A situation may arise where the target changes its appearance from civilian to military or vice versa. For instance, if anti-aircraft batteries are stationed on a school roof or a sniper is positioned in a mosque’s minaret, the protection imparted to the facility by its being a civilian object will be removed, and the attacking party will be allowed to hit it … A reverse situation may also occur in which an originally military objective becomes a civilian object, as for instance, a large military base that is converted to a collection point for the wounded, and is thus rendered immune to attack. 

It may be the case that a target might change its status from civilian to military or vice versa. For example, if an anti-aircraft battery is positioned on the roof of a school or if a sniper takes up a position on the minaret of a mosque, the protection provided for the facility by the virtue of it being [a] civilian target is no longer valid, and the attacker is permitted to attack it. The legal responsibility for the deaths of civilians in such a case is that of the side that made unreasonable use of a civilian target rather than on the side who attacked this target. In the case of incidents in which there is a doubt as to whether the target changed its status from civilian to military, the Additional Protocols determine that it should be assumed that it is not a military target unless proven otherwise.

The opposite situation may occur, in which a target that was originally military changes into a civilian target, such as a large military base converted into a clearing station for the wounded. In such cases, it must not be attacked as it is a medical facility (on the assumption that no military activities are conducted therein, being disguised as treatment for the wounded). 

The loss of absolute protection for a civilian site when it is misused by the adversary as a locus for military operations is broadly recognised in the Law of Armed Conflict. Thus, for instance, the hidden placement of a significant military asset within a civilian building or even the presence of enemy combatants can make the otherwise civilian site amenable to attack. This is a harsh reality of urban warfare.  [footnotes in original omitted]

110. … [A] commander’s intent is critical in reviewing the principle of distinction during armed conflict. Where it is believed in good faith, on the basis of the best available intelligence, that a civilian building has been misused as a sanctuary for military fighters, military intelligence, or the storage and manufacture of military assets, the commander has a legitimate basis for using force against the site. This is so even where judgment is based on limited information in a fluid battlefield situation.

111. The definition of military targets thus could include terrorists who move rapidly throughout a neighbourhood, even where they shelter themselves in civilian dwellings. It does not relieve the commander of the obligation to judge the proportionality of his action. But it makes clear that a civilian site can be converted to a legitimate target by the conduct of the opposing force in using such places for military purposes, including the escape of armed combatants.