International humanitarian law has its origins in the customary practices of armies as they developed over the ages and on all continents. The “laws and customs of war”, as this branch of international law has traditionally been called, was not applied by all armies, and not necessarily vis-à-vis all enemies, nor were all the rules the same. However, the pattern that could typically be found was restraint of behaviour vis-à-vis combatants and civilians, primarily based on the concept of the soldier’s honour. The content of the rules generally included the prohibition of behaviour that was considered unnecessarily cruel or dishonourable, and was not only developed by the armies themselves, but was also influenced by the writings of religious leaders.
The most significant landmark from the point of view of cataloguing these customs in one document was the drafting by Professor Francis Lieber of the Instructions for the Government of Armies of the United States in the Field, promulgated as General Order No. 100 by President Lincoln in 1863 during the American Civil War. The Lieber Code, as it is now known, strongly influenced the further codification of the laws and customs of war and the adoption of similar regulations by other States. Together, they formed the basis of the draft of an international convention on the laws and customs of war presented to the Brussels Conference in 1874. Although this conference did not adopt a binding treaty, much of its work was later used in the development of the 1899 and 1907 Hague Conventions and Declarations. These treaties did not codify all aspects of custom, but its continued importance was reaffirmed in the so-called “Martens clause”, first inserted in the preamble to the 1899 Hague Convention (II), which provides that:
Until a more complete code of the laws of war is issued, the High Contracting Parties think it right to declare that in cases not included in the Regulations adopted by them, populations and belligerents remain under the protection and empire of the principles of international law, as they result from the usages established between civilized nations, from the laws of humanity and the requirements of the public conscience.
The importance attributed to customary law, despite, or because of, its partial codification, was most clearly seen in the reliance placed on it by the various war crimes trials after both the First and Second World Wars.
The driving force behind the development of international humanitarian law has been the International Committee of the Red Cross (ICRC), founded in 1863. It initiated the process which led to the conclusion of the Geneva Conventions for the protection of the victims of war of 1864, 1906, 1929 and 1949. It was at the origin of the 1899 Hague Convention (III) and 1907 Hague Convention (X), which adapted, respectively, the 1864 and 1906 Geneva Conventions to maritime warfare and were the precursors of the Geneva Convention for the Amelioration of the Condition of the Wounded, Sick and Shipwrecked Members of Armed Forces at Sea of 1949. It took the initiative to supplement the Geneva Conventions that led to the adoption in 1977 of two Additional Protocols. The ICRC has both encouraged the development of and been involved in the negotiation of numerous other treaties, such as the 1980 Convention on Certain Conventional Weapons, the 1997 Ottawa Convention banning anti-personnel landmines and the 1998 Statute of the International Criminal Court. Recognition of this role is reflected in the mandate given to the ICRC by the international community to work for “the faithful application of international humanitarian law applicable in armed conflicts” and for “the understanding and dissemination of knowledge of international humanitarian law applicable in armed conflicts and to prepare any development thereof”.
More than 50 years have now passed since the Geneva Conventions of 1949 were adopted and almost 30 years since the adoption of their Additional Protocols. These years have, unfortunately, been marked by a proliferation of armed conflicts affecting every continent. Throughout these conflicts, the Geneva Conventions – and in particular Article 3 common to the four Conventions, applicable in non-international armed conflicts – together with their Additional Protocols have provided legal protection to war victims, namely persons who do not or no longer participate in hostilities (the wounded, sick and shipwrecked, persons deprived of their liberty for reasons related to the conflict, and civilians). Nevertheless, there have been countless violations of these treaties and of basic humanitarian principles, resulting in suffering and death which might have been avoided had international humanitarian law been respected.
The general opinion is that violations of international humanitarian law are not due to the inadequacy of its rules, but rather to a lack of willingness to respect them, to a lack of means to enforce them and to uncertainty as to their application in some circumstances, but also to ignorance of the rules on the part of political leaders, commanders, combatants and the general public.
The International Conference for the Protection of War Victims, convened in Geneva from 30 August to 1 September 1993, discussed, in particular, ways and means to address violations of international humanitarian law but did not propose the adoption of new treaty provisions. Instead, in its Final Declaration, adopted by consensus, the Conference reaffirmed “the necessity to make the implementation of humanitarian law more effective” and called upon the Swiss government “to convene an open-ended intergovernmental group of experts to study practical means of promoting full respect for and compliance with that law, and to prepare a report for submission to the States and to the next session of the International Conference of the Red Cross and Red Crescent”.
To this end, the Intergovernmental Group of Experts for the Protection of War Victims met in Geneva in January 1995 and adopted a series of recommendations aimed at enhancing respect for international humanitarian law, in particular by means of preventive measures that would ensure better knowledge and more effective implementation of the law. Recommendation II of the Intergovernmental Group of Experts proposed that:
The ICRC be invited to prepare, with the assistance of experts in IHL [international humanitarian law] representing various geographical regions and different legal systems, and in consultation with experts from governments and international organisations, a report on customary rules of IHL applicable in international and non-international armed conflicts, and to circulate the report to States and competent international bodies.
In December 1995, the 26th International Conference of the Red Cross and Red Crescent endorsed this recommendation and officially mandated the ICRC to prepare a report on customary rules of international humanitarian law applicable in international and non-international armed conflicts.
The present study is the outcome of the research carried out pursuant to this mandate.
International humanitarian treaty law is well developed and covers a wide variety of aspects of warfare, offering protection to victims of war and limiting permissible means and methods of warfare. The four Geneva Conventions of 1949 and their Additional Protocols of 1977 provide an extensive regime for the protection of persons who do not or no longer participate in armed conflict. The regulation of the means and methods of warfare in treaty law goes back to the 1868 St. Petersburg Declaration, the 1899 and 1907 Hague Conventions and the 1925 Geneva Gas Protocol and has most recently been addressed in the 1972 Biological Weapons Convention, the 1977 Additional Protocols, the 1980 Convention on Certain Conventional Weapons and its five Protocols, the 1993 Chemical Weapons Convention and the 1997 Ottawa Convention banning anti-personnel landmines. The protection of cultural property in the event of armed conflict is regulated in detail in the 1954 Hague Convention and its two Protocols. The 1998 Statute of the International Criminal Court contains a list of war crimes subject to its jurisdiction.
There are, however, two important impediments to applying these treaties to current armed conflicts. First, treaties apply only to the States that have ratified them. This means that different treaties of international humanitarian law apply to different armed conflicts depending on which treaties the States involved have ratified. While nearly all States have ratified the four Geneva Conventions of 1949, Additional Protocol I has not yet gained universal adherence. As the Protocol is applicable only between parties to a conflict that have ratified it, its efficacy today is limited because several States that have been involved in international armed conflicts are not a party to it. Similarly, Additional Protocol II is only applicable in armed conflicts taking place on the territory of a State that has ratified it. While some 150 States have ratified this Protocol, several States in which non-international armed conflicts are taking place have not. In these non-international armed conflicts, common Article 3 of the four Geneva Conventions often remains the only applicable treaty provision.
Secondly, this wealth of treaty law does not regulate a large proportion of today’s armed conflicts in sufficient detail. The primary reason for this is that the majority of current armed conflicts are non-international, which are subject to far fewer treaty rules than international conflicts, although their number is increasing. In fact, only a limited number of treaties apply to non-international armed conflicts, namely the Convention on Certain Conventional Weapons, as amended, the Statute of the International Criminal Court, the Ottawa Convention banning anti-personnel landmines, the Chemical Weapons Convention, the Hague Convention for the Protection of Cultural Property and its Second Protocol and, as already mentioned, Additional Protocol II and Article 3 common to the four Geneva Conventions. While common Article 3 is of fundamental importance, it only provides a rudimentary framework of minimum standards and does not contain much detail. Additional Protocol II usefully supplements common Article 3, but it is still less detailed than the rules governing international armed conflicts contained in Additional Protocol I.
Additional Protocol II contains a mere 15 substantive articles, whereas Additional Protocol I has more than 80. These figures may not be all important, but they nonetheless show that there is a significant difference in terms of regulation between international and non-international armed conflicts, with the latter suffering from a lack of rules, definitions, details and requirements in treaty law. This is the prevailing situation, even though the majority of armed conflicts today are non-international.
Specifically, Additional Protocol II contains only a very rudimentary regulation of the conduct of hostilities. Article 13 provides that “the civilian population as such, as well as individual civilians, shall not be the object of attack … unless and for such time as they take a direct part in hostilities”. Unlike Additional Protocol I, Additional Protocol II does not contain, however, specific rules and definitions with respect to the principles of distinction and proportionality.
Common sense would suggest that such rules, and the limits they impose on the way war is waged, should be equally applicable in international and non-international armed conflicts. The fact that in 2001 the Convention on Certain Conventional Weapons was amended to extend its scope to non-international armed conflicts is an indication that this notion is gaining currency within the international community.
This study provides evidence that many rules of customary international law apply in both international and non-international armed conflicts and shows the extent to which State practice has gone beyond existing treaty law and expanded the rules applicable to non-international armed conflicts. In particular, the gaps in the regulation of the conduct of hostilities in Additional Protocol II have largely been filled through State practice, which has led to the creation of rules parallel to those in Additional Protocol I, but applicable as customary law to non-international armed conflicts.
Knowledge of the rules of customary international law is therefore of use to the many actors involved in the application, dissemination and enforcement of international humanitarian law, such as governmental authorities, arms bearers, international organisations, components of the International Red Cross and Red Crescent Movement and non-governmental organisations. A study on customary international humanitarian law may also be helpful in reducing the uncertainties and the scope for argument inherent in the concept of customary international law.
Knowledge of the rules of customary international law may also be of service in a number of situations where reliance on customary international law is required. This is especially relevant for the work of courts and international organisations. Indeed, courts are frequently required to apply customary international law. This is the case, for example, for the International Criminal Tribunal for the Former Yugoslavia which, pursuant to Article 3 of its Statute, has jurisdiction over violations of the laws and customs of war. As a result, the Tribunal has had to determine whether certain violations of international humanitarian law were violations under customary international law over which the Tribunal has jurisdiction. In addition, in many countries, customary international law is a source of domestic law and can be invoked before and adjudicated by national courts. Customary international law is also relevant to the work of international organisations in that it generally represents the law binding upon all member States.
This study has not sought to determine the customary nature of each treaty rule of international humanitarian law and, as a result, does not necessarily follow the structure of existing treaties. Rather, it has sought to analyse issues in order to establish what rules of customary international law can be found inductively on the basis of State practice in relation to these issues. As the approach chosen does not analyse each treaty provision with a view to establishing whether or not it is customary, it cannot be concluded that any particular treaty rule is not customary merely because it does not appear as such in this study. In this regard, it is important to note that the great majority of the provisions of the Geneva Conventions of 1949, including common Article 3, are considered to be customary law, and the same is true for the 1907 Hague Regulations (see infra). Furthermore, given that the Geneva Conventions have now been ratified by 192 States, they are binding on nearly all States as a matter of treaty law.
It was decided not to research customary law applicable to naval warfare as this area of law was recently the subject of a major restatement, namely the San Remo Manual on Naval Warfare.
The general rules contained in the manual were nevertheless considered useful for the assessment of the customary nature of rules that apply to all types of warfare.
A number of topics could not be developed in sufficient detail for inclusion in this edition, but they might be included in a future update. These include, for example, the Martens clause, identification of specifically protected persons and objects, and civil defence.
Where relevant, practice under international human rights law has been included in the study. This was done because international human rights law continues to apply during armed conflicts, as indicated by the express terms of the human rights treaties themselves, although some provisions may, subject to certain conditions, be derogated from in time of public emergency. The continued applicability of human rights law during armed conflict has been confirmed on numerous occasions by the treaty bodies that have analysed State behaviour, including during armed conflict, and by the International Court of Justice (see introduction to Chapter 32). This study does not purport, however, to provide an assessment of customary human rights law. Instead, human rights law has been included in order to support, strengthen and clarify analogous principles of international humanitarian law. In addition, while they remain separate branches of international law, human rights law and international humanitarian law have directly influenced each other, and continue to do so, and this for mainly three reasons. First, an assessment of conformity with human rights law at times involves a determination of respect for or breach of international humanitarian law. For example, measures taken in states of emergency will be unlawful under human rights law if, inter alia
, they violate international humanitarian law.
Conversely, international humanitarian law contains concepts the interpretation of which needs to include a reference to human rights law, for example, the provision that no one may be convicted of a crime other than by a “regularly constituted court affording all the judicial guarantees which are recognised as indispensable”.
Secondly, human rights-type provisions are to be found in international humanitarian law, for example, Article 75 of Additional Protocol I and Articles 4 and 6 of Additional Protocol II, and humanitarian law-type provisions are to be found in human rights law, for example, the provisions on child soldiers in the Convention on the Rights of the Child and its Protocol on the Involvement of Children in Armed Conflict. Thirdly, and most significantly, there is extensive practice by States and by international organisations commenting on the behaviour of States during armed conflict in the light of human rights law.
The Statute of the International Court of Justice describes customary international law as “a general practice accepted as law”.
It is generally agreed that the existence of a rule of customary international law requires the presence of two elements, namely State practice (usus
) and a belief that such practice is required, prohibited or allowed, depending on the nature of the rule, as a matter of law (opinio juris sive necessitatis
). As the International Court of Justice stated in the Continental Shelf case
: “It is of course axiomatic that the material of customary international law is to be looked for primarily in the actual practice and opinio juris
The exact meaning and content of these two elements has been the subject of much academic writing. The approach taken in this study to determine whether a rule of general customary international law exists is a classic one, set out by the International Court of Justice in a number of cases, in particular in the North Sea Continental Shelf cases.
In the assessment of State practice, two separate issues need to be addressed, namely the selection of practice that contributes to the creation of customary international law and the assessment of whether this practice establishes a rule of customary international law.
Selection of State practice
The practice collected for the purpose of this study, and which is summarised in Volume II, was selected on the basis of the following criteria:
(i) Both physical and verbal acts of States constitute practice that contributes to the creation of customary international law. Physical acts include, for example, battlefield behaviour, the use of certain weapons and the treatment provided to different categories of persons. Verbal acts include military manuals, national legislation, national case-law, instructions to armed and security forces, military communiqués during war, diplomatic protests, opinions of official legal advisers, comments by governments on draft treaties, executive decisions and regulations, pleadings before international tribunals, statements in international organisations and at international conferences and government positions taken with respect to resolutions of international organisations.
The approach to consider both physical and verbal acts as practice follows that taken by leading bodies in the field of international law and by States themselves. The International Court of Justice has taken into consideration official statements as State practice in a number of cases, including the Fisheries Jurisdiction cases
the Nicaragua case
and the Gabčíkovo-Nagymaros Project case
The International Law Commission has similarly considered verbal acts of States as contributing towards the creation of customary international law. It did so, for example, in the context of the Draft Articles on State Responsibility where it considered the concept of a “state of necessity” to be customary.
The International Criminal Tribunal for the Former Yugoslavia has stated that in appraising the formation of customary rules of international humanitarian law, “reliance must primarily be placed on such elements as official pronouncements of States, military manuals and judicial decisions”.
The International Law Association considers that “verbal acts, and not only physical acts, of States count as State practice” and points out that “the practice of the international tribunals is replete with examples of verbal acts being treated as examples of practice. Similarly, States regularly treat this sort of act in the same way.”
Whether physical or verbal, relevant practice only consists of official practice. Hence, the physical acts of parties to armed conflicts contribute only to the creation of rules of customary international law as long as they represent official practice.
Abstention from certain conduct is also noted where relevant. Such omissions will be discussed in more detail below.
(ii) The practice of the executive, legislative and judicial organs of a State can contribute to the formation of customary international law. The State comprises the executive, legislative and judicial branches of government. The organs of these branches can engage the international responsibility of the State and adopt positions that affect its international relations.
In case of conflict between the positions of various organs of a State, the practice is considered internally inconsistent and does not contribute to the formation of customary law.
(iii) Acts do not contribute to the formation of customary international law if they are never disclosed.
This is so as long as such acts are not known to other States and, consequently, do not give them an opportunity, if they so wished, to react to them. In order to count, practice has to be public or communicated to some extent. This does not necessarily mean that the practice has to be published or communicated to the whole world, but at least it should be communicated to one other State or relevant international organisation, including the ICRC. States communicate with the ICRC in the context of its international mandate to assist in the implementation of international humanitarian law and the fact that it may “take cognizance of any complaints based on alleged breaches of [international humanitarian law]”.
Hence, communications to the ICRC, while often confidential, are not purely private acts and count as State practice.
(iv) Although decisions of international courts are subsidiary sources of international law,
they do not constitute State practice. This is because, unlike national courts, international courts are not State organs. Their decisions have nevertheless been included because a finding by an international court that a rule of customary international law exists constitutes persuasive evidence to that effect. In addition, because of the precedential value of their decisions, international courts can also contribute to the emergence of a rule of customary international law by influencing the subsequent practice of States and international organisations.
What States claim before international courts, however, is clearly a form of State practice.
(v) International organisations have international legal personality and can participate in international relations in their own capacity, independently of their member States. In this respect, their practice can contribute to the formation of customary international law.
Therefore, this study has included, for example, the UN Secretary-General’s Bulletin on observance by United Nations forces of international humanitarian law as relevant practice, in particular because “the instructions in the Bulletin reflect the quintessential and most fundamental principles of the laws and customs of war”, even though it is recognised that “the Secretary-General did not consider himself necessarily constrained by the customary international law provisions of the Conventions and Protocols as the lowest common denominator by which all national contingents would otherwise be bound”.
In addition, official ICRC statements, in particular appeals and memoranda on respect for international humanitarian law, have been included as relevant practice because the ICRC has international legal personality.
The practice of the organisation is particularly relevant in that it has received an official mandate from States “to work for the faithful application of international humanitarian law applicable in armed conflicts and … to prepare any development thereof”.
The view that ICRC practice counts is also adopted by the International Criminal Tribunal for the Former Yugoslavia, which has regarded the organisation’s practice as an important factor in the emergence of customary rules applicable to non-international armed conflicts.
In addition, the official reactions which ICRC statements elicit are State practice.
(vi) The negotiation and adoption of resolutions by international organisations or conferences, together with the explanations of vote, are acts of the States involved. With a few exceptions, it is recognised that resolutions are normally not binding in themselves and therefore the value accorded to any particular resolution depends on its content, its degree of acceptance and the consistency of State practice outside it.
The greater the support for the resolution, the more importance it is to be accorded. Information on reasons for abstentions or negative votes is therefore indicated in this study where relevant, for such votes are often based on disagreement with certain parts of the resolution and not necessarily with the resolution as a whole. Likewise, statements made by States during debates on the drafting of resolutions constitute State practice and have been included where relevant.
(vii) The practice of armed opposition groups, such as codes of conduct, commitments made to observe certain rules of international humanitarian law and other statements, does not constitute State practice as such. While such practice may contain evidence of the acceptance of certain rules in non-international armed conflicts, its legal significance is unclear and it has therefore been listed under “Other Practice” in Volume II.
Assessment of State practice
State practice has to be weighed to assess whether it is sufficiently “dense” to create a rule of customary international law.
To establish a rule of customary international law, State practice has to be virtually uniform, extensive and representative. Although some time will normally elapse before there is sufficient practice to satisfy these criteria, no precise amount of time is required. As stated by the International Court of Justice in the North Sea Continental Shelf cases:
Although the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law on the basis of what was originally a purely conventional rule, an indispensable requirement would be that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked; and should moreover have occurred in such a way as to show a general recognition that a rule of law or legal obligation is involved.
(i) The first requirement for State practice to create a rule of customary international law is that it must be virtually uniform. Different States must not have engaged in substantially different conduct, some doing one thing and some another. In the Asylum case, the International Court of Justice was presented with a situation in which practice was not sufficiently uniform to establish a rule of customary international law with respect to the exercise of diplomatic asylum. In this respect, it stated that:
The facts brought to the knowledge of the Court disclose so much uncertainty and contradiction, so much fluctuation and discrepancy in the exercise of diplomatic asylum and in the official views expressed on various occasions, there has been so much inconsistency in the rapid succession of conventions on asylum, ratified by some States and rejected by others, and the practice has been so much influenced by considerations of political expediency in the various cases, that it is not possible to discern in all this any constant and uniform usage, accepted as law.
In the Fisheries case, the International Court of Justice dealt with a similar situation with respect to a ten-mile closing line for bays in which it considered that, although such a line had
been adopted by certain States both in their national law and in their treaties and conventions, and although certain arbitral decisions have applied it as between these States, other States have adopted a different limit. Consequently, the ten-mile rule has not acquired the authority of a general rule of international law.
However, the Court in this case also considered that “too much importance need not be attached to a few uncertainties or contradictions, real or apparent” in a State’s practice when making an evaluation.
It is enough that the practice is sufficiently similar. It was on the basis of such sufficient similarity that the International Court of Justice found in the Continental Shelf cases
that the concept of the exclusive economic zone had become part of customary law. Even though the various proclamations of such a zone were not identical, they were sufficiently similar for the Court to reach this conclusion.
The jurisprudence of the International Court of Justice shows that contrary practice which, at first sight, appears to undermine the uniformity of the practice concerned, does not prevent the formation of a rule of customary international law as long as this contrary practice is condemned by other States or denied by the government itself and therefore does not represent its official practice. Through such condemnation or denial, the original rule is actually confirmed. The International Court of Justice dealt with such a situation in the Nicaragua case in which it looked at the customary nature of the principles of non-use of force and non-intervention, stating that:
It is not to be expected that in the practice of States the application of the rules in question should have been perfect, in the sense that States should have refrained, with complete consistency, from the use of force or from intervention in each other’s internal affairs. The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolute rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule.
This findin is particularly relevant for a number of rules of international humanitarian law where there is overwhelming evidence of verbal State practice supporting a certain rule found alongside repeated evidence of violations of that rule. Where this has been accompanied by excuses or justifications by the actors and/or condemnations by other States, such violations are not of a nature to challenge the existence of the rule in question. States wishing to change an existing rule of customary international law have to do so through their official practice and claim to be acting as of right.
(ii) The second requirement for a rule of general customary international law to come into existence is that the State practice concerned must be both extensive and representative
. It does not, however, need to be universal; a “general” practice suffices.
No precise number or percentage of States is required. One reason why it is impossible to put a precise figure on the extent of participation required is that the criterion is in a sense qualitative
rather than quantitative. That is to say, it is not simply a question of how many States participate in the practice, but also which
In the words of the International Court of Justice in the North Sea Continental Shelf cases
, the practice must “include that of States whose interests are specially affected”.
This consideration has two implications: (1) if all “specially affected States” are represented, it is not essential for a majority of States to have actively participated, but they must have at least acquiesced in the practice of “specially affected States”; (2) if “specially affected States” do not accept the practice, it cannot mature into a rule of customary international law, even though unanimity is not required as explained.
Who is “specially affected” will vary according to circumstances. Concerning the question of the legality of the use of blinding laser weapons, for example, “specially affected States” include those identified as having been in the process of developing such weapons. In the area of humanitarian aid, States whose population is in need of such aid or States which frequently provide such aid are to be considered “specially affected”. With respect to any rule of international humanitarian law, countries that participated in an armed conflict are “specially affected” when their practice examined for a certain rule was relevant to that armed conflict. Notwithstanding the fact that there are specially affected States in certain areas of international humanitarian law, it is also true that all States have a legal interest in requiring respect for international humanitarian law by other States, even if they are not a party to the conflict (see commentary to Rule 144). As a result, the practice of all States must be considered, whether or not they are “specially affected” in the strict sense of that term.
This study has taken no view as to whether it is legally possible to be a “persistent objector” in relation to customary rules of international humanitarian law. Apart from the fact that many authorities believe that this is not possible in the case of rules of jus cogens
, there are also authorities that doubt the continued validity of this doctrine.
If one accepts that it is legally possible to be a persistent objector, the State concerned must have objected to the emergence of a new norm during its formation and continue to object afterwards; it is not possible to be a “subsequent objector”.
(iii) The third requirement is related to the time necessary to form a rule of customary international law through the adoption of virtually uniform, extensive and representative practice. As indicated above, while some time will normally elapse before there is sufficient practice to satisfy these criteria, there is no specific time requirement. It is all a question of accumulating a practice of sufficient density, in terms of uniformity, extent and representativeness.
The second requirement for the existence of a rule of customary international law, opinio juris, relates to the need for the practice to be carried out as of right. The particular form in which the practice and this legal conviction needs to be expressed may well differ depending on whether the rule involved contains a prohibition, an obligation or merely a right to behave in a certain manner.
Practice establishing the existence of a prohibition, for example, the rule that it is prohibited to declare that no quarter will be given (see Rule 46), includes not only statements that such behaviour is prohibited and condemnations of instances where the prohibited behaviour did take place, possibly combined with justifications or excuses from the criticised State, but also physical practice abstaining from the prohibited behaviour. If the practice largely consists of abstention combined with silence, there will need to be some indication that the abstention is based on a legitimate expectation to that effect from the international community.
Practice establishing the existence of an obligation, for example, the rule that the wounded and sick must be cared for (see Rule 110), can be found primarily in behaviour in conformity with such a requirement. The fact that it is a legal requirement, rather than one reflecting courtesy or mere comity, can be found by either an expression of the need for such behaviour, or by criticism by other States in the absence of such behaviour. It may also be that, following criticism by other States, the criticised State will explain its abstinence by seeking justification within the rule.
Practice establishing the existence of a rule that allows a certain conduct, for example, the rule that States have the right to vest universal jurisdiction in their courts over war crimes (see Rule 157), can be found in acts that recognise the right to behave in such a way without actually requiring such behaviour. This will typically take the form of States undertaking such action, together with the absence of protests by other States.
During work on the study it proved very difficult and largely theoretical to strictly separate elements of practice and legal conviction. More often than not, one and the same act reflects practice and legal conviction. As the International Law Association has pointed out, the International Court of Justice “has not in fact said in so many words that just because there are (allegedly) distinct elements in customary law the same conduct cannot manifest both. It is in fact often difficult or even impossible to disentangle the two elements.”
This is particularly so because verbal acts count as State practice and often reflect the legal conviction of the State involved at the same time.
When there is sufficiently dense practice, an opinio juris
is generally contained within that practice and, as a result, it is not usually necessary to demonstrate separately the existence of an opinio juris
. Opinio juris
plays an important role, however, in certain situations where the practice is ambiguous, in order to decide whether or not that practice counts towards the formation of custom. This is often the case with omissions, when States omit to act or react but it is not clear why. An example of such a situation was analysed by the Permanent Court of International Justice in the Lotus case
in which France disputed Turkey’s right to prosecute for a collision on the high seas. France argued that the absence of such prosecutions proved a prohibition under customary international law to prosecute, except by the flag State of the ship on board which the wrongful act took place. The Court, however, disagreed because it was not clear whether other States had abstained from prosecuting because they thought they had no right to do so or because of some other reason, for example, lack of interest or belief that a court of the flag State is a more convenient forum. The Court stated there was no evidence of any “conscious[ness] of having a duty to abstain”.
Another situation of ambiguity was analysed by the International Court of Justice in the North Sea Continental Shelf cases
in which Denmark and the Netherlands argued that a customary rule existed requiring a continental shelf to be delimited on the basis of the equidistance principle, inter alia
, because a number of States had done so. The Court considered that the basis of the action of those States remained speculative and that no inference could be drawn that they believed themselves to be applying a rule of customary international law.
In other words, the States that had delimited their continental shelf on the basis of the equidistance principle had behaved in accordance with that principle but nothing showed that they considered themselves bound by it. It is basically in such cases, where practice is ambiguous, that both the International Court of Justice and its predecessor, the Permanent Court of International Justice, have looked in particular at whether they could separately establish the existence of an opinio juris
that would indicate that the ambiguous practice in fact counted towards the establishment of customary international law.
In the area of international humanitarian law, where many rules require abstention from certain conduct, omissions pose a particular problem in the assessment of opinio juris because it has to be proved that the abstention is not a coincidence but based on a legitimate expectation. When such a requirement of abstention is indicated in statements and documents, the existence of a legal requirement to abstain from the conduct in question can usually be proved. In addition, such abstentions may also occur after the behaviour in question created a certain controversy, which also helps to prove that the abstention was not coincidental, although it is not always easy to conclude that the abstention occurred because of a sense of legal obligation. A particular example of this problem is abstention from certain conduct in non-international armed conflicts when a clear rule to abstain from such conduct can only be found in treaty law applicable to international armed conflicts. This is, for example, the case for abstention from the use of certain weapons in non-international armed conflicts, when the prohibition of the use of these weapons was agreed to by treaty a long time ago when rules in relation to non-international armed conflicts were not as readily thought about or accepted as they are now. Abstention from such use or of prohibited behaviour is not likely to lead other States to comment, and this is particularly the case in relation to non-international armed conflicts in which other States are not directly affected. The process of claim and counterclaim does not produce as much clarity with respect to non-international armed conflicts as it does with respect to international armed conflicts because in the latter case, two or more States are directly affected by each other’s behaviour, while in the former case, usually only one State is directly affected.
It appears that international courts and tribunals on occasion conclude that a rule of customary international law exists when that rule is a desirable one for international peace and security or for the protection of the human person, provided that there is no important contrary opinio juris
Examples of such conclusions are the finding by the International Military Tribunal at Nuremberg that the Hague Conventions of 1907 had hardened into customary law,
and the finding by the International Court of Justice in the Nicaragua case
that the rule of non-intervention in the internal and external affairs of other States was part of customary international law.
However, when there was clear evidence of contrary opinio juris
by a number of States, including specially affected ones, international case-law has held that the existence of a rule of customary international was not proven, for example, the advisory opinion of the International Court of Justice in the Nuclear Weapons case
on the issue of whether the use of nuclear weapons was illegal,
and the ruling of the sole arbitrator in the Texaco v. Libya case
on the issue of a possible change in the law relating to compensation for expropriation.
This aspect of the assessment of customary law is particularly relevant for international humanitarian law, given that most of this law seeks to regulate behaviour for humanitarian reasons. In some instances, it is not yet possible to find a rule of customary international law even though there is a clear majority practice in favour of the rule and such a rule is very desirable.
Impact of treaty law
Treaties are also relevant in determining the existence of customary international law because they help assess how States view certain rules of international law. Hence, the ratification, interpretation and implementation of a treaty, including reservations and statements of interpretation made upon ratification, are included in the study. In the North Sea Continental Shelf cases
, the International Court of Justice clearly considered the degree of ratification of a treaty to be relevant to the assessment of customary international law. In that case, the Court stated that “the number of ratifications and accessions so far secured  is, though respectable, hardly sufficient”, especially in a context where practice outside the treaty was contradictory.
Conversely, in the Nicaragua case
, the Court placed a great deal of weight, when assessing the customary status of the non-intervention rule, on the fact that the UN Charter was almost universally ratified and that relevant UN General Assembly resolutions had been widely approved, in particular Resolution 2625 (XXV) on friendly relations between States, which was adopted without a vote.
It can even be the case that a treaty provision reflects customary law, even though the treaty is not yet in force, provided that there is sufficiently similar practice, including by specially affected States, so that there remains little likelihood of significant opposition to the rule in question.
In practice, the drafting of treaty norms helps to focus world legal opinion and has an undeniable influence on the subsequent behaviour and legal conviction of States. This reality was recognised by the International Court of Justice in the Continental Shelf case:
It is of course axiomatic that the material of customary international law is to be looked for primarily in the actual practice and opinio juris
of States, even though multilateral conventions may have an important role to play in recording and defining rules deriving from custom, or indeed in developing them.
The Court thus recognised that treaties may codify pre-existing customary international law but may also lay the foundation for the development of new customs based on the norms contained in those treaties. The Court has even gone so far as to state that “it might be that … a very widespread and representative participation in [a] convention might suffice of itself, provided it included that of States whose interests were specially affected”.
The International Law Association has summarised this case-law, stating that a (multilateral) treaty may thus interact in four different ways with custom: it can provide evidence of existing custom; it can provide the inspiration or model for the adoption of new custom through State practice; it can assist in the so-called “crystallisation” of emerging custom; and it can even give rise to new custom of “its own impact” if the rule concerned is of a fundamentally norm-creating character and is widely adopted by States with a view to creating a new general legal obligation. There can be no presumption that any of these interactions has taken place and in each case it is a matter of examining the evidence.
This study takes the cautious approach that widespread ratification is only an indication and has to be assessed in relation to other elements of practice, in particular the practice of States not party to the treaty in question. Consistent practice of States not party has been considered as important positive evidence. Contrary practice of States not party, however, has been considered as important negative evidence. The practice of States party to a treaty vis-à-vis States not party is also particularly relevant.
This study has not, however, limited itself to the practice of States not party to the relevant treaties of international humanitarian law. To limit the study to a consideration of the practice of only the 30-odd States that have not ratified the Additional Protocols, for example, would not comply with the requirement that customary international law be based on widespread and representative practice. Therefore, the assessment of the existence of customary law takes into account the fact that, at the time of writing, Additional Protocol I has been ratified by 162 States and Additional Protocol II by 158 States. Similarly, the assessment of customary law also takes into account the fact that the Geneva Conventions have been ratified by 192 States and this is not repeated in the commentaries.
Lastly, the most important judicial decisions on the customary nature of humanitarian law provisions are not repeated in the commentaries which cite the rules held to be customary. This applies in particular to the finding by the International Military Tribunal at Nuremberg that the 1907 Hague Regulations “undoubtedly represented an advance over existing international law at the time of their adoption … but by 1939 these rules laid down in the Convention were recognized by all civilized nations, and were regarded as being declaratory of the laws and customs of war”.
It also applies to the Nicaragua case
, in which the International Court of Justice held that common Article 3 of the Geneva Conventions reflected “elementary considerations of humanity” constituting a “minimum yardstick” applicable to all armed conflicts.
It further applies to the finding of the International Court of Justice in the Nuclear Weapons case
that the great majority of the provisions of the 1949 Geneva Conventions represent customary international law.
In the same vein, it is important to stress, though it is not repeated in the commentaries, that with regard to the Statute of the International Criminal Court, there was a “general agreement that the definitions of crimes in the ICC Statute were to reflect existing customary international law, and not to create new law”.
To determine the best way of fulfilling the mandate entrusted to it, the ICRC consulted a group of academic experts in international humanitarian law who formed the Steering Committee of the study. The Steering Committee consisted of Professors Georges Abi-Saab, Salah El-Din Amer, Ove Bring, Eric David, John Dugard, Florentino Feliciano, Horst Fischer, Françoise Hampson, Theodor Meron, Djamchid Momtaz, Milan Šahović and Raúl Emilio Vinuesa. The Steering Committee adopted a Plan of Action in June 1996 and research started in October 1996. Pursuant to the Plan of Action, research was conducted using both national and international sources reflecting State practice. Research into these sources focused on the six parts of the study as identified in the Plan of Action:
Principle of distinction
Specifically protected persons and objects
Specific methods of warfare
Treatment of civilians and persons hors de combat
The measure of access to national and international sources largely explains the research method adopted.
Research in national sources of practice
Since national sources are more easily accessible from within a country, it was decided to seek the cooperation of national researchers. To this end, nearly 50 countries were selected (9 in Africa, 15 in Asia, 11 in Europe, 11 in the Americas and 1 in Australasia) and in each a researcher or group of researchers was identified to report on State practice (see Annex I). The Steering Committee selected the countries on the basis of geographic representation, as well as recent experience of different kinds of armed conflicts in which a variety of methods of warfare had been used. The result was a series of reports on State practice. Significant practice of other countries was identified through research into international sources and ICRC archives (see infra).
The sources of State practice collected by the national researchers include military manuals, national legislation, national case-law, instructions to armed and security forces, military communiqués during war, diplomatic protests, opinions of official legal advisers, comments by governments on draft treaties, executive decisions and regulations, pleadings before international tribunals, statements in international organisations and at international conferences and government positions taken with respect to resolutions of international organisations.
The military manuals and national legislation of countries not covered by the reports on State practice were also researched and collected. This work was facilitated by the network of ICRC delegations around the world and the extensive collection of national legislation gathered by the ICRC Advisory Service on International Humanitarian Law. The purpose of the additional research was also to make sure that the study would be as up-to-date as possible and would, to the extent possible, take into account developments up to 31 December 2002. In some cases, it has been possible to include more recent practice.
Research in international sources of practice
State practice gleaned from international sources was collected by six teams, each of which concentrated on one part of the study. These teams consisted of the following persons:
Part I. Principle of distinction
Rapporteur: Georges Abi-Saab
Researcher: Jean-François Quéguiner
Part II. Specifically protected persons and objects
Rapporteur: Horst Fischer
Researchers: Gregor Schotten and Heike Spieker
Part III. Specific methods of warfare
Rapporteur: Theodor Meron
Researcher: Richard Desgagné
Part IV. Weapons
Rapporteur: Ove Bring
Researcher: Gustaf Lind
Part V. Treatment of civilians and persons hors de combat
Rapporteur: Françoise Hampson
Researcher: Camille Giffard
Part VI. Implementation
Rapporteur: Eric David
Researcher: Richard Desgagné
These teams researched practice in the framework of the United Nations and of other international organisations, in particular the African Union (formerly the Organization of African Unity), Council of Europe, Gulf Cooperation Council, European Union, League of Arab States, Organization of American States, Organization of the Islamic Conference and Organization for Security and Cooperation in Europe. The practice of the Commonwealth of Independent States, Inter-Parliamentary Union and Non-Aligned Movement was also researched. Access to the practice of these organisations was facilitated by the ICRC delegations which maintain contacts with them.
State practice at the international level is reflected in a variety of sources, including in resolutions adopted in the framework of the United Nations, in particular by the Security Council, General Assembly and Commission on Human Rights, ad hoc investigations conducted by the United Nations, the work of the International Law Commission and comments it elicited from governments, the work of the committees of the UN General Assembly, reports of the UN Secretary-General, thematic and country-specific procedures of the UN Commission on Human Rights, reporting procedures before the Human Rights Committee, the Committee against Torture, the Committee on the Elimination of Discrimination Against Women and the Committee on the Rights of the Child, travaux préparatoires of treaties, and State submissions to international and regional courts.
International case-law was also collected to the extent that it provides evidence of the existence of rules of customary international law.
Research in ICRC archives
To complement the research into national and international sources, the ICRC looked into its own archives relating to nearly 40 recent armed conflicts, some 20 of which occurred in Africa, 8 in Asia, 8 in Europe and 2 in the Americas (see Annex II). In general, these conflicts were selected so that countries and conflicts not yet dealt with by a report on State practice would also be covered.
The result of this three-pronged approach – that is, research into national, international and ICRC sources – is that practice from all parts of the world is cited. In the nature of things, however, this research cannot purport to be complete. Research for the study focused in particular on practice from the last 30 years to ensure that the result would be a restatement of contemporary customary international law, but, where still relevant, older practice has also been cited.
Consolidation of research results
Upon completion of the research, all practice gathered was summarised and consolidated into separate parts covering the different areas of the study. This work was carried out by the six international research teams for the part which concerned them. The chapters containing this consolidated practice were subsequently edited, supplemented and updated by a group of ICRC researchers, and are published in Volume II, “Practice”. The reason for publishing such voluminous chapters is twofold. First, those consulting the study should be able to verify the basis in State practice for each rule of customary international law. Each rule in Volume I refers to the chapter and section in Volume II where the practice on which that rule is based can be found. Secondly, it was considered useful to publish the wealth of information that has been compiled. Many practitioners and scholars will thus be able to use the practice gathered for their own professional purposes.
In a first round of consultations, the ICRC invited the international research teams to produce an “executive summary” containing a preliminary assessment of customary international humanitarian law on the basis of the practice collected. These executive summaries were discussed within the Steering Committee at three meetings in Geneva (see Annex III). On the basis of this first round of consultations, the “executive summaries” were updated, and during a second round of consultations, they were submitted to a group of academic and governmental experts from all the regions of the world invited in their personal capacity by the ICRC to attend two meetings with the Steering Committee (see Annex III). During these two meetings in Geneva, the experts helped to evaluate the practice collected and indicated particular practice that had been missed.
Writing of the report
The assessment by the Steering Committee, as reviewed by the group of academic and governmental experts, served as a basis for the writing of the final report. The authors of the study re-examined the practice, reassessed the existence of custom, reviewed the formulation and the order of the rules, and drafted the commentaries. The draft texts were submitted to the ICRC Legal Division, whose members provided extremely helpful comments and insights. Moreover, each Part was reviewed by an additional reader: Maurice Mendelson for the introductory part on the assessment of customary international law, Knut Dörmann for Part I, Theodor Meron for Part II, Horst Fischer for Part III, the Mines and Arms Unit of the ICRC led by Peter Herby for Part IV, William Fenrick for Part V and Antonio Cassese for Part VI. On the basis of their comments and those of the ICRC Legal Division, a second draft was prepared, which was submitted for written consultation to the Steering Committee, the group of academic and governmental experts and the ICRC Legal Division. The text was further updated and finalised taking into account the comments received.
This study was initiated under the supervision of Louise Doswald-Beck, then Deputy Head and later Head of the ICRC Legal Division. Jean-Marie Henckaerts has been responsible for the overall management of the study and drafted Parts I, II, III and V of Volume I. Louise Doswald-Beck drafted Parts IV and VI, as well as Chapters 14 and 32, of Volume I. The introductory parts were drafted by both of them. In drafting the text they received important contributions from Carolin Alvermann, Knut Dörmann and Baptiste Rolle. The authors, jointly, bear the sole responsibility for the content of the study.