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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 IV: Hostilities#Bombardment. - Art. 23.
Art. 23. Any air bombardment carried out for the purpose of enforcing requisitions in kind or payments of contributions in ready money, is forbidden.