War criminals have been prosecuted at least since the time of the ancient Greeks, and probably well before that. The idea that there is some common denominator of behaviour, even in the most extreme circumstances of brutal armed conflict, confirms beliefs drawn from philosophy and religion about some of the fundamental values of the human spirit. The early laws and customs of war can be found in the writings of classical authors and historians. Those who breached them were subject to trial and punishment. Modern codifications of this law, such as the detailed text prepared by Columbia University professor Francis Lieber that was applied by Abraham Lincoln to the Union army during the American Civil War, proscribed inhumane conduct and set out sanctions, including the death penalty, for pillage, raping civilians, abuse of prisoners and similar atrocities. Prosecution for war crimes, however, was only effected by national courts, and these were and remain ineffective when those responsible for the crimes are still in power and their victims remain subjugated. Historically, the prosecution of war crimes was generally restrained to the vanquished or to isolated cases of rogue combatants in the victor's army. National justice systems have often proven themselves to be incapable of being balanced and impartial in such cases.

The first genuinely international trial for the perpetration of atrocities was probably that of Peter von Hagenbach, who was tried in 1474 for atrocities committed during the occupation of Breisach. When the town was retaken, von Hagenbach was charged with war crimes, convicted and beheaded. But what was surely no more than a curious experiment in medieval international justice was soon overtaken by the sanctity of State sovereignty resulting from the Peace of Westphalia of 1648.