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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. 30.
Art. 30. When a belligerent commander deems the presence of aircraft to be able to endanger the success of the operations in which he is engaged, he may forbid the passage of neutral aircraft in the immediate vicinity of his own forces or impose on them an itinerary. A neutral aircraft which does not comply with such prescription of which it has cognizance through a publication of the belligerent commander, exposes itself to being fired at.