The right of belligerents to have recourse to bombardment against fortresses and other places in which the enemy is intrenched is not contestable, but humanity requires that this form of violence shall be so restrained as to limit as much as possible its effects to the armed forces of the enemy and to their defenses.
Hence,
33. The commander of an attacking force must do everything in his power to intimate to the local authorities his intention of bombarding, before the bombardment commences, except when bombardment is coupled with assault.
34. In cases of bombardment, all necessary measures ought to be taken to spare, so far as possible, buildings devoted to religion, the arts, sciences, and charity, hospitals, and places in which sick and wounded are kept; provided always that such buildings are not at the same time utilized, directly or indirectly, for defense.
It is the duty of the besieged to indicate these buildings by visible signs, notified to the besieger beforehand.
(b) Of the sanitary matériel
The rules (Arts. 10 and those following) for the protection of the wounded would be insufficient if special protection were not also given to hospitals. Consequently, in accordance with the Convention of Geneva,
35. The ambulances and hospitals used by armies are recognized as being neutral, and must be protected and respected as such by the belligerents, so long as there are sick and wounded in them.
36. A like rule applies to private buildings, or parts of private buildings, in which sick and wounded are collected and cared for.
Nevertheless,
37. The neutrality of ambulances and hospitals ceases to exist if they are guarded by a military force, a police post being alone permissible.