Waging war

There are three bodies of international law that are relevant to how wars must be waged. First, general international law prohibits states from starting a war in most circumstances. There is a general prohibition on the use of force, which is enshrined in the charter of the United Nations.

© Sean Pavone via Shutterstock

There are some limited exceptions, like self-defense, but generally we have a law that is sometimes called jus ad bellum, which prohibits starting wars. Second, we have a body of law that prescribes how wars must unfold. It is mostly called international humanitarian law or the laws of war. Sometimes it is called jus in bello, and it proscribes certain conduct on the battlefield and puts certain behaviors off limits. Some violations of international humanitarian law are criminalized, so if an individual violates international humanitarian law in a certain way, they may later be held to account by a criminal court or tribunal. That is regulated in international criminal law, the third body of law. Of course, law is only one factor that shapes how the wars that we witness day-to-day unfold. Most of the time, war follows imperatives of strategy, of politics; there are strong affective or emotional dynamics on battlefields, and ideally, ethics or morality would also shape how people conduct wars and when wars start. Law is one factor that helps us understand and make sense of the violence we witness in international relations every day.

Enforcing international law

International law is not enforced in the way domestic law is enforced. The blueprint for legal enforcement that most of us have in our mind is that if you threaten someone with violence or use violence against somebody – for instance, in the United Kingdom – the police would come, a court would then decide whether you broke the law, potentially issue an arrest warrant and the police would execute this arrest warrant and put you in jail. This is not how it works in the international system. We have courts that can adjudicate whether a state broke the law, but these courts do not have jurisdiction over every legal question that might arise in international relations. For instance, the International Court of Justice, when it has jurisdiction, can determine that a state violated international law, for example by going to war illegally or by breaking another rule of international law. The Court can command that state to cease the illegal conduct; it can impose a demand of reparations, but there is no international police force that will execute the Court’s orders.

International law is enforced in a “horizontal” system, which means states are supposed to enforce law vis-à-vis each other, rather than a “vertical” system where a sovereign with a police force enforces law vis-à-vis the subjects. Generally, when a state is found to have violated international law, other states are called upon to impose sanctions, cease trade and protest diplomatically. There is a rule in international law that states are not allowed to support another state in violating international law. Of course, that system of “horizontal” enforcement is often disrupted by politics, because states may find that they do not have an interest in enforcing a particular international law. There is also some vertical enforcement in international law: The UN Security Council is in charge of enforcing the international law that prohibits the use of force. The Security Council can mandate the defensive use of force in response to an aggression.

© Senior Airman JoAnn S. Makinano via Wikimedia

Of course, the Security Council is made up of states. They vote in line with their interests, so oftentimes enforcement is disrupted by politics, and it is therefore not entirely reliable. Whether or not a state that breaks international law actually faces consequences for the breach is often a matter of politics and not only a question of whether we can reliably and convincingly demonstrate that they violated international law.

Lawful use of force

One of the greatest achievements of international law was the UN Charter in 1945, articulating a blanket prohibition on the use of force by states in international relations. Up until this point, war between states was considered a tool of sovereign statecraft. When states had conflicts, at some point they would go to war, and that was more or less accepted. Following a process of gradual delegitimization of war, which culminated in the UN Charter, Article two, paragraph four now demands that states resolve their conflicts peacefully and not use force in their international relations. There is a customary prohibition that echoes the treaty. There are some exceptions to the prohibition on the use of force. Two are particularly important. One is that the Security Council can mandate the use of force in response to an aggression. The Security Council can diagnose that there is a threat to the peace, breach of the peace, or act of aggression, and then mandate a response that includes the use of force. States that take up that mandate would then be going to war in compliance with international law rather than in contravention of it.

© Gorodenkoff via Shutterstock

For that, the Security Council must vote in favor – none of the permanent members must veto it – and then a state must be willing to take up that mandate. Mandated use of force is relatively rare in international relations. More common is the second exception, which is self-defense. While states must generally not use force in their international relations, when a state is under armed attack by another state, the attacked state can resort to force in self-defense. It has an inherent right of self-defense, and not surprisingly, this is a justification that states very frequently invoke, even when it is not entirely plausible.

Mandate and self-defense

In 1990, Iraq invaded its neighboring country Kuwait, and in response, the Security Council mandated that Kuwait could act in collective self-defense so that states cooperating with Kuwait could assist in repelling the armed attack by Iraq. That is one example of a mandate by the Security Council for the use of force and for a war, in this instance led by the United States, that was covered by a Security Council mandate. Self-defense is often invoked, and it is not necessarily always confirmed by the Security Council whether its invocation is appropriate.

Russia first invaded sovereign Ukrainian territory in contravention of the prohibition on the use of force in 2014 when it seized the peninsula of Crimea. In 2022, it then launched a full-scale invasion of Ukraine.

© Ruslan Lytvyn via Shutterstock

This is a classic case of one state launching an armed attack against another state, and Ukraine's response to that armed attack is covered by Ukraine's inherent right of self-defense.

Limits on the right of self-defense

It is in principle possible that a state is attacked with military force and cannot respond in self-defense under international law. An armed attack is considered to be a particularly grave use of military force. For instance, a border skirmish might remain below the threshold of an armed attack, and the affected state would not then be permitted to respond to it with force covered by the right of self-defense. The more practically relevant question is whether it necessarily has to be a state that launches the armed attack for the right of self-defense to be triggered. In the traditional understanding, a state has a right of self-defense against an armed attack by another state. We now live in a world where non-state armed groups, often called terrorist groups, use violence across borders. The question is can a state that is attacked at the threshold of gravity of an armed attack by a non-state armed group respond to that attack in self-defense? The question received urgent attention when the United States was attacked by al-Qaeda on 9/11.

© Robert J. Fisch via Wikimedia

The conduct of al-Qaeda was not easily attributable to another state. The group was considered not to be under the effective control of another state, but it obviously operated in other states. The allegation was in Afghanistan and Iraq. The question then was whether the United States could invade these countries in self-defense to counter the attack by al- Qaeda. At that point, the United States articulated a doctrine called “unwilling or unable”, which argued that if the state on whose territory a group operates is unwilling or unable to neutralize the threat the group poses, then the right of self-defense includes a permission to use force on that state's territory. This doctrine was contested and still is. It is also relevant in the context of the right of self-defense that Israel claims against Hamas in Gaza; Hamas is generally not considered to be under the effective control of the Palestinian Authority, which is the government that represents the sovereign state of Palestine for those who accept the existence of that state. Hamas is a non-state armed group, and one complication about the claim that Israel is acting in self-defense is the uncertainty about how states must respond to armed attacks by non-state armed groups. There are other open doctrinal questions in this particular case, but this is one area where international law is in flux or contested – a question of open doctrinal contestation.

The Geneva Conventions

When it comes to how states or armed groups must behave on the battlefield, we look to a series of treaties called the Geneva Conventions. They contain many provisions that are also customary law. The Geneva Conventions and Protocols are the most important treaties regulating how wars must unfold, how parties to a war must use force against each other, and how they must treat detainees. The Geneva Conventions also regulate humanitarian operations and occupation. For the conduct of hostilities – how two belligerent parties at war engage each other on the battlefield and use kinetic force against each other – we look at three principles in particular, called the principles of necessity, distinction and proportionality.

© Everett Collection via Shutterstock

The principle of distinction is the most fundamental. It states that civilians must never directly or intentionally be attacked, while combatants can always be attacked in war. Civilian objects must never be intentionally or directly attacked, and military objectives, which are objects that have a military function, can always be attacked or destroyed. Concretely, every time a soldier releases their weapon in war, the attack must be directed against a target the attacker knows or has reasonable grounds to believe is either a military objective or a combatant/fighter. It is never permissible to direct an attack against a person that is a civilian or against a civilian object.

Civilian harm

In many attacks that are targeted appropriately, the attacker will be able to foresee that the attack causes some damage to civilian infrastructure or harm to civilians as a side effect.

© Oleksii Synelnykov via Shutterstock

Militaries call this “collateral damage”. The law calls it “incidental civilian harm”, and it must be minimized as much as possible. This is the principle of necessity. Only incidental civilian harm that is necessary for the achievement of a military advantage, and that cannot be avoided, is permissible. Parties to a war are called upon to take all feasible precautions to minimize incidental civilian harm, for instance through the choice of weapon, the time or the angle of attack. Warnings must also be issued where possible so that civilians can move away from military objectives. Collateral damage that is necessary and unavoidable must also be proportionate. It must not be excessive in relation to the anticipated military advantage of the attack. The attacker must weigh the military advantage against foreseeable and unavoidable civilian harm. It is possible for an attack to be targeted appropriately and for precautions to be taken but still be illegal if the expected incidental civilian harm is disproportionate.

Morality vs the law

There are divergences between what international law permits or does not prohibit and what a moral approach to permissible killing would imply. International humanitarian law does not prohibit attacks against combatants. A combatant can simply be someone conscripted into the armed forces of their state and compelled to fight for an unjust cause.

© Getmilitaryphotos via Shutterstock

From a moral perspective, someone who had no choice in fighting, or who fights for a just cause, is not necessarily morally liable to harm. Law does not make that distinction. Law does not make these distinctions between voluntary and forced combatants or between those who fight for a just or those who fight for an unjust cause. From the perspective of law, soldiers on both sides are equally liable to killing and harming, even if they had no choice but to fight.

Example of divergence

In the war between Russia and Ukraine, Russia is conducting an illegal and immoral aggression while Ukrainian soldiers are acting in self-defense. From a moral point of view, soldiers who act in self-defense should not be killed, as they have done nothing wrong; however, international humanitarian law addresses Russian and Ukrainian soldiers with the exact same rules and permits both sides to kill each other in equal measure. This is a significant divergence between morality and law. The divergence exists for a moral reason: if law established one set of rules for the just defender and another for the unjust aggressor, no state would admit to being an unjust aggressor. The restrictive rules for aggressors would never be applied, and the unjust aggressor would use the permissive rules for defenders. For law to be complied with, it must be the same for both sides, even if this allows unjust combatants to kill just ones.

Human rights law vs humanitarian law

International humanitarian law, or the laws of war, is very old. It evolved specifically to regulate conduct in war. In the Geneva Conventions we have a modern articulation of quite old laws. Still, they are not particularly morally ambitious because they must elicit compliance in the reality of war. The laws of war do not prohibit conduct that may be morally abhorrent; for instance, they do not protect just soldiers, and they do not prohibit killing civilians foreseeably as a side effect of necessary and proportionately targeted attacks. Our intuitions about permissible violence are better reflected in human rights law, which regulates relations between states and citizens in peacetime. The traditional understanding was that human rights law applies in peacetime and humanitarian law applies during war. That understanding has been criticized with the view gaining traction that human rights obligations continue to apply in war, at least where discharging them is possible. The dominant understanding now is that humanitarian law is the primary source of regulation for the conduct of hostilities, but where a state has effective control over individuals during war – for example, when detaining them or as an occupying power exercising effective control over territory – the state must also meet its human rights obligations.

© SGT Craig J. Shell via Wikimedia

This is somewhat contested, but more states accept that human rights inhere in people who carry their rights into war, even if it is not always possible to fully protect these rights during war.

Compliance with wartime law

The fact that international law is not reliably enforced is not the primary reason for non-compliance. States comply with international law for many reasons, and fear of sanctions is only one of them. Law bestows legitimacy; it helps coordination; and it carries moral authority. The most important way to improve compliance with international law is for states to consistently demand compliance, not only from adversaries but also from allies. Enforcement is most effective vis-à-vis allies, over whom states have the most influence. The current crisis of compliance in International Humanitarian Law is not due to weakness of that body of law but to a lack of political courage. States must be willing to impose consequences on allies who violate the law.

Lack of political courage

There are several misconceptions about international law in war. The most important is that highly legalized war is still a moral catastrophe.

© Yanya via Shutterstock

Law is not a panacea that makes war morally acceptable, but it plays an important role in curbing violence and lessening the moral evil that war is. We must insist on compliance and work hard to bring law and war into better alignment. When there is non-compliance, as in the wars in Ukraine, Gaza and Sudan, the cause is not usually the inadequacy of law but politics and lack of political courage.

Editor’s note: This article has been carefully edited in conjunction with the author for added clarity and accuracy. Edit date: 2025

Discover more about

international law in wartime

Dill, J. (2015). Legitimate targets? Social construction, international law and US bombing. Cambridge University Press.

Bohrer, Z., Dill, J. & Duffy, H. (2020). Law applicable to armed conflict. Cambridge University Press.

Dill, J. & Schubiger, L. I. (2021). Attitudes toward the use of force: Instrumental imperatives, moral principles and international law. American Journal of Political Science, 65(3), 612–633.

Dill, J. (2019). Distinction, necessity and proportionality: Afghan civilians’ attitudes toward wartime harm. Ethics & International Affairs, 33(3), 315–342.

Dill, J. (2019). Do attackers have a legal duty of care? Limits to the individualization of war. International Theory, 11(1), 1–25.

United Nations (1945). United Nations Charter. United Nations

United Nations (1949). Geneva Convention relative to the protection of civilian persons in time of war. United Nations

Law and morality in war