The old woman reached into her plastic bucket and pulled out a triangular shard of human bone, still pink and glistening, a gruesome reminder of her daily struggle to find water.

She had been waiting in line for water, at a standpipe protected, she had thought, by the ruins of an old school, when a mortar bomb crashed through a hole in the roof. Seven people were killed and twelve wounded, including the elderly woman herself, who had a slight graze on her forehead and a serious case of shattered nerves.

Collecting water was one of the most dangerous and dispiriting daily tasks in Sarajevo between April 1992 and December 1996. The secessionist Serb forces frequently cut off the city’s water supply (and its gas and electricity as well). And even when they permitted the water to flow, they routinely fired on those queuing up for it.

As a general principle, in both internal and international armed conflict it is lawful to attack only military objectives. From this derives the rule stated in Article 54 of the first of the two 1977 Additional Protocols to the Geneva Conventions that “starvation of civilians as a method of warfare is prohibited.” Denying the civilian population water is just as illegal as denying them food. Article 54 states that “it is prohibited to attack, destroy, remove or render useless objects indispensable to the survival of the civilian population,” and includes not only foodstuffs, livestock, and the like, but “drinking water installations and supplies and irrigation works.”

But though the idea would have made little sense to the people of besieged Sarajevo, water supplies do not enjoy absolute protection under international law. If water supplies are being used exclusively by civilians, legally they are supposed to be immune. But if they are being used by both combatants and noncombatants, the picture changes.

The premise of all laws of war is that it is perfectly legal to attack legitimate military targets. So when water and waterworks are used exclusively to sustain military forces they can be targeted. Moreover, according to the language of Additional Protocol I, where water and waterworks are used “in direct support of military action” they can be destroyed. For example, if a water installation is being used by soldiers as a firing position or to conceal supplies then it, like other generally protected spaces such as hospitals, is not legally protected from attack.

Nonetheless, these exceptions are not as broad as they first appear and contain their own limitations and exceptions. The laws state that any harm to civilians must not be excessive compared to a concrete and direct military advantage. And Additional Protocol I, Article 54 states that “in no event” shall actions against targets such as waterworks be undertaken when they may be “expected to leave the civilian population with such inadequate food or water as to cause its starvation or force its movement.” Military necessity alone does not give soldiers license to destroy a water installation if it is indispensable to the survival of the civilian population.

Whether what went on in Sarajevo when the Serbs cut off the water was a war crime is problematic. Often the real crime seems to have been sniping at and shelling the people lined up for water, rather than the cut-off itself. A deliberate and systematic cutting off of water to the civilian population, however, would be a war crime.

Fearing that the Sarajevo experience is the shape of things to come, and convinced that in an increasing number of conflicts the lack of clean water kills more people than bullets or bombs, the International Committee of the Red Cross (ICRC) is campaigning to have a blanket ban on attacking waterworks. It is pressing to have the immunity from attack, of the sort given to medical staff, given in practice to water engineers and other personnel seeking to keep water supplies flowing or to repair water systems. Such personnel are protected under Additional Protocol I because they fall into the category of civil defense personnel. The ICRC position asserts “the absolute imperative” of water for the survival of the civilian population.