They had fled from Srebrenica, the world’s first UN-declared civilian safe area, after it had fallen to the Bosnian Serbs on July 11, 1995, and the tales several dozen of these Bosnian Muslim men told were chillingly consistent.

The Dutch UN peacekeepers and the NATO jets the UN Security Council had promised would protect Srebrenica had put up scant resistance. The men knew that they were earmarked for death and they had fled, joining a three-mile-long, single-file column of fifteen thousand mostly unarmed men trying to make their way across thirty miles of enemy territory to reach Bosnian government lines. Over and over again along the line of march, hundreds had been killed in a series of well-planned ambushes by Bosnian Serb forces. After that, Bosnian Serb soldiers wearing stolen UN uniforms and driving stolen UN vehicles announced over megaphones that that they were UN peacekeepers and that they were prepared to oversee the Bosnian Muslims’ surrender and guarantee they would not be harmed.

Disoriented and exhausted, many Bosnian Muslims fell for the lie. It was only after they had surrendered that they discovered their fatal mistake. For in surrendering, they were going to their deaths. Those whom the Serbs got their hands on were killed by firing squad.

Srebrenica was the worst massacre in Europe since World War II. The shock at what took place there was so great that to separate war crimes from entirely licit military actions seemed, and in many ways still seems, almost obscene. And yet from the point of view of international humanitarian law, the ambushes the Serbs sprang on the fleeing Bosnians were legal ruses soldiers can employ in wartime. Investigators for the International War Crimes Tribunal for the Former Yugoslavia said the fact that many of those in the retreating column were Bosnian government soldiers made the column a military threat and thus a legitimate target. In legal terms, the Bosnian Serb ambushes did not lull the Bosnian Muslims into a false sense of protection under international law, rather they led them to miscalculate the nature of the threat.

What was entirely criminal was the Bosnian Serbs’ use of UN emblems and matériel to lure the fleeing Muslims to surrender—a clear example of a war crime. The prohibition in modern times against what is alternatively called “perfidy” and “treachery” goes back to the American Civil War. But the definitive statement banning the kind of deceptions the Bosnian Serbs engaged in on the roads out of Srebrenica can be found in Articles 37, 38, and 39 of the 1977 Additional Protocol I to the Geneva Conventions.

Article 37 of Protocol I states that “acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence, shall constitute perfidy.” And Article 38 explicitly prohibits the use of “the distinctive emblem of the United Nations, except as authorized by that organization.” It also prohibits the “improper use of the distinctive emblems of the red cross, red crescent or red lion,” which, if used perfidiously, is a grave breach.

The difference between perfidy and treachery is the difference between wrongful deception and betrayal. Perfidy is making someone believe a falsehood, while betrayal involves an act that actually harms the person. In international law, treachery and perfidy are used interchangeably.

Other examples of perfidy are feigning to negotiate under a flag of truce or surrender, feigning to be incapacitated by wounds or sickness, and feigning of civilian, noncombatant status. Article 39 prohibits the use of flags, military emblems, insignia, or uniforms of the opposing side at the time of an attack in order to protect or impede military operations.

But the protocol states explicitly that ruses of war are not prohibited. A ruse is an act that is intended to mislead an adversary or to “induce him to act recklessly” but which infringe no rule of armed conflict and do not attempt to gain his confidence by assuring protection under law. Camouflage, decoys, mock operations, and misinformation are all permitted ruses. An example of a legal ruse was when U.S. forces gathered at sea during the Gulf War to trick Iraq into thinking an amphibious assault was imminent; the attack eventually came by land. Another example might be sending a bomber toward barracks in order to draw air defense away from a shipyard.

In the case of the Muslim men of Srebrenica, the textbook definition of perfidy, which is actually extremely narrow, was all too tragically fulfilled. The Bosnian Muslims who surrendered did so because they were tricked by the Serbs into believing they were in the presence of UN peacekeepers. This was not a ruse, which is legal in warfare, but treachery pure and simple, and it cost thousands of people their lives.

The killing of all those who surrendered, who were hors de combat, is, of course, the gravest breach of all.