Photo: Angelica Alzona (GMG)

This piece is part of a recurring series that aims to be a complete guide to the laws of war. You can read previous entries here.

The law on the conduct of hostilities serves to protect civilians in times of armed conflict and prevent unnecessary suffering for combatants. Warring parties are allowed to attack enemy combatants and military objectives, but that right is not unlimited. The only legitimate objective of war is to weaken the military capacity of the enemy. Adverse effects to the civilian population and civilian objects must be limited as much as possible.

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Broadly speaking, there are two domains in International Humanitarian Law: the so-called law of the Hague, which regulates the use of armed force in the conduct of hostilities, and the so-called law of Geneva, which provides protections to persons in the power of the enemy. Depending on which domain applies, the same act may be either lawful or unlawful: the killing of an enemy combatant is permissible in the conduct of hostilities but a war crime if he or she is being held as a prisoner of war. Even within the conduct of hostilities, different circumstances may alter the legality of an attack: the presence of a large number of civilians at a location may make an attack unlawful where it would be otherwise permitted.

Although these branches of law were named after the cities in which their foundational treaties were concluded, modern day Hague law is largely reflected in Additional Protocol I of 1977, a supplementary treaty to the four 1949 Geneva Conventions. Although AP I has “only” been ratified by 174 nations, the laws on the conduct of hostilities contained within it are largely reflective of customary international law which is binding on all nations in both international armed conflict and non-international armed conflict.

There are five factors which cumulatively determine whether an attack is lawful—factors which are systematically analyzed every single day by military personnel all around the world. First, the principle of distinction provides that only permitted objectives may be targeted. Secondly, an attack is prohibited if the civilian harm is disproportionate to the military advantage anticipated. Thirdly, feasible precautions must be taken to reduce the adverse effects to the civilian population and the natural environment. Fourthly and fifthly, a party may only employ permitted means and methods of warfare—which we will discuss in depth in the next article in this series. Let’s now consider the first three factors in turn.

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Principle of Distinction

Parties to an armed conflict must abide by the principle of distinction. In the third article in this series, we looked at this principle as it relates to persons—enemy combatants may be attacked, the civilian population may not be, and individual civilians may only be attacked if and for such time as they directly participate in hostilities. Additional specific categories of persons are protected from attack: persons hors de combat (such as combatants who have been captured, have surrendered, or are defenseless because of wounds or sickness), medical and religious personnel, persons lawfully using the protective emblems of the Geneva Conventions, and United Nations peacekeeping personnel are all protected.

The principle of distinction applies in a conceptually similar fashion to objects—military objectives may be attacked, while civilian objects may only be attacked if and for such time as they are military objectives. Note that an “attack” encompasses all acts of violence against an adversary, whether in offense or defense.

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Which objects are classified as military objectives? There are two limbs to the definition, which we will unpack. First, an object must by its nature, location, purpose or use make an effective contribution to military action. Secondly, the object’s total or partial destruction, capture or neutralization must, in the circumstances ruling at the time, offer a definite military advantage. All objects which are not military objectives are civilian objects.

Many objects directly used by the armed forces will come within the first limb of the definition by their nature: weapons, equipment, transports, buildings occupied by armed forces, communication networks, and so on. An object may contribute to military action by its location because its seizure is strategically important.

An object’s purpose refers to its intended future use, while its use naturally refers to its present function. For example: in the normal course of events, a bridge is a civilian object, being used for everyday transportation of civilians (and their vehicles and possessions). In an armed conflict, a bridge may be used by an armed force to transport its personnel and weapons, or as a passageway to launch attacks, and would in such circumstances make an effective contribution to military action. Similarly, a civilian residential building would not ordinarily be a military objective, but in an armed conflict may be used by forces as a base to conduct operations or to launch attacks. Likewise, the building would therefore make an effective contribution to military action by its use.

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If an object is used for military purposes, it is irrelevant for the principle of distinction that it may be simultaneously used for civilian purposes (such as the bridge and residential apartment building in the examples above). That fact will be relevant for the proportionality analysis below, when we’ll take into account the anticipated civilian death and destruction to determine whether the attack would be lawful. For this step in our calculus, however, we’re only concerned that the object makes an effective contribution to military action and that its attack offers a definite military advantage.

To the second limb of our definition: it is not lawful to launch an attack which offers no military advantage, or only potential or indeterminate advantage, in the circumstances ruling at the time. A factory which manufactures parts for military naval vessels will contribute to military action by its use, but its destruction may not offer any advantage if a particular conflict is being fought exclusively on land or in the air. The bombing of a bridge may not offer any advantage if there is another bridge nearby which can just as easily be used. However, the military advantage should be considered in relation to an attack as a whole, and not just from isolated parts thereof. In this scenario, attacks on multiple bridges in a location may produce a definite military advantage where an attack on one may not.

The definite advantage sought must be of a military character, and not solely political or economic. Mixed-purpose attacks are permissible so long as the above definition is satisfied. Part of NATO’s strategy in its intervention in the former Yugoslavia in the late 1990s was to bomb the personal property and businesses of Slobodan Milošević’s allies in order to pressure them to withdraw their support for his regime—the so-called “espresso machine war”, where it was thought that support would be withdrawn only when allies were affected personally, when the lack of electricity meant they couldn’t get their daily caffeine hit. While some such attacks were directed at objects which may have had a mixed use (such as Belgrade’s television station, which—putting aside the proportionality assessment until the next step below—was used for communicating tactical information), purely civilian objects such as cigarette and plastics factories doubtful made an effective contribution to military action, and attacks on them unlikely yielded a definite military advantage.

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Civilian objects are protected against attack, unless and for such time as they are military objectives. We can therefore see that many objects which will “ordinarily” be civilian objects may be protected one moment but targetable the next, losing and regaining protection according to shifting facts on the ground. The inquiry must be made according to the prevailing circumstances at the time of the attack—if an object was previously being used to contribute to military action but is no longer so, it cannot be targeted. In the case of doubt whether an object normally dedicated to civilian purposes (such as a place of worship, house, or school) is being used to make an effective contribution to military action, it must be presumed not to be so used.

There are several defined classes of specially protected objects which are singled out for extra protection. These only lose their protection (and consequently become eligible for attack) in certain circumstances.

It is prohibited to attack non-defended localities, areas designated as hospital and safety zones and localities, and areas which have been agreed upon between parties to a conflict as demilitarized zones. In addition, attacking, destroying, removing, or rendering useless objects indispensable to the survival of the civilian population is prohibited. Such objects may include water installations, crops, or livestock.

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Certain specific works or installations containing dangerous forces—namely dams, dykes and nuclear electrical generating stations—cannot be attacked even where they are military objectives, if the attack may cause the release of dangerous forces and consequent severe losses among the civilian population. Other military objectives located in the vicinity of works or installations containing dangerous forces may not be attacked if the same adverse consequences are expected. In each case, these objects may only be attacked if they are being used in regular, significant, and direct support of military operations and if attack is the only feasible way to terminate that support.

The general principles on the conduct of hostilities apply to the natural environment, including that no part of the natural environment may be attacked, unless it is a military objective.

International humanitarian law provides special protection to cultural heritage of peoples affected by conflict. Both Additional Protocol I and the 1954 Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict lay down protection for cultural objects and places of worship. Cultural property is defined in the latter treaty to cover movable or immovable property of great importance to the cultural heritage of every people (such as architecture, art, archaeological sites, manuscripts, and scientific collections) and buildings which preserve or exhibit cultural property (such as museums, libraries, or archives). Cultural objects must not be attacked and may not be used for military purposes. Even if they are so used, their immunity may only be waived in cases of imperative military necessity. In 2016 the International Criminal Court convicted a Malian national for the war crime of attacking religious and historic buildings in Timbuktu.

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In addition to the prohibition on direct attacks, indiscriminate attacks are also prohibited—being those not directed at a specific military objective and consequently are of a nature to strike military objectives and civilians or civilian objects without distinction. Attacks by bombardment which treats a number of clearly separated and distinct military objectives in an area as a single military objective (carpet or saturation bombings) are an example of prohibited indiscriminate attacks.

It is a grave breach of Additional Protocol I—such that every nation in the world is obliged to search for and prosecute or hand over for trial persons on their territory suspected of their commission—to attack the civilian population or individual protected civilians, works or installations containing dangerous forces, non-defended localities and demilitarized zones, or items of cultural heritage. In addition, it is a war crime to intentionally attack any of the following: the civilian population or civilians not taking a direct part in hostilities; civilian objects; personnel and material involved in a United Nations humanitarian assistance or peacekeeping mission; undefended localities; items of cultural heritage which are not military objectives; and personnel and material lawfully using the protective emblems (either the red cross, the red crescent, or the red crystal).

Proportionality

Having established that our target is indeed a military objective for the purposes of the principle of distinction, we must now consider whether the attack would be proportionate. This is perhaps the single most consequential rule in all of international humanitarian law—the entirety of the laws of war are aimed at striking a balance between military necessity and humanitarian requirements, and this compromise has either spared or condemned of hundreds of millions of human lives over the years.

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An attack will be disproportionate if it may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.

Let’s break that down. The principle of proportionality is indeed a legal balancing exercise, and so we consider it in three component parts: the two counterweights and the fulcrum.

On one side of the ledger, the incidental loss of civilian life, injury to civilians, and damage to civilian objects (which I’ll collectively refer to as “civilian harm”) expected from an attack may arise in many ways. Most simply, if there are large numbers of civilians or many important civilian objects in the vicinity of the target at the time of the attack, the incidental civilian harm as a result of the attack on the military objective can be expected to be large. The civilian harm will vary depending on the type of weaponry used—a large bomb will cause more extensive destruction than a small bomb. Poor weather conditions may decrease visibility and accuracy and therefore increase the expected danger of an attack. The nature of the specific military objective aimed at may mean that the kinetic force of an attack causes further civilian harm, for example from explosion of a fuel reservoir or the release of radioactive material from a nuclear power station. Targets located in certain geographic areas may cause landslides, bushfires, or floods. It is accepted that parties must take into account reasonably foreseeable reverberating effects of an attack, for example the consequential suffering of the civilian population caused by attacks on water purification stations. However, not all butterfly effects must be considered, so in attacking a weapons factory a warring party does not need to take into consideration the loss of the factory workers’ wages.

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On the other side of the ledger, the anticipated concrete and direct military advantage is taken into account—the enemy combatants killed and maimed and the military objectives destroyed, the territory gained, and consequences which directly enhances friendly military operations or hinder those of the enemy. The advantage must be of a military character—advantages which are solely political, psychological, economic, financial, social or moral must be excluded from the equation. The specific wording of the rule was chosen to show that the advantage should be substantial and relatively close, and that advantages which are hardly perceptible or which would only appear in the long term should be disregarded. The military advantage must be foreseeable by the perpetrator at the relevant time, and such advantage may or may not be temporally or geographically related to the object of the attack.

Negative military consequences of a successful attack must be taken into account—in many types of modern conflict, an attack which causes incidental civilian harm may mean that enemy forces intensify their operations and take revenge against civilians, may make the local civilian population reluctant to provide valuable intelligence, and may inspire new recruits to join enemy forces or become sympathizers.

Finally, the standard by which the two sides of the ledger are to be compared: the civilian harm must not be excessive in relation to the military advantage anticipated. This is in many ways quite a sobering rule—your nation has agreed that you, dear reader, may be lawfully killed if an enemy nation gains a sufficient military advantage from doing so.

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There is no set formula for what constitutes “excessive” civilian harm—it may be thought of as an “objective subjective” test, whereby a hypothetical reasonable military commander must assess the standard in good faith. It is not simply a matter, for example, of counting that there are to be more enemy casualties than civilian casualties (or some other defined multiplier). By and large, military manuals around the world provide examples of attacks which are either clearly proportionate or clearly disproportionate rather than discussing the general principles of the analysis—Canada’s manual states that complete destruction of a town in order to eliminate a small pocket of opposing forces would be disproportionate, but an air strike on an ammunition dump where there is a farmer ploughing an adjacent field would be proportionate.

The fact that proportionality is set in terms of attacks “which may be expected” to cause excessive harm indicates that the inquiry is to be gauged at the time of the attack, based on the information available to the actor at the time, rather than by looking back with hindsight.

Wilfully conducting a disproportionate attack is a grave breach of Additional Protocol I. In the International Criminal Court, the corresponding war crime is based on a slightly different definition of proportionality, incorporating widespread, long-term and severe damage to the natural environment on one side of the ledger, and requiring that civilian harm must not be “clearly” excessive.

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Precautions

Having established that a target is a military objective, and that an attack in the prevailing circumstances would not be disproportionate, a party to an armed conflict must take precautionary measures in attack. The general rule is that in the conduct of military operations constant care must be taken to spare the civilian population, civilians, and civilian objects.

Those who plan or decide upon attack must do everything feasible to verify that a target is a military objective, that it is not subject to special protection, and that the attack is not prohibited. They must also take all feasible precautions in the choice of means and methods of attack (which we will discuss in detail in the next entry in this series) with a view to avoiding, and in any event to minimizing, incidental civilian harm. Most simply, a large bomb should not be used where a small one would do, and a weapons factory staffed by civilians should not be targeted during working hours where it could just as easily be targeted at night.

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If it becomes apparent that a target is not a military objective or that an attack on it would be disproportionate, an attack must be cancelled or suspended. This may happen in the planning phase, for example if an armed force gains intelligence that a target is not or is no longer being used for military purposes, or in the execution phase, for example if a military pilot observes a substantial number of civilians in the vicinity of a target which were not expected and therefore not factored into the proportionality analysis. In the latter case there is nothing preventing the pilot from carrying out the attack at a later time when the civilians have dispersed, if at that time the principles of distinction and proportionality are respected.

If a choice is available between attacking different military objectives to obtain a similar military advantage, the one which is expected to cause the least danger to civilian lives and objects must be selected.

If an attack may affect the civilian population, effective advance warning must be given, unless circumstances do not permit. Warnings are a crucial factor in limiting civilian casualties and can often transform a proposed attack from being disproportionate to lawful. The factors that make a warning “effective” will differ from case to case—less time is required to evacuate a bridge than an entire town, for example. During World War II, warnings were made by radio or by dropping pamphlets, while in modern times “roof-knocking” has been controversially used by Israel in Palestine and by the United States in Iraq whereby low-impact devices are dropped on the roofs of civilian homes shortly before full-strength attacks. Circumstances would plainly not permit advance warnings when launching an attack on enemy combatants or when taking a location by assault.

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Nations must take precautionary measures in defense as well as in attack. They should avoid constructing fixed military objectives (such as bases) within or near densely populated areas. In time of conflict, nations should endeavor to remove the civilian population under their control from the vicinity of military objectives and generally protect them against the dangers resulting from military operations. Note that these obligations may apply to a nation even when it is not party to any armed conflict—the former is largely a peacetime obligation, and the latter may occur while a foreign armed force is fighting a rebel armed group on its territory.


Death and destruction in war are always tragic, but may often be lawful according to the laws that the nations of the world have agreed to be bound by. Attacks which cause indiscriminate or disproportionate civilian harm are among the most serious war crimes that may be committed, and their authors must be held to account.

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The next installment of this series will cover the means and methods of warfare.