More than any of the other ancients, the Romans understood war in ways that foreshadowed our own. Marcus Tullius Cicero’s De Officiis first raised questions of just war that concern us even today. Cicero first introduced the idea that war should advance some good beyond merely self-interested expansion. For Cicero, the natural state of mankind was of peace; war was an unnatural rupture. “Wars, then, are to be waged in order to render it possible to live in peace without injury,” wrote Cicero. A Roman leader, therefore, must have a just cause (iusta causa) to go to war. “No war can be undertaken by a just and wise state, unless for faith or self-defense,” Cicero wrote. If not for self-defense, war should come only in response to an earlier wrong, such as an attack on allies or ambassadors, a breach of treaties, or support for an enemy. Cicero acknowledged that just causes might also include punishment of the enemy, but the unifying idea remained that Roman war should respond to an earlier wrong.
Although Rome marked a just war with religious ceremony, Cicero thought war should be just without reference to religious sanction. Just war flowed from natural law. “In a republic the laws of war are to be maintained to the highest degree. For there are two ways of deciding an issue, one through discussion, the other through force,” Cicero wrote. “The former [is] appropriate for human beings, the latter for beasts.” Cicero believed that Rome must fight honorably and show mercy to the conquered, even though Roman rules of war permitted the seizure of property and enslavement. For Rome, a war must be iustum piumque: just and pious.
In practice, however, just war did not meaningfully restrict Rome. Her leaders focused more on observing the forms of the law than its substance. While there were instances where Rome obeyed just war principles, there were many, if not more, examples where Roman leaders manufactured events to justify war. As Polybius observed, Rome always sought a pretext for war so as not to be the public aggressor.
The First and Third Punic Wars provide examples. At the time of the former in 264 BCE, Carthage rivaled Rome in the Mediterranean, but she carefully avoided giving any cause for hostilities. Roman intervention in a war between Syracuse and Messina sparked the conflict. While the Second Punic War might have been inevitable, Carthage no longer posed a threat by the time of the Third Punic War. Rome destroyed her old enemy anyway. Julius Caesar’s conquest of Gaul provides another example. In his Gallic Wars, Caesar did not produce an arguable case of harm done to Rome. Conquest drove the war, a fact that did not go unnoticed among Caesar’s critics. By the end of the Republic, Roman wars seemed to honor the just war ideal only in the breach.
Cicero proposed an answer to the question of the enforcement of just war ideas. He speculated that the Roman Republic deserved its end because of the lack of causus belli in the many conflicts. “So long as the sway of the Roman people was maintained by the bestowal of benefits, not by injustice,” Cicero wrote, “our sovereignty might then have been termed the patronage, rather than the government, of the world.” But once this policy of just war was “abandoned” in the years after Sulla’s dictatorship, “we are being justly punished” with civil wars and “the republic we have completely lost.” Just war theory might have been just that: a theory. Romans may have liked to claim that they had fulfilled their religious or legal duties, but in reality, the doctrine crumbled before republican and, later, imperial demands. But it was Cicero who suggested that, by ignoring its own just war ideals, the Republic would deserve its fate.