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The Laws of War on Land. Oxford, 9 September 1880.
Part II : Application of general principles #I. Hostilities #A. Rules of conduct with regard to individuals #(f) Spies - Art. 24.
Art. 24. Individuals may not be regarded as spies, who, belonging to the armed force of either belligerent, have penetrated, without disguise, into the zone of operations of the enemy, -- nor bearers of official dispatches, carrying out their mission openly, nor aeronauts (Article 21