(The following was published in “The Crimes of War: What the Public Should Know,” a handbook about war crimes.)

Slobodan was being a gracious host. Whenever we entered an exposed stretch of territory, he would stop, listen like a terrier for signs of trouble, and then sprint ahead, waving me on when he felt it was safe. It was the winter of 1993, the Bosnian war was in full throttle, and Slobodan was taking me to his place of work, a ransacked apartment in a bombed-out building on the frontline around Sarajevo.

Slobodan was a Bosnian Serb sniper. Because he was off duty when he showed me around, he was armed only with a pistol. Once we reached his perch, though, he cheerfully pointed it at the Sarajevans running across exposed ground a few hundred yards away. “I can shoot!” he said in excited English. “Look, look people, pistol, pop-pop!” Then he calmed down and smiled. “No problem, no problem. No shoot people. No, no shoot.”

What he actually was saying was that he didn’t shoot civilians, only soldiers. This was improbable. I had been in Sarajevo long enough to know that civilians were pretty much the only targets of snipers like Slobodan. I had talked to people who were shot by snipers, I saw a youth get shot near the Holiday Inn near the frontline, and I knew, as everyone did, that the cold weather and lack of food in Bosnia’s besieged capital were not the worst killers. It was the snipers you worried about the most, because they were the ones firing all those bullets that found their way into so many arms and legs and heads and hearts.

There is no prohibition on sniping at combatants during wartime, but the intentional killing of civilians is a war crime. And it was likely that Slobodan and his sniping pals were guilty of willful killing-the legal term for that crime-many, many times over.

The 1977 Additional Protocol I to the Geneva Conventions makes it clear that civilian deaths that are incidentally, even if foreseeably, caused by justifiable military operations are legal, subject to the principle of proportionality. But if the killing of a civilian, a noncombatant, is intentional or is not justified by military necessity, a war crime has been committed. For example, the execution of hostages or prisoners would be such a crime. In an international conflict, the violation could be prosecuted as willful killing under the grave breaches provisions of the Geneva Conventions; in an internal conflict, the crime could be prosecuted as murder under domestic law or under Article 3 Common to the Four Geneva Conventions of 1949.

The sorts of people covered under the willful killing and murder rubrics included, among others, not just civilians in the ordinary sense of the word, but prisoners of war, sick or wounded or surrendering soldiers, and medical and religious personnel.

The crime of willful killing is an active component of international law. The International Criminal Tribunal for the Former Yugoslavia is empowered to prosecute acts of willful killing, and several defendants have been charged with the crime. However, in a controversial decision in the case of Dusko Tadic, a Serb prison guard, the tribunal ruled that Tadic could not have committed acts of willful killing because the Bosnian war was not international in character. As a result, Tadic was declared not guilty of the willful killing charges, although the ruling is under appeal.

With the Tadic case in mind, what penalties might a sniper like Slobodan face? If the Bosnian war is ultimately determined to have been an international conflict, he could be charged with willful killing for each protected person he shot. If it is determined to be an internal conflict, he could be charged with murder. But whatever the charge, he would be subject to multiple counts. On the day he served so politely as my tour guide, I asked whether he had shot anyone. “Today, no,” he replied. “Yesterday, yes. Pop, pop!”