Proportionality in International Humanitarian Law: A Principle and a Rule

by | Oct 24, 2022


This post is based on research supporting a doctoral dissertation in progress at the University of Geneva.

Proportionality plays a key role in international humanitarian law (IHL). It is essential to regulating the conduct of hostilities, requiring that the expected incidental harm is not excessive in relation to the anticipated military advantage (art. 51(5)(b), 57(2)(a)(iii) and (b) Additional Protocol I). But proportionality plays a greater role than this, appearing in many other provisions of the 1949 Geneva Conventions (GCs) and the 1977 Additional Protocols (APs).

This post demonstrates the different legal forms that proportionality takes, as both a principle and a rule of IHL. First, it outlines the general characteristics of legal principles to distinguish them from legal rules. Second, it analyzes the principle of proportionality, confirming its generality and proving its status as a general principle of IHL. Finally, the post considers the quality of the rule of proportionality and seeks to define its limitations.

Legal Principles

Legal principles are characterized by their high degree of generality (para. 79). Principles prescribe unspecific actions, whereas rules are formulated with greater precision. A principle may also have an indefinite scope of application.

Some legal scholars question this distinction because rules can also be characterized by their generality. For instance, rules might address an unspecified number of people and apply to an undefined body of facts, in contrast with decisions that apply to a specific person in a specific situation. Rules, however, have a greater degree of specificity than principles.

To illustrate, consider rules on road traffic, in particular speed restrictions. The maximum speed limit is a rule. It is general because it addresses an unlimited number of people (drivers) in an unlimited number of situations (driving at any time and speed). This contrasts with an individual and concrete decision (e.g., an imposed fine) which concerns a particular person, driving at a certain time at a defined speed above the limit. A principle, however, exhibits a greater degree of generality. It can be applied across different legal regimes, as the principle of good faith, the principle of non-retroactivity, and the principle of proportionality illustrate. Thus, the generality of principles is very broad and exceeds the generality of legal rules.

The Principle of Proportionality in IHL

Given its generality, the principle of proportionality appears in many provisions of the GCs and APs beyond the context of targeting. Here some examples of provisions where proportionality applies:

Art. 12(5), GC I: The Party to the conflict which is compelled to abandon wounded or sick to the enemy shall, as far as military considerations permit, leave with them a part of its medical personnel and material to assist in their care.

Art. 8(3), GC III, IV: The representatives or delegates of the Protecting Powers shall not in any case exceed their mission under the present Convention. They shall, in particular, take account of the imperative necessities of security of the State wherein they carry out their duties.

Art. 48(3), GC III: Mail and parcels addressed to their former camp shall be forwarded to them without delay. The camp commander shall take, in agreement with the prisoners’ representative, any measures needed to ensure the transport of the prisoners, community property and of the luggage they are unable to take with them in consequence of restrictions imposed by virtue of the second paragraph of this Article.

Art. 75(3), GC III: In the absence of special agreements, the costs occasioned by the use of such means of transport shall be borne proportionally by the Parties to the conflict whose nationals are benefited thereby.

These examples not only show that proportionality in IHL contains a high degree of generality that allows it to be applied widely (but not everywhere). They also illustrate that the principle takes a number of different forms.

The first relates to the correlation that must exist between the means invested and the objective to be achieved. This form of proportionality is known as “proportionality of means to ends.” For example Article 89(4) of GC IV provides that internees who work shall receive additional rations “in proportion” to the kind of labour which they perform. It requires a correlation between the means and the objectives sought and does not require an additional comparison of antinomic interests, and so no weighing of interest. This type of proportionality and “necessity” are closely linked, if not merged. Indeed, necessity (p. 168) is described as, inter alia, the capacity of a measure to serve the desired purpose and the search for the least onerous means. But it must be distinguished from military necessity, which can only be invoked within the framework authorized by IHL, when a norm provides for it by a reference.

The second type of proportionality corresponds to a “balance of interests.” To illustrate, Article 42(1) of GC IV provides that a detaining power may order the internment of a protected person “only if the security of the Detaining Power makes it absolutely necessary.” This form of proportionality requires an evaluation of conflicting values, in this case the rights of the protected person against the security of the detaining power. A further example is the IHL rule of proportionality applicable to attacks. This rule prescribes that the anticipated incidental damage must not be excessive in relation to the expected military advantage. It pits incidental damage versus the military advantage anticipated.

A third form of proportionality concerns the connection between the seriousness of a crime committed and the punishment that would allow justice to be done. For instance, Article 50(1) of GC II provides that the High Contracting Parties must enact legislation to provide “effective penal sanctions” for persons committing or ordering the commission of grave breaches of the GCs. This form of proportionality is referred to as “inherent proportionality” in the penal domain. The punishment must be proportionate to the seriousness of the crime committed.

Finally, the fourth form of proportionality consists of a calculation called “mathematical proportionality.” Article 75(4) of GC III, relating to the distribution of the costs of special transport, provides for mathematical proportionality in the sense of a pro rata share. The provision states: “In the absence of special agreements, the costs occasioned by the use of such means of transport shall be borne proportionally by the Parties to the conflict whose nationals are benefited thereby.”Other mathematical calculations involving the application of proportionality also appear in IHL, such as Article 60(3)(b) of GC III relating to advances of pay.

The Normative Quality of the Principle of Proportionality

Proportionality is relevant beyond IHL. Although it does not apply in the same way in every field, it always contains the common idea of a balance between elements which, if they change, must all guarantee the same ratio of magnitude. Often, this equilibrium concerns interests that oppose each other.

Proportionality is omnipresent in public international law. For example, it can be found in international investment law and arbitration, maritime delimitation, WTO law, the protection of human rights bodies (IACHR, ECHR, ACHPR), European Union law, international criminal law, jus ad bellum, and jus in bello.

More specifically, proportionality is a fundamental principle of IHL, alongside the principles of distinction and precautions in attack. It governs the protection of civilians and civilian objects in the conduct of hostilities. It has a limiting role (p. 4) that impacts targeting, including in the choice of weapons used, the precautionary measures required to be taken, and the anticipated incidental civilian harm, which must not be disproportionate to the expected military advantage.

Norms of IHL embody a delicate balance between military necessity and the principle of humanity. When adopting IHL’s provisions, States sought to find an appropriate balance between these competing interests. In this sense, the St. Petersburg Declaration of 1868 explicitly recognizes the need to reconcile the necessities of war with the laws of humanity. Therefore, each legal rule and principle of the law of armed conflict manifests and incorporates a balance (p. 41) of these opposing interests to reach an equilibrium. Proportionality is the link between those countervailing interests when they lead to conflicting results.

Proportionality is a general principle of IHL (p. 48), and a source of international law (p. 81). But general principles can perform other roles. They can operate as a tool to fill gaps in conventional and customary international law (avoiding non liquet) or as a means of interpreting other rules of international law, or of supporting legal reasoning. Moreover, principles of law can act as a foundation of the international legal system or as a means of strengthening its systematic character.

The Rule of Proportionality in the Conduct of Hostilities

In addition to being a principle of IHL, proportionality is a rule relating to the protection of civilians in the conduct of hostilities. The three elements that lead to its characterization as a rule are its codification, its specific scope, and its all-or-nothing operation.

Codification consists of adopting a norm into writing. Proportionality devoted to the protection of civilians and civilian property in the conduct of hostilities, as enshrined in articles 51(5)(b), 57(2)(a)(iii), and 57(2)(b) of AP I, is the first codification (p. 6) of the rule of proportionality. It codifies the general principle of proportionality in a particular case that allows it to regain the quality of a rule. The rule is recognized as customary based on State practice and opinio juris. Moreover, its codification clearly attests to its status as law. This avoids the long and laborious demonstration of State practice and opinio juris required to establish uncodified customary international law. It is also the first necessary step in demonstrating its status as a rule.

In addition to being codified, the rule of proportionality has a specific scope. It applies in a particular context, in the conduct of hostilities and prescribes a specific type of proportionality: the balance of interests. It also determines the elements to be weighed. The rule of proportionality requires that the anticipated incidental loss of human life and damage to civilian objects should not be excessive in relation to the concrete and direct military advantage expected from the destruction of a military objective. The balance is composed of, on the one hand, the military advantage expected from the destruction of a military objective, and on the other hand, the incidental damage caused by the military intervention, i.e., the harm to civilians and civilian property.

The limited circumstances to which the rule of proportionality applies demonstrate its specific scope. The rule applies only when a military objective (legitimate target) is the object of an attack and incidental damage is foreseeable. Moreover, the rule applies only to an attack. Not every military operation in an armed conflict constitutes an attack.

Finally, the proportionality rule operates according to an “all-or-nothing” (p. 24) model. This means that once the conditions for its application are met, it must be complied with in full. The principle of proportionality, on the contrary, offers variable solutions since it operates according to the “plus-or-minus” (p. 427) scheme. Principles are norms that require something to be achieved to the greatest extent possible according to the factual possibilities and within the limits imposed by other competing principles. They are “optimization requirements”(p. 47) characterized by the fact that they can be satisfied to varying degrees and the appropriate level of satisfaction depends not only on factual, but also on legal and political possibilities. Rules, on the other hand, are either satisfied or not satisfied, with no room for discretion. They are called “fixed points” (p. 2) in the field of factual and legal possibilities.

Article 51(5)(b) AP I requires an “all or nothing” assessment of proportionality. Its violation automatically leads to a breach of the obligation. The fact that manifest disproportionality constitutes a war crime further demonstrates this “all-or-nothing” aspect. Weighing the elements in the balance is certainly delicate, but the consequence of the violation is pre-determined. Either the rule is respected because the incidental effects are legally justified, or the rule is violated because the anticipated harm is disproportionate to the expected advantage to be gained.

From this perspective, the implementation of the principle of proportionality in IHL differs from that of the rule. The principle is adjacent to the norm to which it relates. It can be seen as a complementary legal instrument. Its violation does not systematically constitute a breach of the relevant norm, or an independent offence. The principle of proportionality applies to many provisions, but unlike the rule of proportionality it is not a provision in itself.


Proportionality in IHL takes the form both of a rule and a principle. The principle of proportionality has a broad scope and can take various forms. The rule of proportionality applies solely to the protection of civilians in the conduct of hostilities. In a subsequent post, I will address the application of the principle of proportionality outside the sphere of targeting using examples from the current conflict between Russia and Ukraine.


Anaïs Maroonian is a Ph.D. candidate in international humanitarian law at the Faculty of Law of the University of Geneva.


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