Distinction, Proportionality, Precaution: The Rules That Govern Every Strike

The delegates who signed the four Geneva Conventions on August 12, 1949, had all lived through a war in which the line between soldier and civilian had nearly vanished. The Second World War had killed tens of millions of civilians: in bombed cities, in occupied villages, in camps built for murder, in famines engineered or ignored. The diplomats meeting in Geneva that summer, at a conference convened by the Swiss government with the International Committee of the Red Cross as its intellectual engine, were trying to write rules that would hold the next time. They produced four treaties: one protecting wounded and sick soldiers on land, one for the wounded, sick and shipwrecked at sea, one for prisoners of war, and, for the first time, a whole convention devoted to protecting civilians. Every country in the world has since ratified them. It is easy to be cynical about laws that are broken so often. It is worth knowing what they actually say first, because the rules are clearer and more demanding than most public debate about war suggests, and because they apply to everyone.

In short: International humanitarian law, the law of armed conflict, rests on three targeting rules. Distinction means attacks may be aimed only at military objectives, never at civilians. Proportionality forbids attacks expected to cause civilian harm excessive compared with the concrete military advantage anticipated. Precaution requires every feasible step to spare civilians. These bind every state and armed group, regardless of who started the war or how just its cause.

A battlefield in Italy and a Swiss businessman

The modern law of war has an odd founding scene. In June 1859, a Swiss businessman named Henry Dunant was travelling in northern Italy, hoping to secure support for a business venture, when he came upon the aftermath of the Battle of Solferino, fought between the armies of France and Sardinia on one side and Austria on the other. Tens of thousands of soldiers lay dead or wounded, with almost no medical care. Dunant organized local people, many of them women from the nearby town, to tend to the wounded of both armies, under the principle that a wounded soldier was no longer an enemy but a person in need.

He wrote a book about what he had seen, A Memory of Solferino, published in 1862, and proposed two things: a permanent volunteer relief organization in every country, and an international agreement protecting the wounded and those who cared for them. The first became the Red Cross movement. The second became the first Geneva Convention, in 1864. Dunant later shared the first Nobel Peace Prize, in 1901.

Around the same time, in the middle of the American Civil War, the Union Army issued General Orders No. 100, drafted by the legal scholar Francis Lieber and signed by President Lincoln in 1863. The Lieber Code was one of the first attempts to set down the laws of land warfare in a single document, and it influenced later international rules.

From those beginnings came a long series of treaties. The 1949 conventions were the great consolidation after the Second World War. In 1977, two Additional Protocols filled in the rules on how attacks may be conducted, which the 1949 treaties had mostly left to custom. Most of the rules discussed below are written most precisely in the first of those protocols. Some major military powers, including the United States, have not ratified it, but they accept that many of its targeting rules reflect customary international law that binds them anyway. The US Defense Department's own Law of War Manual sets out distinction, proportionality and precaution as obligations.

Distinction: the line at the centre of everything

The first rule is the most basic. Parties to a conflict must at all times distinguish between civilians and combatants, and between civilian objects and military objectives, and direct their operations only against military objectives.

That sounds obvious. Its consequences are not small. Civilians may never be the object of attack. Attacks intended to spread terror among the civilian population are forbidden. So are indiscriminate attacks, meaning attacks that are not directed at a specific military objective, or use weapons that cannot be directed at one, or whose effects cannot be limited. Firing unguided rockets toward a city, or bombing an area as a single target when it contains separate military objectives among civilians, falls on the wrong side of the line.

A military objective is defined by what it does, not what it is. It is an object that by its nature, location, purpose or use makes an effective contribution to military action and whose destruction, capture or neutralization offers a definite military advantage in the circumstances at the time. A school is a civilian object. If fighters turn it into a firing position, it can lose that protection while they use it. When they leave, the protection returns. In cases of doubt, the rule is that an object normally dedicated to civilian purposes, like a house, a place of worship or a school, is presumed not to be making a military contribution.

People work the same way. Civilians are protected unless and for such time as they take a direct part in hostilities. A farmer is a civilian; a farmer firing a weapon at soldiers is, while doing so, a legitimate target. The phrase "for such time as" carries a lot of weight, and states and scholars disagree about exactly how far it extends, particularly for members of armed groups who fight intermittently. But the starting point is protection.

Proportionality: the hardest judgment

Distinction says what may be targeted. Proportionality limits how. Even an attack on a legitimate military objective is prohibited if it may be expected to cause incidental loss of civilian life, injury to civilians or damage to civilian objects, or a combination of them, that would be excessive in relation to the concrete and direct military advantage anticipated.

Several features of that rule are often misunderstood.

It does not forbid all civilian casualties. The law accepts, grimly, that attacks on military targets may kill civilians nearby. What it forbids is harm that is excessive compared with the military advantage expected.

It is judged in advance, on the information reasonably available to the commander at the time. A strike that kills far more civilians than expected because of information nobody could have had is not automatically unlawful. A strike that kills many civilians because the commander ignored obvious information is a different matter.

The advantage must be "concrete and direct," not speculative or long-term. Weakening the enemy's morale or pressuring its population is not the kind of advantage that can justify civilian deaths.

And it is a comparison of things that cannot be measured on the same scale: lives on one side, military gain on the other. That is why proportionality is the most contested rule in practice. Reasonable military lawyers disagree at the margins. But at the extremes, the rule has real force. When very large numbers of civilians are killed to strike a single low-level fighter, it is hard to see how any reading of "excessive" would permit it.

Precaution: the rule that asks the most

The third principle asks the most of the people planning an attack. In the conduct of military operations, constant care must be taken to spare the civilian population, civilians and civilian objects.

In practice, attackers must do everything feasible to verify that targets are military objectives. They must choose means and methods of attack that minimize incidental civilian harm, such as a smaller bomb, a different angle of approach or a different time of day. They must cancel or suspend an attack if it becomes clear that the target is not military or that the harm would be disproportionate. They must give effective advance warning of attacks that may affect the civilian population, unless circumstances do not permit. And when there is a choice between several military objectives offering similar advantage, they must choose the one expected to cause the least danger to civilians.

"Feasible" is the key word. It means what is practicable, taking into account all the circumstances, including humanitarian and military considerations. It does not require the impossible. It does require commanders to use the tools and information they actually have.

There are precautions on the defending side too. Parties must, to the extent feasible, avoid placing military objectives within or near densely populated areas and take other steps to protect civilians under their control. Using civilians to shield military objectives from attack is a separate and serious violation.

The rule people most want to forget: it binds everyone equally

The most important thing about these rules may be the one that is hardest to accept in wartime. They apply equally to every party, regardless of which side started the war, which side has the more just cause, and whether the other side is obeying them.

This principle is deliberate. If the rules applied only to the aggressor, every party would claim to be the victim, and the rules would protect no one. The law separates the question of whether a war is lawful to begin with from the question of how it is fought. A state defending itself against an unprovoked invasion must still observe distinction, proportionality and precaution. A state that was attacked first gains no licence to target civilians in return.

Nor is compliance conditional on reciprocity. Common Article 1 of the 1949 conventions commits states to respect and ensure respect for the conventions "in all circumstances." If one party uses human shields, that party commits a violation, but the attacker's obligations to assess proportionality and take precautions do not disappear. The civilians are still civilians, whoever put them in danger.

The rules also bind non-state armed groups. Common Article 3, the short article appearing in all four 1949 conventions, sets minimum protections in conflicts that are not between states, including civil wars and wars between states and armed groups. It forbids murder, torture, hostage-taking and humiliating treatment of people not taking part in hostilities, and it binds "each Party to the conflict." The customary rules of distinction, proportionality and precaution apply to armed groups as well.

That symmetry is why these rules can be the common ground in debates that otherwise have none. Whether the subject is Russia's war in Ukraine, the war in Gaza that followed the Hamas-led attack on Israel on October 7, 2023, the fighting between Israel and Iran, or conflicts that get far less attention in Sudan, Myanmar and elsewhere, the same questions apply to every party. Was the target military? Was the expected civilian harm excessive? Were feasible precautions taken? Were hostages taken, prisoners mistreated, or aid blocked? Applying those questions consistently, to friends and adversaries alike, is what separates a principled position from a partisan one. And it is how we keep in view that civilians on every side are people, and that governments and armed groups are not the same as the peoples they claim to represent.

When the rules are broken

A serious violation of these rules can be a war crime. The 1949 conventions require states to search for people alleged to have committed grave breaches and either prosecute them or hand them over to a state that will. The Rome Statute of the International Criminal Court, adopted in 1998, lists intentionally directing attacks against civilians, and launching an attack knowing it will cause clearly excessive civilian harm, among the crimes the court can try.

Enforcement is the system's weakest point. Many major powers, including the United States, Russia, China and Israel, are not members of the court. National courts often decline to prosecute their own forces. Investigations take years. Critics of the system, from different directions, argue that it is either toothless or applied selectively against weaker countries and their leaders.

Those criticisms have weight. But they are arguments for enforcing the rules consistently, not for abandoning them. The rules do more than provide a basis for prosecution. They shape military training, legal review of targets and the procedures militaries use every day. They give journalists, investigators and human rights organizations a common standard for documenting what happens. And they give the public a vocabulary for asking hard questions of their own governments.

We follow these questions, including humanitarian access and the protection of civilians in current conflicts, on our human rights and foreign aid page, and the debate over American arms transfers and military power on our Pentagon and militarism page.

What you can do

• Ask your members of Congress how they check that American weapons are used lawfully. US law already bars security assistance to units credibly implicated in gross violations of human rights. Look up your representative and senators, their contact forms and recent votes at find your representatives, and ask whether they support strong reporting on how American-supplied weapons are used.
• Support humanitarian and human rights groups. Organizations that deliver aid in war zones and document violations rely on public support. Our organizations directory lists nonprofits by cause in every state, including human rights and immigrant-support groups that work with refugees from these conflicts.
• Apply the same three questions to every side. When you read about a strike, ask whether the target was military, whether civilian harm was excessive, and whether precautions were taken, and ask them of the side you sympathize with as firmly as of the other.