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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 VII: Search, capture and confiscation. - Art. 50.
Art. 50. Belligerent military aircraft have the right to give order to public non-military aircraft to land or to alight on water, or to repair to a suitable and reasonably accessible place for the purpose of being searched.
The refusal, after warning, to obey the order to land or to alight on water at such place for the purpose of being searched, exposes the aircraft to being fired at.