Related Rule
Belgium
Practice Relating to Rule 107. Spies
Belgium’s Law of War Manual (1983) defines a spy as:
an individual who gathers or attempts to gather, clandestinely or on false pretences, information in the zone of operations of a belligerent with the intention of communicating it to the adverse party. The “zone of operations” comprises zones where no land operations are taking place but which may be hit by aerial bombardment (including bombardment by long-range missiles). This interpretation is very wide. Neutral territory on which a spy may operate cannot, however, be considered as a “zone of operations”. 
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. 21.
Belgium’s Law of War Manual (1983) states:
Spying is not contrary to the law of war and, as a result, does not constitute a war crime. Most countries provide, however, that spying is a crime [under domestic law] in order to protect their national interests and the interests of their armed forces. A person who is caught spying for the enemy is liable to punishment, but only after being tried … In general, civilians act as spies. This activity, by itself, does not give them the status of combatant … Members of the armed forces who perform spying missions in the zone of operations will be treated, if captured, either as prisoners of war or as spies, depending on whether they accomplished their mission wearing their uniform or disguised as civilians wearing civilian clothes. 
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. 21–22.