Wikipedia Deep Dive

Law of war

13 min read

Here is one of humanity's strangest contradictions: we have spent millennia writing rules for how to kill each other properly.

Think about that for a moment. War, by its very nature, is about breaking the most fundamental rule of civilization—thou shalt not kill. And yet, since at least 1750 B.C., humans have been drafting elaborate codes specifying exactly when killing is acceptable, who may be killed, and which methods of killing are simply beyond the pale. It seems absurd. It is absurd. And it has saved countless lives.

The Ancient Roots of Wartime Restraint

The Code of Hammurabi, that famous Babylonian law carved in stone nearly four thousand years ago, contains what might be the first recorded laws of war. King Hammurabi himself explained the purpose with striking clarity: "I prescribe these laws so that the strong do not oppress the weak."

The Hebrew Bible's Book of Deuteronomy, written centuries later, gets surprisingly specific about environmental regulations during siege warfare. You can cut down trees that don't bear fruit—you need the lumber for siege equipment, after all. But fruit trees? Those stay standing. The logic was practical: you might want to eat from those trees once you've conquered the place.

Even more remarkably, Deuteronomy addresses what happens to female captives who were forced to marry their captors. If the conquering soldier later decides he no longer wants her, he cannot sell her into slavery. She must be freed to go wherever she chooses. In the brutal context of ancient warfare, this represented a genuine, if limited, attempt at protecting the vulnerable.

In the early seventh century, the first Sunni Muslim caliph, Abu Bakr, issued instructions to his army that sound remarkably modern. No mutilating corpses. No killing children, women, or the elderly. No burning trees, especially fruit-bearing ones. No slaughtering the enemy's livestock except for food. And if you encounter people who have devoted their lives to religious service? Leave them alone.

The Christian Contradiction

Early Christianity posed an interesting problem for laws of war: many early Christian writers argued that Christians simply could not be soldiers at all. War was incompatible with following Christ.

This created obvious difficulties once Christianity became the official religion of the Roman Empire. Enter Augustine of Hippo, the influential fourth-century theologian who developed what would become known as "just war" doctrine. Augustine didn't celebrate war, but he acknowledged that sometimes—under specific circumstances—it could be morally justified.

What circumstances? Augustine was deliberately vague, which left plenty of room for later interpreters. But the basic framework he established would shape Western thinking about warfare for the next sixteen centuries: some wars are just, others are not, and even just wars must be conducted according to certain rules.

In 697, an Irish abbot named Adomnan gathered kings and church leaders from Ireland and Scotland at a place called Birr. There, he presented them with what he called the "Law of the Innocents." Its core prohibitions were simple: no killing women and children in war, and no destroying churches. The fact that this needed to be explicitly stated tells you something about the standards of the time.

Two Different Questions

Modern lawyers who study the laws of war distinguish between two fundamentally different questions, and the distinction matters enormously.

The first question is called jus ad bellum—Latin for "right to war." When is it legal, or morally permissible, to start a war in the first place? Can you attack another country because they threaten you? Because they're oppressing their own people? Because they have something you want? Different eras and different legal frameworks have answered these questions very differently.

The second question is jus in bello—the "law in war." Once hostilities have begun, regardless of whether the war itself was justified, how must combatants behave? This is the realm of rules about prisoners, civilians, permissible weapons, and treatment of the wounded.

These two concepts are intentionally separate. A soldier fighting in an unjust war—perhaps his country was the aggressor—is still protected by the laws of war if captured. And a soldier fighting in a perfectly justified defensive war can still commit war crimes. The righteousness of your cause doesn't give you license to commit atrocities.

The Golden Age of Written Rules

The historian Geoffrey Best identified the period from 1856 to 1909 as the law of war's "epoch of highest repute." This was when the modern system really took shape.

Before this period, laws of war relied heavily on custom, chivalric codes, and religious teachings. Everyone sort of knew the rules, but they weren't written down in binding treaties. The late nineteenth century changed that. International conferences became the forum where nations debated and agreed upon explicit rules. Multilateral treaties—formal agreements signed by many countries—became the mechanism for making those rules binding.

The transformation began with smaller agreements. In 1848, the Treaty of Guadalupe Hidalgo, ending the Mexican-American War, included specific provisions about protecting civilians and treating prisoners of war. During the American Civil War, the Union promulgated something called the Lieber Code, named after the legal scholar Francis Lieber who drafted it. This was a comprehensive set of instructions for Union soldiers about the rules of land warfare—one of the first attempts to codify such rules for a modern army.

In 1864, twelve countries signed the Geneva Red Cross Convention, protecting medical personnel and facilities. If you've ever wondered why attacking a hospital is considered especially heinous, this is where that prohibition took legal form.

The Hague Conferences

In 1898, Tsar Nicholas II of Russia made a remarkable proposal. He called for an international conference to discuss limiting armaments and finding peaceful ways to resolve disputes between nations. His stated goal was "insuring to all peoples the benefits of a real and durable peace, and, above all, of putting an end to the progressive development of the present armaments."

Was Nicholas sincere? Historians debate this. Russia was struggling to keep pace with the arms race, and calling for disarmament may have been strategic as much as humanitarian. But regardless of motive, the result was the first Hague Peace Conference in 1899, followed by a second in 1907.

These conferences produced an extraordinary body of detailed international law. The Hague Conventions covered everything from the treatment of prisoners to the conduct of naval warfare to the prohibition of certain weapons. They established principles that still govern armed conflict today.

There was genuine optimism in the air. Many believed that civilization had advanced to the point where war might eventually be abolished entirely. International arbitration and adjudication would replace armed conflict. Humanity was progressing beyond such barbarism.

Then came 1914.

The Five Principles

Despite the catastrophic wars of the twentieth century—or perhaps because of them—the international community continued developing the laws of war. Today, five core principles are most commonly cited as governing the legal use of force.

Military necessity means that military action must actually serve a military purpose. You cannot attack something just because you can, or just because it belongs to the enemy. The action must be intended to help defeat the enemy, it must target a legitimate military objective, and any harm to civilians must be proportional to the military advantage gained. This is a significant constraint. It means armies cannot simply destroy everything in their path.

Distinction requires combatants to distinguish between fighters and protected civilians. You cannot deliberately target civilians, hospitals, schools, or other protected sites. This principle sounds straightforward but becomes horrifyingly complicated in practice. What about a factory that makes both consumer goods and ammunition? What about a bridge used by both military convoys and civilian commuters? What about fighters who don't wear uniforms and blend in with the civilian population?

Proportionality demands that even when attacking legitimate military targets, the expected harm to civilians cannot be excessive relative to the anticipated military advantage. This is not, importantly, about responding to attacks in kind. As international law professor Robbie Sabel has noted, "if you are attacked with a rifle, there is no rule that stipulates that you can only shoot back with a rifle." You can absolutely respond with greater force than was used against you. Proportionality is about weighing civilian harm against military benefit, not about matching your enemy's weapons.

Humanity—sometimes called the prohibition on unnecessary suffering—restricts weapons and methods designed to cause pain or injury manifestly disproportionate to their military usefulness. Many countries now review new weapons before deploying them to ensure compliance with this principle. The purpose is to prevent suffering that serves no military purpose. Killing an enemy soldier achieves your military objective; torturing him first does not.

Honor—sometimes called chivalry—demands a certain fairness between adversaries. Parties must recognize that their right to harm each other is not unlimited. They must not take advantage of an enemy's adherence to the law by falsely claiming its protections. (Pretending to surrender and then opening fire, for instance, is prohibited.) And they must understand that they fight not out of personal hostility but as representatives of their respective states.

The Problem of Enforcement

Here is the fundamental challenge with laws of war: who enforces them?

Domestic law works because the state has a monopoly on legitimate violence. Break the law, and the police arrest you, courts try you, and prisons hold you. The state is stronger than any individual criminal.

International law has no equivalent. There is no world government, no global police force capable of arresting the army of a major power. The laws of war rely largely on states policing themselves—and on the threat that other states will respond to violations.

After World War II, the Nuremberg Trials established that individuals could be held personally accountable for war crimes. The International Criminal Court, established in 2002, has jurisdiction over war crimes, crimes against humanity, and genocide. But the Court has no army of its own. It depends on states to arrest suspects and hand them over. Major powers like the United States, Russia, and China have not ratified the treaty creating the Court, limiting its reach.

The Nuremberg Tribunal did establish one crucial principle that affects how laws of war operate. The judges held that the Hague Convention of 1907, having been widely accepted by "all civilised nations" for about half a century, had become part of customary international law. This meant it was binding on all parties—even those who hadn't signed it.

This concept matters because it means laws of war aren't just contracts between signatories. When a practice becomes sufficiently widespread and accepted, it becomes binding on everyone. The prohibition on torture, for instance, is now considered a norm of customary international law. No country can opt out by simply refusing to sign the relevant treaties.

The Laws Keep Changing

International humanitarian law is not static. It evolves as technology changes, as tactics change, and as the international community's understanding of what is acceptable changes.

Consider depleted uranium ammunition. In 2001, the chief prosecutor for the International Criminal Tribunal for the former Yugoslavia noted that while there was no specific treaty ban on depleted uranium projectiles, scientific concern about their effects was growing. She suggested that in the future, a consensus might emerge that such weapons violate general principles of the law of armed conflict.

This is how the law often develops. New weapons or tactics emerge. Some states express concern. International debate follows. Eventually, either a new treaty explicitly bans the practice, or a consensus develops that existing principles prohibit it. The process can take decades.

The "war on terror" has prompted similar evolution. Targeted killing of specific individuals—what critics call assassination—was traditionally considered outside the bounds of legitimate warfare. But in the context of combating non-state terrorist organizations, several countries have adopted policies of targeted killing. Whether these policies comply with international law remains hotly contested.

Why Bother?

Given the difficulty of enforcement, why do the laws of war exist at all? Why would nations agree to limit their own military options?

Several reasons.

First, reciprocity. If you treat enemy prisoners humanely, your enemy is more likely to treat your soldiers humanely if they're captured. Mutual restraint benefits both sides.

Second, morale. Soldiers fight better when they believe their cause is just and their conduct is honorable. Armies that commit atrocities often suffer from discipline problems and desertion.

Third, post-war reconciliation. Wars eventually end. If one side has committed widespread atrocities, making peace becomes much harder. The occupied population will resist indefinitely. Former enemies cannot become trading partners or allies. Restraint during war makes peace after war more achievable.

Fourth, reputation. States that flagrantly violate the laws of war become international pariahs. Other countries may impose sanctions, refuse cooperation, or intervene militarily.

Fifth, and most fundamentally, shared humanity. Even enemies recognize each other as human beings with rights. The laws of war express a belief that some things are simply wrong, regardless of the strategic situation—that there are lines we should not cross even when fighting for our survival.

The Central Paradox

We return to where we began: the strange idea that there are rules for something as lawless as war.

The paradox is real but not debilitating. Consider an analogy: there are rules for boxing too. Two people have agreed to punch each other repeatedly. That's violent. That's potentially dangerous. But we still have rules—no hitting below the belt, no biting, no strikes after the bell. The existence of the fight doesn't mean anything goes.

War is far more complex and consequential than boxing, of course. The stakes are incomparably higher. But the same logic applies. Once hostilities begin, there are still choices to be made. Soldiers can target military installations or they can bomb hospitals. They can accept surrender or execute prisoners. They can minimize civilian casualties or maximize them.

The laws of war shape those choices. They don't prevent all atrocities—far from it. But they establish a framework of expectations, a vocabulary for condemnation, and legal mechanisms for accountability. They express humanity's conviction that even in our darkest moments, we can choose to be better than our worst impulses.

Nearly four thousand years after Hammurabi inscribed his code to prevent the strong from oppressing the weak, we are still trying to write rules that limit the violence humans inflict on each other. We have not succeeded in abolishing war. We have not prevented atrocities. But we have established principles that, when followed, reduce suffering. We have created institutions that, when functioning, hold the worst offenders accountable. We have articulated a vision of warfare that, however imperfectly realized, recognizes the humanity of enemies and the imperative to protect the innocent.

It is not enough. It will never be enough. But it is something—and in the brutal mathematics of war, something matters enormously.