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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 #(e) Who may be made prisoners of war - Art. 21.
Art. 21. Individuals who form a part of the belligerent armed force, if they fall into the hands of the enemy, are to be treated as prisoners of war, in conformity with Articles 61
The same rule applies to messengers openly carrying official dispatches, and to civil aeronauts charged with observing the enemy, or with the maintenance of communications between the various parts of the army or territory.