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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 V: On Military Authority over enemy and neutral aircraft and persons on board. - Art. 33.
Art. 33. Belligerent non-military aircraft, whether public or private, flying within the jurisdiction of their State, are exposed to being fired at if they do not land at the nearest suitable point when an enemy military aircraft is approaching.