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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 VI: Duties of belligerents towards neutral States and duties of neutrals towards belligerent States. - Art. 44.
Art. 44. It is forbidden to a neutral government to supply a belligerent Power, whether directly or indirectly, with aircraft, component parts thereof or material or ammunition for aircraft.