ICRC databases on international humanitarian law
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1949 Conventions and Additional Protocols, and their Commentaries
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Project of an International Declaration concerning the Laws and Customs of War. Brussels, 27 August 1874.
Spies - Art. 19.
Art. 19. A person can only be considered a spy when acting clandestinely or on false pretenses he obtains or endeavours to obtain information in the districts occupied by the enemy, with the intention of communicating it to the hostile party.