Related Rule
Belgium
Practice Relating to Rule 45. Causing Serious Damage to the Natural Environment
Belgium’s Law of War Manual (1983) prohibits the use of “methods or means of warfare … which cause such damage to the natural environment that they prejudice the health or survival of the population”. The manual specifically prohibits “methods or means of warfare that are intended or may be expected to cause widespread, long-term and severe damage to the natural environment”. 
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, pp. 27–28.
With respect to weapons, the manual states that the basic principle whereby the only legitimate goal in war is to weaken the enemy’s military forces would be violated if weapons or other means of warfare were used which “would cause widespread, long-term and severe damage to the natural environment”. 
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. 37.
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 … widespread, long-term and severe damage to the natural environment, which would be excessive in relation to the concrete and direct 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 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 … widespread, long-term and severe damage to the natural environment, which would be excessive in relation to the concrete and direct 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).