Related Rule
Belgium
Practice Relating to Rule 4. Definition of Armed Forces
Belgium’s Law of War Manual (1983) defines armed forces as comprising
all members of organized armed forces, under a responsible command and an internal disciplinary system which ensures compliance with the laws and customs of war. Members of organized resistance movements are also considered to be combatants provided they:
a) are subject to internal discipline;
b) wear a fixed distinctive sign recognisable from a distance;
c) carry arms openly;
d) comply with the laws and customs of war. 
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. 20.
A report submitted to the Belgian Senate in 1991 noted that two elements were essential in the definition of armed forces: first, they must be integrated into a military organization (that is, a hierarchical structure) subject to an internal disciplinary system; second, this organization must operate under a command structure responsible to a party for the conduct of its subordinates. If these two conditions were fulfilled, the concept of armed forces could be extended to groups of combatants who were left behind in an occupied territory to perform acts of sabotage, to gather intelligence or to take part in guerrilla warfare. The report recalled that this was the position of the Belgian government in exile during the Second World War. From its base in London, the government adopted legislation authorizing the executive power to nominate agents in charge of action or intelligence missions in a foreign country, occupied area or zone evacuated by the enemy. These agents had the status of combatants and were allowed to carry arms. The government in exile, however, was very reticent about resistance cells or individuals over whom it had no direct control. 
Belgium, Senate, Report, Enquête parlementaire sur l’existence en Belgique d’un réseau de renseignements clandestin international, 1990–1991 Session, Doc. 1117-4, 1 October 1991, §§ 19 and 20.
Resistance networks operating behind enemy lines would not be protected, according to the report, if composed of civilians that were neither part of a hierarchical structure nor subject to an internal disciplinary system. 
Belgium, Senate, Report, Enquête parlementaire sur l’existence en Belgique d’un réseau de renseignements clandestin international, 1990–1991 Session, Doc. 1117-4, 1 October 1991, § 25.
On the basis of the report, the Report on the Practice of Belgium concludes that the definition given in Article 43 of the 1977 Additional Protocol I is recognized by Belgium and that the central criterion is State control over the combatants. 
Report on the Practice of Belgium, 1997, Chapter 1.1.
Upon ratification of the 1977 Additional Protocol I, Belgium notified the High Contracting Parties of the duties assigned to the Belgian Gendarmerie (constabulary) in time of armed conflict. Belgium considered that this notification fully satisfied any and all requirements of Article 43 pertaining to the Gendarmerie. It informed the High Contracting Parties that the Gendarmerie was formed to maintain law and order and was, according to national legislation, a police force which was part of the armed forces within the meaning of Article 43 of the 1977 Additional Protocol I. Consequently, members of the Gendarmerie had the status of combatant in time of international armed conflict. 
Belgium, Interpretative declarations made upon ratification of the 1977 Additional Protocol I, 20 May 1986, § 2.
An Act of Parliament of 18 July 1991 has, however, put an end to this situation as it has disconnected the Gendarmerie from the armed forces. 
Belgium, Law on Demilitarization of the Gendarmerie, 1991.