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Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923.
Part II: Rules of Air Warfare#Chapter III: Belligerents. - Art. 16.
Art. 16. No aircraft other than a belligerent military aircraft may take part in hostilities in any form whatever.
The term "hostilities" includes the transmission, during a flight, of military information for the immediate use of a belligerent.
No private aircraft may be armed in time of war outside the jurisdiction of its own country.