ANNEX TO THE CONVENTION : Regulations respecting the laws and customs of war on land #Section II : On hostilities #Chapter II : On spies - Art. 29.
Art. 29. An individual can only be considered a spy if, acting clandestinely, or on false pretences, he obtains, or seeks to obtain information in the zone of operations of a belligerent, with the intention of communicating it to the hostile party.
Thus, soldiers not in disguise who have penetrated into the zone of operations of a hostile army to obtain information are not considered spies. Similarly, the following are not considered spies: soldiers or civilians, carrying out their mission openly, charged with the delivery of despatches destined either for their own army or for that of the enemy. To this class belong likewise individuals sent in balloons to deliver despatches, and generally to maintain communication between the various parts of an army or a territory.