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Convention (IV) respecting the Laws and Customs of War on Land and its annex: Regulations concerning the Laws and Customs of War on Land. The Hague, 18 October 1907.
Annex to the Convention: Regulations respecting the laws and customs of war on land - Section II : Hostilities - Chapter I : Means of injuring the enemy, sieges, and bombardments - Regulations: Art. 26.
Art. 26. The officer in command of an attacking force must, before commencing a bombardment, except in cases of assault, do all in his power to warn the authorities.