law of War Manual (1983) states: “Declaring that no quarter will be given is forbidden.” 
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. 33.
Belgium’s Teaching Manual for Officers (1994) provides that it is forbidden “for military commanders to conduct hostilities on the basis that there shall be ‘no quarter’, i.e. no survivors at the end of combat. The threat to use this method of combat is also prohibited.” 
Belgium, Droit de la Guerre, Manuel d’Instruction pour Officiers, Etat-Major Général, Division Opérations, 1994, Part I, Title II, p. 34.
Belgium’s Penal Code (1867), as amended in 2003, provides:
War crimes envisaged in the 1949 [Geneva] Conventions … and in the [1977 Additional Protocols I and II] … , as well as in Article 8(2)(f) of the [1998 ICC Statute], and listed below, … constitute crimes under international law and shall be punished in accordance with the provisions of the present title … :
28. declaring that no quarter will be given. 
Belgium, Penal Code, 1867, as amended on 5 August 2003, Chapter III, Title I bis, Article 136 quater, § 1(28).
Belgium’s Law relating to the Repression of Grave Breaches of International Humanitarian Law (1993), as amended in 2003, provides:
War crimes envisaged in the 1949 [Geneva] Conventions … and in the [1977 Additional Protocols I and II] … , as well as in Article 8(2)(f) of the [1998 ICC Statute], and listed below, … constitute crimes under international law and shall be punished in accordance with the provisions of the present title … :
15 ter. declaring that no quarter will be given. 
Belgium, Law relating to the Repression of Grave Breaches of International Humanitarian Law, 1993, as amended on 23 April 2003, Article 1 ter, § 1(15 ter).
The Sergeant W. case before Belgium’s Court-Martial of Brussels in 1966 concerned the murder of an unarmed Congolese woman by a senior member of the Belgian staff who had been sent to provide assistance to the army in the Democratic Republic of the Congo. In his defence, the accused argued that he had been ordered by his commanding officer (Major O.) to “shoot all suspect elements on sight” in the zone forbidden to civilians and that he had shot the woman on the basis that he had interpreted this order as meaning that he should “take no prisoners and to ‘kill’ everything we come across in here”. The Court found:
As interpreted by the accused in practice – viz. the right or even the obligation to kill an unarmed person in his power – the order was patently illegal. Executing or causing to be executed without prior due trial a suspect person or even a rebel fallen into the hands of the members of his battalion was obviously outside the competence of Major O., and such an execution was a manifest example of voluntary manslaughter. The illegal nature of the order thus interpreted was not in doubt and the accused had to refuse to carry it out … The act perpetrated by the accused constitutes not only murder within the meaning of Articles 43 and 44 of the Congolese Criminal Code and Articles 392 and 393 of the Belgian Criminal Code, but is also a flagrant violation of the laws and customs of war and the laws of humanity. 
Belgium, Court-Martial of Brussels, Sergeant W. case, Judgment, 18 May 1966.