Share This Article

From ancient origins the informal rules of war have developed into a complex code designed to curb man’s lawless violence. But does it work?

All’s fair in love and war. In love, perhaps—in war almost never.

Despite the impression that fighting in our era of terrorism and guerrilla warfare is no-holds-barred, armed conflicts between sovereign states do have generally accepted rules both formal and informal. The notion that rules go out the window as soon as the shooting starts is exaggerated, though no set of rules, even if scrupulously observed, can change war’s basic violent nature.

Still, some wars are bloodier than others. Culture, political aims, social organization, economic means and technology shape the style of warfare in a particular epoch. Wars for honor, power, territory and natural resources can differ markedly from wars of religion or race in terms of ferocity and intensity. Humanitarian concerns and chivalry—the sense of fair play—interact with military necessity in shaping rules, although civil wars, revolts and conflicts with supposed “inferiors” can affect the rules’ application. Observance of rules could depend on one’s enemy, whether a neighbor sharing the same culture or a total stranger. Even the pre-state warfare of primitive peoples, despite incidents of massacre and annihilation, may have had ritualistic aspects with restraints on violence.

The Western military tradition has its origins in Greco-Roman antiquity, an age that already knew certain customary laws of war. Greeks and Romans equated international norms with natural law. The Latin terms for international law, jus gentium (“the law of nations”), and the law of war, jus belli, imply a fundamental universal law. Current headlines about war crimes, the right to prisoner of war status, and appropriate and inappropriate means of combat echo the concerns of ancient Greeks and Romans.

In the Middle Ages the tradition of Roman law combined with Christian interpretations of Greek thought and Scripture to produce a new sense of Western community—Christendom, headed by the pope. Rules of conduct for Christians, however, need not apply to relations with non-Christians. Later, during the Reformation and an age of religious wars, Dutch jurist Hugo Grotius tried in his foundational On the Law of War and Peace (1625) to isolate discussions of international law from concepts of both natural law and Catholic doctrine. This secularization of international law in Europe coincided with a new emphasis on treaties and conventions as sources for international norms. But then, in the 18th- and 19th-century era of colonial empires, a new dichotomy about the rules’ application appeared: civilized vs. uncivilized nations. Against the latter, the former’s rules could be bent or ignored. Only with the establishment of the United Nations in 1945 did such distinctions—officially at least—subside.

Modern laws of war embrace two aspects: the right to war (jus ad bellum) associated with a legitimate cause (i.e., a just war), and the rules of conduct in war (jus in bello), including prohibition of specific acts. Many of these rules reflect a long tradition, with perhaps the earliest written code appearing in the Old Testament. The Deuteronomic historians, active in the late 7th century BC, attributed to Moses a set of rules (in Deuteronomy, Chapter 20): A city under siege had to be offered the opportunity to surrender. If accepted, its inhabitants became the Israelites’ servants; but if a battle was required, the Israelites were to slaughter all adult males, enslave the women and children, and take all property as booty. Destruction of fruit trees was specifically prohibited. Historians traditionally date the code’s purported historical context, the Israelite acquisition of Canaan, to the Late Bronze Age (c. 13th– 12th centuries BC). Such rules, however, applied only to distant populations. Conquered peoples within the Promised Land—Hittites, Amorites, Canaanites and others—were to be annihilated. As elsewhere in the ancient Near East, the acknowledged god(s) sanctioned war; jus ad bellum was not a concern.

The Greeks established the Western tradition of laws of war. The Archaic period (750–490 BC) saw development of the “common laws of the Greeks,” unwritten but generally recognized rules for interstate relations, and some of those directly relate to war. Like the Roman use of jus, these rules reflect traditional practices, not legislation, and some probably long antedated the Archaic period. In 430 BC Pericles told the Athenians that violation of the unwritten laws was shameful, and his contemporary Protagoras postulated shame and justice as twin restraints on war’s violence. Among the Greek common laws:

  • War should be openly declared and have a legitimate cause if the gods’ favor was to be expected.
  • A pledged word must be kept. Oaths, sworn to the gods and accompanied by a sacrifice or libations, guaranteed treaties, truces and other agreements. Absence of an oath, or a flaw in administering one, nullified the agreement. Oath-breakers would incur the gods’ punishment.
  • Heralds were sacrosanct. Associated with Hermes, the messenger god, heralds often accompanied diplomatic missions, and their persons were inviolate, a privilege in Greek practice not always extended to ambassadors. Heralds announced the declaration of war on an opposing city or accompanied embassies to the enemy for negotiations. The significance of oaths and heralds is already clear in the earliest Greek literature, Homer’s Iliad and Odyssey.
  • Temples, with their treasuries, and other religious precincts were sacrosanct during hostilities, and violation of sacred property was akin to waging war on the gods. The stipulation in modern laws of war to protect “cultural property” is an ancient idea.
  • A general truce suspended hostilities during festivals at such Panhellenic centers as Olympia or Delphi.
  • Prisoners who surrendered on the battlefield received quarter, but the fate of the surrendered after a siege was at the victor’s discretion.
  • A truce had to be granted the defeated for recovery and burial of the dead.

These laws functioned within unwritten conventions of limited warfare between major city-states of the Greek mainland. As border disputes were then the most common cause of war, annihilation of the opposing army or capture of its city were not war aims. Wars, usually local affairs, reflected the short-term service of a city’s army, essentially a militia providing its own arms and equipment for combat as heavy infantry (hoplites) in a phalanx. Except for the Spartans, Greek hoplites received little or no formal training, and Greek cities lacked logistic capabilities for lengthy and distant campaigns. Hence these wars, affairs of the summer season, usually amounted to a single bloody clash of rival phalanges on an open plain. Battle could be provoked by invasion of the opponent’s territory and destruction of his crops. Often a formal challenge preceded combat. War and especially battle, however, were affairs of honor, requiring an open, face-to-face trial of strength. The ferocity of the actual fighting knew no restraints, but honor proscribed surprise attacks. Tactical maneuvers in battle were limited not only by honor but also by the inflexible character of the original hoplite phalanx and the lack of command and control after the initial disposition of forces. Commanders fought in the front ranks themselves and scarcely had an overview of the battle.

“Rules” for hoplite combat probably derived from an earlier period, when duels of champions decided border disputes. The memory of such duels survived in the Classical era (490–323 BC). Just as a formal duel specified boundaries for the action, so the plain of a hoplite battle marked its “lists.” Final possession of the battlefield determined the winner, and pursuit of the defeated beyond the immediate scene of action was discouraged. The loser conceded victory in requesting a truce to recover its dead. The Greek failure to develop concepts of strategy, techniques of siege-craft beyond starving a city with a blockade or even rudimentary reconnaissance in field operations reflects this system of limited warfare with a clear set of rules.

Yet these rules did not apply everywhere or under all circumstances. On the periphery of the Greek world, especially in colonial wars against non-Greeks, the rules need not be observed. Similarly, the conventions of formal field battles did not apply to raids and small-scale operations. More significant, the Greeks recognized a special category of war “without heralds” or “without truce,” that is, war without formal rules about restraints on behavior. Similarly, in civil wars no rules restrained behavior.

In the 5th century BC the Greeks’ conflicts with the Persians, who did not recognize the Greek “rules of the game,” and the creation of a maritime Athenian empire—which expanded Athens’ power far beyond that of a typical Greek polis—began to change the operational conventions of battles and campaigns. The culmination of that process came in the showdown of the Athenian empire with Sparta and its coalition in the Peloponnesian War (431–404 BC). While the laws of the Greeks remained unchanged, the increased ferocity and intensity of the struggle bent, broke or flouted operational niceties. Trickery and deception, never entirely absent before, clashed with concepts of honor, as war came to be conceived as a rational activity subject to human calculation. Belief in the gods’ role in the ultimate outcome remained, but apparently the gods also helped those who helped themselves.

If the Greco-Persian wars had heightened a sense of Greek self-awareness, a generation of slaughter and destruction in the Peloponnesian War aroused a notion of Panhellenism: Greek energies should be turned from fratricide between Greeks to defense against such barbarians as the Persians. Destruction of a Greek city, for example, came to be seen as a barbarian act. Fourth century BC intellectuals and historians, looking back across the bloody gulf of the Peloponnesian War, idealized the limited warfare of the Archaic period. A supposed oath taken by all member cities of the religious league administrating Apollo’s temple at Delphi stipulated that no member should destroy, starve or cut off from running water another member city, on penalty of its own destruction.

Similarly, the fervent Panhellenist historian Ephorus invented the story of a prohibition on missile weapons in a late 8th century BC war between the cities of Chalcis and Eretria—perhaps in reaction to the introduction of catapults, a mechanization of weaponry that threatened traditional notions of honor and heroism in combat. Even Plato in his Republic (c. 380 BC) devised a new Panhellenic set of rules for Greek warfare, prohibiting enslavement of Greek cities or decoration of temples with spoils from defeated Greek armies, and limiting destruction of territory to crops, while exempting lands and dwellings. In the same spirit Aristotle’s lost Just Acts of War (c. 334 BC, the earliest referenced monograph on just war) defended wars against barbarians, approved wars for self-defense and proposed means to settle border disputes.

Harsh realities, however, contrasted with such Panhellenic sentiments. The phrase “law of war” (nomos polemou) first appeared soon after the Peloponnesian War. It denoted the right of the victor to dispose of a defeated enemy and his property as he saw fit, thus asserting the loser had no rights. After a successful siege, execution of adult males and the enslavement of women and children became common practice, moderated only by a victor’s political expediency or generosity. This “right of the victor” is the most common meaning of the phrase “law of war,” when used in the singular, in both Greek and Latin sources.

Alexander the Great’s conquest of the Persian empire (334– 323 BC) opened a new cosmopolitan age with the incorporation of the Near East into the Greek world. The massive Macedonian empire fragmented after Alexander’s death into several military monarchies founded by his generals, whose legitimacy rested solely on the right of conquest. Despite bloody battles of now mainly mercenary forces, a sense of competition among Alexander’s successors tempered the destruction and enslavement of cities. Such acts could alienate the conquered, their new subjects, and damage property, now the victor’s own. After 280 BC, when competition yielded to an equilibrium of power among dynasties, the consequent intrigues, deceit and trickery in diplomacy and on the battlefield prompted the historian Polybius to bemoan the despicable conduct of Greek interstate relations.

In the broadened geographical horizon of the Hellenistic period, the old “laws of the Greeks” became the “laws of men.” Peripatetic philosophers, followers of Aristotle, wrote works on oaths, treaties and even military affairs. The new philosophical school of the Stoics—believers in a brotherhood of mankind and much concerned with ethics—discussed proper behavior in war. Cicero later incorporated Stoic views about war in his On Duties (44 BC). His work became an important source for early modern theorists of international law like Grotius. Likewise, later Roman-era historians, such as Livy and Plutarch, reflected Stoic views in writing (in the plural) of “laws of war.” In a famous example, the Roman general Marcus Furius Camillus, when besieging the town of Falerii in 394 BC, refused the local schoolteacher’s offer to surrender to the Romans the children in his charge as potential hostages to leverage the town’s surrender. Such treachery repulsed Camillus, who evidently believed human society must incorporate laws of war that dictated honor and proper behavior. Impressed by Camillus’ rejection of a traitor’s offer, the town voluntarily surrendered.

In assessing diplomatic and military behavior of the late 3rd and 2nd centuries BC, Polybius provided the most detailed discussions of laws of war: keeping oaths; good faith in both treaties and truces; the necessity of a declaration of war; the inviolability of ambassadors; sparing prisoners taken on the battlefield; the immunity of religious sanctuaries and their treasuries; and limiting destruction or capture to such elements that support the opponent’s war effort, such as forts, harbors, cities, men, ships and crops—but sparing noncombatants or items of long-term value like fruit trees. He also invoked issues of honor, regarding ambushes, nocturnal attacks and feigned retreats as shameful. For the rationalist Polybius, observance of these rules depended upon a state’s expediency and wise policy.

Rome’s rise to prominence brought its own version of laws of war that, like the Roman constitution, remained unwritten. Some coincided with Greek practice, while others reflected Rome’s legalistic tendencies. Its armies scrupulously observed the inviolability of ambassadors and sanctuaries, unless an enemy’s cults were seen as potential inspiration for later revolt. Above all, Romans prided themselves as people of good faith (bona fides) who always kept their word.

Just war (bellum justum), the doctrine most associated with Rome, underwent considerable interpretation in later periods by Saints Augustine (354–430) and Thomas Aquinas (1225–74) and others. According to Roman tradition, a priesthood, the fetiales, oversaw the proper procedures for declaring war—procedures initially applied only to local conflicts. War could be properly declared only after another state had declined a Roman demand to correct a grievance. Refusal gave Rome a just cause for war. The meaning of justum remained ambiguous: whether “just” from an ethical and moral perspective, or “just” in the sense that appropriate procedures were followed. Roman expansion outside Italy rendered the fetial procedure cumbersome, and it eventually vanished. To the stricter sense of law of war—the right of the victor—Romans added their own wrinkle: the “battering ram rule.” During a siege Rome would accept surrender under favorable terms only before a battering ram first struck the city’s walls. Thereafter little or no quarter could be expected.

The concept of hostis justus—a regular, legally defined enemy—emerged as a lasting contribution to the Western tradition of laws of war, and it clearly remains important today. Bandits, pirates and rebels were not regular enemies, so rules of proper warfare did not apply to them. Barbarians largely fell outside the status of regular enemies, although by the 3rd century Germans and the Iranian Parthians had achieved that status. The idea of a hostis justus survives in current controversies over the right of terrorists and guerrillas to prisoner of war status.

Enforcement of rules of war, not surprisingly, has always been problematic. Great powers can flout rules with impunity. Among the ancients, punishment for violations lay with the gods, the victor’s discretion or the court of public opinion. Then, as now, a nation risked both its reputation and continued influence in the international community for alleged war crimes. Ancient punishments for war crimes are a matter of record: In 405 BC, after the Spartan coalition defeated the Athenian fleet at Aegospotami, the Spartans executed several thousand prisoners, including Philocles, an Athenian general who in a previous engagement had ordered the captured crews of two Spartan triremes thrown overboard; he subsequently initiated a decree that thereafter prisoners of the Spartan coalition would have their right hands cut off. Later, around 240 BC, a general of the Achaean League, Aratus of Sicyon, was tried in absentia and fined for his surprise attack on the city of Argos in a bid to oust a tyrant—though no evidence exists he ever paid the fine.

Even the Romans were sensitive to what would now be viewed as war crimes. The Roman senator Cato the Younger wanted Julius Caesar surrendered to the Germans for his violation of a truce leading to a massacre, a preliminary to his 55 BC crossing of the Rhine. The historian Sallust had to defend Roman general Gaius Marius against the appearance of a war crime. In 107 BC he had captured by surprise attack the Numidian city of Capsa. Despite the town’s surrender, Marius burned it and massacred or enslaved its inhabitants, citing the city’s strategic importance and the population’s untrustworthiness. An argument from “military necessity” also appears at the siege of Jerusalem in AD 70. Titus, the Roman commander and later emperor, held a council of war to decide the fate of the Jewish Temple. Some urged its destruction under the law of war: As a potential rallying point for the rebels, it had to be demolished. Others thought it should be saved unless used for military purposes. The latter view corresponds to modern practice on destruction of cultural property.

As unwritten customs, the ancient laws of war were open for dispute or even sometimes invented as a pretext. Under the common laws of the Greeks, the legality of consulting an oracle about a war between Greeks was disputed, as was the legality of economic warfare. Striking, too, is the appeal to different laws by opposing sides: After the 424 BC Battle of Delium the victorious Boeotians refused to grant a truce for return of the Athenian dead, since the Athenians had occupied and fortified the nearby sanctuary of Apollo, which included a sacred spring. The Athenians replied that they held the sanctuary by right of conquest and used the spring through military necessity, an act pardonable by the gods. Such appeals to military necessity as an excuse for violation of laws of war have an all-too-familiar modern ring.

Laws of war are intended to structure and restrain war’s violence and to introduce humanitarian considerations. The Greeks and Romans initiated this practice in the Western military tradition. They set the parameters within which “the game” was to be played and provided many precedents for later developments. Laws, however, are subject to not only enforcement or application but also interpretation. How wars are actually conducted in all ages, whether “fairly” or not, has a history all its own.


Author-historian Everett Wheeler is the scholar in residence in the Department of Classical Studies at Duke University. For further reading he recommends his own Stratagem and the Vocabulary of Military Trickery; his section on “Ruses and Stratagems” in Vol. 5 of The International Military and Defense Encyclopedia; and David J. Bederman’s International Law in Antiquity.

Originally published in the July 2013 issue of Military History. To subscribe, click here.