For centuries, armies have tried to capture cities by surrounding them, blocking access roads, and then bombarding the encircled territory until the townspeople, or their defenders, give up. Constantinople was besieged at one time or another by the Persians, the Arabs, the Bulgars, and the Russians. The siege of Leningrad (St. Petersburg) during World War II lasted 872 days and resulted in the deaths of more than a million civilians.

Fifty years later, it was Sarajevo’s turn. In May 1992, the Bosnian Serb Army, having failed in an attempt to overrun the city, closed all roads leading in and out of Sarajevo, blocked commerce, and began to pound city neighborhoods with artillery and sniper fire from all directions. Human rights groups, relief agencies, and some governments said the Serb war tactics around Sarajevo subjected the city to a siege, immorally and illegally.

Under international humanitarian law (IHL), siege is not prohibited per se. The capture of an enemy-controlled city is a legitimate military aim, and army commanders have often seen siege as less costly than the alternative—fighting house to house, street by street. Historically, a key element of siege warfare has been to reduce a town’s defenses and force its surrender by cutting off its vital supplies and leaving the population, civilian and military alike, to starve. Cruel as this tactic is, the laws of war permitted it at least until the end of World War II, under the rationale of military necessity.

The last revision of the U.S. Army field manual on the law of land warfare, for example, informed commanders that civilians who are fleeing a besieged city can, as an “extreme measure,” be turned around and forced back into the city for the specific purpose of “hastening its surrender.” Implicitly, this principle permitted the deliberate starvation of the civilian population, if only as a means of demoralizing the city’s armed defenders. Under pre-World War II Hague Regulations, “undefended” cities could not be bombarded, but siege tactics are normally used against defended places, so this prohibition did not exclude sieges.

The laws pertaining to siege warfare, however, have changed radically in the post-World War II era. Though the word “siege” is never mentioned as a term of law, the Additional Protocols of 1977 impose restrictions on warfare that, if enforced, would effectively make siege illegal. Besieging forces are not allowed to target civilians or starve them “as a method of warfare,” and relief agencies are authorized to provide aid to needy populations.

The most important limitations are the rules regarding “objects indispensable to the survival of the civilian population,” including food and water supplies. The Fourth Geneva Convention of 1949 upheld the traditional view that an army may legally block food or other relief shipments into a besieged city if the aid would result in more goods becoming available to the local military forces. But Article 54 of the First Additional Protocol contains an absolute ban on the starvation of civilians as well as forbidding the destruction of foodstuffs, crops, livestock and drinking water supplies that a civilian population relies on for sustenance. This provision may require a besieging force to allow relief supplies to enter a besieged city, even if some of the supplies will inevitably be shared with the defenders. A besieging army is also forbidden, for example, from destroying a city’s drinking water supply.

Comparable rules are found in the Second Additional Protocol, applicable in internal conflict. Even when the protocols do not apply, prohibitions on the starvation of civilians are now widely seen as part of customary law. In addition, rules against the deliberate targeting of civilians would apply during sieges as at other times.

In Sarajevo, officials of the United Nations Protection Force (UNPROFOR) questioned whether the city was genuinely besieged, since Serb forces occasionally allowed UN relief convoys to pass into the city. Comdr. Barry Frewer, the UNPROFOR spokesman, summarized the official view while briefing journalists in July 1993: “The Serbs have encircled the city,” he acknowledged. “They are in a position to bring force to bear on the city. You call it a siege. We say they are deployed in a tactically advantageous position.”

The dispute was both ridiculous and irrelevant. No word other than “siege” as effectively described the city’s condition, and in any case the terminology did not resolve the question of whether Serb military actions were lawful. The critical issue was whether the means of Sarajevo’s continued encirclement violated IHL. Some parts of the war record leave little doubt. Indiscriminate shelling of residential areas is clearly illegal. Serb army commanders had modern artillery, a clear line of sight toward their Sarajevo targets, and three years of practice. UN military observers regularly reported deliberate civilian targeting by Serb gunners.

The attacks on Sarajevo prompted war crime indictments against Serb commanders judged responsible for firing “on civilian gatherings that were of no military significance.” In December 2003, the Yugoslav war crimes tribunal convicted Major General Stanislav Galic, commander of the Bosnian Serb army unit around Sarajevo from 1992 to 1994, of the war crime of conducting a campaign of terror against the civilian population of the city, and of the crimes against humanity of murder and inhumane acts. He was sentenced to twenty years’ imprisonment.

Might some army find a way to besiege a city without violating IHL? The limits have been tested by Taliban forces in Afghanistan, Hutu rebels in central Africa, and by Russian forces in Chechnya. Lawful or not, siege warfare remains as popular as ever.