The idea that war needs a justification, and that even a justified war has limits, did not begin with any single people. It grew in the law of Rome, the theology of the Latin church, the jurisprudence of Islam, the philosophy of ancient China and the law books of India. The rules differed, but the questions were strikingly similar.

Rome and the church

In his book On Duties, written in 44 BCE, Cicero set out a principle that later writers quoted for centuries: "The only excuse, therefore, for going to war is that we may live in peace unharmed." Cicero held that the only excuse for war is to live in peace unharmed, not glory or plunder. He also drew on Rome's old fetial law, under which priests carried Rome's grievances to an enemy before any war began. Cicero said no war is just unless a formal demand for redress has been made and war has been declared.

Augustine, bishop of Hippo in North Africa from 395 until his death in 430, never wrote a treatise on war. Writing to a Roman general named Boniface, he put his view compactly: "We do not seek peace in order to be at war, but we go to war that we may have peace." For Augustine, peace is the aim that any war must serve.

Augustine also moved the moral question inward. In a reply to a critic named Faustus he wrote: "The real evils in war are love of violence, revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power, and such like." Augustine located the real evils of war in inner motives such as cruelty and the lust for power. He doubted that a private person could rightly kill even in self-defense, since that seemed to put love of one's own body above love of one's attacker. Killing under public authority, to protect others and restore order, was different.

Around 1140 the legal scholar Gratian gathered Augustine's scattered texts into a great collection of church law. In the following century Thomas Aquinas condensed the result in his Summa Theologiae. Aquinas named three conditions for a just war: sovereign authority, a just cause and right intention. Private lords could not wage war, and the war had to aim at good, not at cruelty or gain.

Islam, China and India

Muslim jurists developed a body of law called siyar, the Islamic law governing war and relations with other peoples. Its classic early text is by Muhammad al-Shaybani, who died in 805. Jurists commonly forbade the killing of women, children, the elderly and monks who took no part in fighting.

In China, the philosopher Mozi, who lived in the fifth century BCE, wrote a famous attack on aggressive war. A man who steals a neighbor's peaches is condemned, he observed, and a man who kills an innocent person is condemned even more. Yet when a ruler invades another state and kills thousands, people call it righteous. Mozi charged that people condemn a single murder yet praise conquest that kills thousands. His followers did more than argue: Mohists offered their skills in defending besieged cities, becoming experts in fortification so that attack would not pay.

In India, the law book known as the Laws of Manu, compiled roughly two thousand years ago, sets rules for a king in battle. He must not use weapons that are concealed, barbed, poisoned or tipped with fire. Manu forbids striking an enemy who joins his palms in surrender, who is asleep, or who has lost his armor. The great epic Mahabharata dramatizes how hard such rules are to keep when victory is at stake.

What the comparison shows

Two lessons stand out. First, restraint in war is not the invention of any single culture; similar limits appear independently in places that had little or no contact. Second, the circle of protection was often narrow. Many early rules protected only enemies seen as members of the same moral community: fellow Christians, fellow believers, warriors of the same class. Outsiders, heretics and rebels were often left outside. Much of the later history of the laws of war is the slow widening of that circle.

Worked example: Abu Bakr's instructions

A collection of early Islamic law compiled by Malik ibn Anas in the eighth century reports what the first caliph, Abu Bakr, told a commander leading an army toward Syria around 633. Do not kill women, children or the old and infirm, he said. Do not cut down fruit-bearing trees or destroy inhabited places. Do not slaughter sheep or camels except for food. And you will find people who say they have devoted themselves to God in their cells: leave them to what they have devoted themselves to.

Set beside Manu and Cicero, these instructions show the same instinct at work: separate the people who fight from those who do not, and avoid destruction that serves no military purpose. They also show the tradition's limits. Rules about conduct do not settle whether a war of conquest is just in the first place; they restrain how it is fought. Historians add a caution about sources: the report was written down well over a century after the events. Its value lies as much in showing what early jurists believed a righteous commander should say as in recording his exact words.