Related Rule
Belgium
Practice Relating to Rule 14. Proportionality in Attack
Belgium’s Law of War Manual (1983) states:
An attack against a military objective must not be launched when it is to be expected that such an attack will cause incidental loss or damage to civilians and civilian objects which would be excessive in relation to the concrete and direct military advantage expected. 
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. 26; see also p. 28.
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 … :
22. intentionally launching an attack in the knowledge that such attack will cause incidental loss of life, injury to civilians or damage to civilian objects … which would be clearly excessive in relation to the concrete and direct overall military advantage anticipated. 
Belgium, Penal Code, 1867, as amended on 5 August 2003, Chapter III, Title I bis, Article 136 quater, § 1(22).
Belgium’s Law concerning the Repression of Grave Breaches of the Geneva Conventions and their Additional Protocols (1993), as amended in 1999, provides that it is a crime under international law to launch
an indiscriminate attack affecting the civilian population or civilian objects in the knowledge that such attack will cause loss of human life, injury to civilians or damage to civilian objects which would be excessive in relation to the concrete and direct military advantage anticipated, without prejudice to the criminal nature of an attack whose harmful effects, even where proportionate to the military advantage anticipated, would be inconsistent with the principles of international law derived from established custom, from the principles of humanity and from the dictates of public conscience. 
Belgium, Law concerning the Repression of Grave Breaches of the Geneva Conventions and their Additional Protocols, 1993, as amended in 1999, Article 1(3)(12).
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 … :
12. intentionally launching an attack in the knowledge that such attack will cause incidental loss of life, injury to civilians or damage to civilian objects … which would be clearly excessive in relation to the concrete and direct overall military advantage anticipated. 
Belgium, Law relating to the Repression of Grave Breaches of International Humanitarian Law, 1993, as amended on 23 April 2003, Article 1 ter, § 1(12).
Belgium’s Law of War Manual (1983) states that, when deciding whether or not to launch an attack, “the commander must consider the advantage of the attack as a whole (and not the advantages of specific or separate parts of the attack)”. 
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. 29.
In an explanatory memorandum submitted to the Belgian Parliament in 1985 in the context of the ratification procedure of the 1977 Additional Protocols, the Belgian Government stated: “The military advantage must be assessed in the light of the attack considered as a whole.” 
Belgium, House of Representatives, Explanatory memorandum on a draft bill for the approval of the Additional Protocols, 1984–1985 Session, Doc. 1096-1, 9 January 1985, p. 11.
Upon ratification of the 1977 Additional Protocol I, Belgium stated that the term “military advantage” as used in the proportionality test of Articles 51 and 57 of the Protocol was understood to refer to the advantage anticipated from the attack considered as a whole and not only from isolated or particular parts of the attack. 
Belgium, Interpretative declarations made upon ratification of the 1977 Additional Protocol I, 20 May 1986, § 5.
Belgium’s Law of War Manual (1983) states:
It will not always be easy for a commander to evaluate this situation [whether an attack will be disproportionate] with precision. On the one hand, he must take into account the elements which are available to him, related to the military necessity necessary to justify an attack, and on the other hand, he must take into account the elements which are available to him, related to the possible loss of human life and damage to civilian objects. 
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. 29.
[emphasis in original]
In an explanatory memorandum submitted to the Belgian Parliament in 1985 in the context of the ratification procedure of the 1977 Additional Protocols, the Belgian Government stated: “The military advantage must be assessed … in the light of what a military commander can foresee on the basis of the available and relevant information which is available at the time of the assessment.” 
Belgium, House of Representatives, Explanatory memorandum on a draft bill for the approval of the Additional Protocols, 1984–1985 Session, Doc. 1096-1, 9 January 1985, p. 11.
Upon ratification of the 1977 Additional Protocol I, Belgium stated:
With respect to Part IV, Section I, of the Protocol, the Belgian Government wishes to emphasize that, whenever a military commander is required to take a decision affecting the protection of civilians or civilian objects or objects assimilated therewith, the only information on which that decision can possibly be taken is such relevant information as is then available and that it has been feasible for him to obtain for that purpose. 
Belgium, Interpretative declarations made upon ratification of the 1977 Additional Protocol I, 20 May 1986, § 3.