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The Laws of War on Land. Oxford, 9 September 1880.
Part II : Application of general principles #I. Hostilities #B. Rules of conduct with regard to things #(a) Means of injuring -- Bombardment - Art. 33.
' If it is incontestable that belligerents have the right to resort to bombardment against fortresses and other places in which the enemy is intrenched, considerations of humanity require that this means of coercion be surrounded with certain modifying influences which will restrict as far as possible the effects to the hostile armed force and its means of defense. This is why '
Art. 33. The commander of an attacking force, save in cases of open assault, shall, before undertaking a bombardment, make every due effort to give notice thereof to the local authorities.