11.1 Introduction

In the two decades that have lapsed since the entry into force of the Rome Statute, the field of international criminal law has grown in both vibrancy and reach. Whilst the International Criminal Court (ICC) is at the centre of the system created by the Rome Statute, States are called to play a key role in investigating and prosecuting core international crimes, in accordance with the principle of complementarity.Footnote ¹ In order to do so, States need to adopt national implementing legislation incorporating the crimes at the domestic level. To date, many States have adopted legislation to implement the Rome StatuteFootnote ² and have, therefore, offered their interpretation of the Statute in the process. However, this engagement has often led to divergent approaches, with several States deviating from the Statute. This chapter explores what happens when there is a clash between what is in the Rome Statute and when core international crimes are interpreted differently within and outside the ICC. It further looks into what happens at the interface. Unpicking this interaction not only offers useful insights into the practice and evolution of the core international crimes provisions but may also contribute to a better understanding of the rules of international criminal law interpretation, as well as pave the way for possible Rome Statute amendments. Whereas many works have focused on the implementation of the Rome Statute, looking at this interface from the interpretation angle has been overlooked, to date.

This chapter will start with a brief consideration of those institutions that gave rise to international criminal law – namely, the Nuremberg and Tokyo International Military Tribunals (IMTs) and the ad hoc Tribunals for the former Yugoslavia and Rwanda. It will consider the influence of customary international law in defining and interpreting core international crimes, as well as its relationship with the principle of legality, a key principle underpinning international criminal justice. Moving then to one of the major instruments of international criminal law of our time – the Rome Statute establishing the ICC – the chapter will discuss to what extent the definitions of crimes contained in Articles 6–8bis of the Rome Statute codify customary international law or rather constitute progressive development in the field. Although these categories of crimes can be regarded as largely reflective of customary international law of the time, it is important to note, as recognised under Article 10 of the Statute, that the exclusion of certain crimes from the Rome Statute or the adoption of retrograde definitions must not be read as limiting or precluding the existing customary international law framework or the progressive development of new customary law outside the Statute. Customary international law in this area has considerably developed alongside the Rome Statute, including through the national laws of States. Notably, when adopting domestic implementing legislation, State Parties have often significantly limited or expanded the scope of application of core international crimes enshrined in the Statute. It is, therefore, crucial to consider how different national core crimes provisions can influence the interpretation of the definitions of core international crimes crystallised in the Rome Statute when the ICC is required to step in owing to the national legal system being unwilling or unable genuinely to investigate or prosecute, in line with the principle of complementarity.

This chapter will consider how this potential new state practice and emerging customary law interacts with the treaty provisions and what this means for their interpretation. This chapter will argue that a distinction should be made between creating new substantive law and an interpretation within the scope of existing law. Subsequent practice or customary international law cannot, therefore, be invoked to unacceptably restrict, broaden, or modify the text of the Statute. Nevertheless, ICC judges need to remain cognisant of how core international crimes have been interpreted through implementation at the domestic level.

11.2 The IMTS and ad hoc Tribunals: Between Customary International Law and the Principle of Legality

The birth of modern international criminal law can be traced back to the Nuremberg and Tokyo IMTs, established by the victorious Allied Powers following the Second World War.Footnote ³ Notably, the definitions of the core crimes in international criminal law – namely, genocide, crimes against humanity, war crimes, and aggression – have been developed at the international level ever since the inception of the field with the creation of these two Tribunals. Core international crimes have since become established categories of crimes in international law, enabling the prosecution of alleged perpetrators at the international level.

At Nuremberg, the Military Tribunal observed that ‘the law of war is to be found not only in treaties, but in customs and practices of States which gradually obtained universal recognition’ and that ‘in many cases, treaties do no more than express and define for more accurate reference the principles of law already existing’.Footnote ⁴ Therefore, the Nuremberg IMT explicitly referred to customary international law in defining the law applicable to the crimes falling within its jurisdiction.Footnote ⁵ Likewise, the Tokyo Tribunal engaged in the practice of determining whether the crimes listed in the Far East Charter constituted customary international law.Footnote ⁶

It was said at the time that ‘the [Nuremberg] Charter is not an arbitrary exercise of power on the part of the victorious nations, but … it is the expression of international law existing at the time of its creation’.Footnote ⁷ Efforts were made to demonstrate respect for the principle of nullum crimen sine lege – that is, the principle of legality.Footnote ⁸ Nevertheless, the inclusion of certain crimes in the Nuremberg and Tokyo Charters raised concerns about retroactivity and compliance with the principle of legality.Footnote ⁹ Notably, it is questionable whether crimes against peace and crimes against humanity constituted crimes under international law at the time of their occurrence during the Second World War.Footnote ¹⁰ Therefore, the Nuremberg and Tokyo IMTs have received a great deal of criticism on account of the fact that individuals were prosecuted for crimes that had not existed at the time they were committed.Footnote ¹¹ Confronted with such a challenge, the Nuremberg judgment developed a relatively weak conception of the principle of legality, which can operate as a ‘principle of justice’ rather than as a ‘limitation of sovereignty’ and can, therefore, be outweighed by considerations of substantive justice.Footnote ¹²

In response to the so-called victors’ justice criticism of the Nuremberg and Tokyo paradigm, and after a long period of inactivity owing to the Cold War,Footnote ¹³ at the time of the establishment of the ad hoc Tribunals for the former Yugoslavia (ICTY) and Rwanda (ICTR) in the 1990s, every effort had been made for core crimes definitions enshrined in their Statutes to reflect existing customary international law in the field.Footnote ¹⁴ The UN Secretary-General’s report that preceded the establishment of the ICTY expressly stated that the Tribunal’s material jurisdiction is limited to conducts that are, ‘beyond any doubt’,Footnote ¹⁵ recognised under customary international law, so that ‘the problem of adherence of some but not all States to specific conventions does not arise’.Footnote ¹⁶ However, in the case of the ICTR, the Security Council adopted a more expansive approach to the choice of the applicable law, including within its subject-matter jurisdiction ‘international instruments regardless of whether they were considered part of customary international law’.Footnote ¹⁷

The UN Secretary-General’s report on the ICTY reveals a presumption in favour of customary international law, according to which ‘provisions of the Statute defining the crimes within the jurisdiction of the Tribunal should always be interpreted as reflecting customary international law, unless an intention to depart from customary international law is expressed in the terms of the Statute, or from other authoritative sources’.Footnote ¹⁸ However, concerns were raised at the time on whether breaches of the Geneva Conventions in a non-international armed conflict constituted customary international law,Footnote ¹⁹ and whether customary international law for crimes against humanity required a nexus with an armed conflict.Footnote ²⁰

In any case, the primary role played by customary international law in the ad hoc international criminal Tribunals’ experience has been deemed necessary to comply with the principle of nullum crimen sine lege and, specifically, with the prohibition against retroactive application of criminal law. By recognising that ‘the tribunal’s power to convict an accused of any crime listed in the Statute depends on its existence qua custom at the time this crime was allegedly committed’,Footnote ²¹ the drafters wanted to avoid risking a possible violation of the principle of legality in the event that a party to the conflict did not ratify a relevant treaty.Footnote ²² For the same reason, a few years later, the UN Secretary-General’s report on the establishment of a Special Court for Sierra Leone expressly recognised that the international crimes enumerated in the draft statute ‘are crimes considered to have had the character of customary international law at the time of the alleged commission of the crime’, linking the presumption of interpretation consistent with customary law with the principle of legality and the ban against the retroactive application of the law.Footnote ²³

11.3 The Rome Statute Crimes: Codification of Custom or Progressive Development?

The adoption of the Rome Statute of the ICC in 1998 provided a further opportunity for crystallising the customary international law of the time. In developing the definitions of core international crimes,Footnote ²⁴ the intention of the drafters at Rome was to limit the Court’s jurisdiction to crimes existing under customary international law.Footnote ²⁵ To reach the widest possible consensus among States and promote its universal application, it was understood that the Rome Statute was not the ‘right place for progressive development of the law into uncertain areas, or for the elaboration of new and uncertain international criminal law’.Footnote ²⁶ As stated by Philippe Kirsch, the Court’s first president, ‘the definitions of crimes in the ICC Statute were to reflect existing customary international law, and not create new law’.Footnote ²⁷ Although the Rome Statute does not expressly specify whether the crimes listed in Articles 6–8bis are declaratory of custom,Footnote ²⁸ these categories of crimes can be regarded as largely reflective of customary international law at the time.Footnote ²⁹ This ensures consistency with the principle of legality, as formulated in Article 22 of the Rome Statute.

However, the drafting of an international treaty is always an arduous process. It is hard to hold the narrow line between a pure codification of existing customary law and progressive or retrogressive developments.Footnote ³⁰ The Rome Statute was the product of intense multilateral negotiation,Footnote ³¹ which revealed the existence of profound and diverging national positions. The substantive provisions contained in Articles 6–8bis ‘represent a consensus reached by a large number of States following a lengthy and complex drafting process’.Footnote ³² Therefore, the numerous compromises that were made to build consensus among States led to divergence from the then existing content of customary international law.Footnote ³³

While the provisions on genocide,Footnote ³⁴ crimes against humanity,Footnote ³⁵ war crimes,Footnote ³⁶ and aggressionFootnote ³⁷ are generally considered in line with customary law,Footnote ³⁸ the Rome Statute shows, in places, both (limited) progressive development, as well as instances of regression – in particular when looking at the definitions of crimes under Articles 7 and 8 of the Rome Statute. Indeed, as it has been observed by the ICTY in the Furundžija case, ‘depending on the matter at issue, the Rome Statute may be taken to restate, reflect or clarify customary rules or crystallise them, whereas in some areas it creates new law or modifies existing law’.Footnote ³⁹ Therefore, the Rome Statute can be regarded as largely, but not completely, reflective of customary law.Footnote ⁴⁰

Provisions that have been considered by some authors as going beyond customary international law include, among war crimes, the conscription or enlistment of children under fifteen years of age into the national armed forcesFootnote ⁴¹ and the indirect transfer by an occupying power of its civilian population into the occupied territory.Footnote ⁴² Moreover, the customary law status of certain acts amounting to crimes against humanity has been called into question, such as forced pregnancy,Footnote ⁴³ enforced disappearance and apartheid,Footnote ⁴⁴ as well as the long list of grounds of persecution.Footnote ⁴⁵ Lastly, the policy element required by Article 7(2)(a) of the Rome StatuteFootnote ⁴⁶ has also been seen to narrow the customary definition of crimes against humanity.Footnote ⁴⁷ Similar reasoning has been employed regarding the qualification of the concept of persecution as occurring ‘in connection with any act or any crime within the jurisdiction of the Court’.Footnote ⁴⁸ However, such progressive definitions, which were not regarded as declaratory of customary international law at the time of the Statute’s adoption, are likely to have gradually evolved and crystallised into custom as a result of other international and domestic courts, or domestic implementing legislation, accepting them as ‘encapsulating the world community’s opinio juris on the matter’.Footnote ⁴⁹

On the other hand, the lack of prohibition on conspiracy to commit genocide constitutes a retrograde deviation from custom.Footnote ⁵⁰ Moreover, in the context of war crimes, the Statute has inadequately codified prohibited weapons and clearly excluded the legal prohibition on the use of weapons of mass destruction.Footnote ⁵¹ The Statute also retains some retrograde distinctions between international and non-international armed conflicts, with significantly fewer categories of war crimes included in this latter section. For instance, Article 8(2)(e) lacks a comparable provision to that of Article 8(2)(b)(xx) on the use of weapons that cause superfluous injury and unnecessary suffering, or weapons that are inherently indiscriminate. Other crimes that are not replicated in internal armed conflicts include compelling a prisoner of war or other protected persons to serve in the armed forces of a hostile power;Footnote ⁵² intentionally directing attacks against civilian objects;Footnote ⁵³ intentionally launching indiscriminate attacks likely to cause incidental loss of life, injury to civilians or damage to civilian objects, or widespread, long-term, and severe damage to the natural environment;Footnote ⁵⁴ attacking undefended places, which are not military objectives;Footnote ⁵⁵ improper use of flags and markings;Footnote ⁵⁶ use of human shields.Footnote ⁵⁷

11.4 Temporality of International Criminal Law

Codifying customary international criminal law, as was largely the case with the adoption of the Rome Statute, ensures legal clarity, as well as better consistency and coherence of the law. However, it should be stressed that ‘customary law continues to exist and develop alongside treaty law.’Footnote ⁵⁸ International criminal law is a dynamic system. It is not intended to be frozen in time but evolves and develops continuously. As it has been observed by the Nuremberg IMT, ‘the laws of the war are not static, but by continual adaptation follow the needs of a changing world’.Footnote ⁵⁹ Concerns have therefore been expressed that the definitions of crimes adopted by the Rome Statute might have ‘frozen customary definitions in a process of rapid evolution’, thus leading to the ‘stagnation, if not decline’ of the field.Footnote ⁶⁰

The Rome Statute itself recognises that the ICC does not operate in a vacuum. Indeed, Article 10 stipulates that: ‘Nothing in this part shall be interpreted as limiting or prejudicing in any way existing or developing rules for international law for purposes other than this Statute.’Footnote ⁶¹ This provision, which had been proposed by the late M. Cherif Bassiouni on behalf of the Egyptian delegation and with the strong support of the ICRC,Footnote ⁶² sought to mitigate concerns regarding the codification of some but not all customary international crimes contained in the Statute.Footnote ⁶³ Likewise, Article 22(3) of the Statute confirms that the exclusion of criminal liability – in application to the principle of legality – for those conducts not criminalised under the Rome Statute ‘shall not affect the characterization of any conduct as criminal under international law independently of this Statute’. Therefore, the exclusion of certain crimes from the Rome Statute, or departure from customary law, is not intended to prejudice the existing customary international law framework or the progressive development of new customary law in the field.Footnote ⁶⁴

While such provisions are intended to restrict the influence of the Statute on the development of customary law, they are not aimed at determining the role of custom for interpretation purposes. Such an approach is, however, of particular relevance, given that customary international criminal law has further developed alongside the Rome Statute, including through the adoption of national implementing legislation by States.

11.5 The Adoption of Divergent Domestic Implementing Legislation

With the principle of complementarity being the cornerstone of the Rome system of justice and the ICC operating as a court of last resort,Footnote ⁶⁵ the primary responsibility to investigate and prosecute core international crimes rests with States.Footnote ⁶⁶ The ICC can only intervene when States are ‘unwilling’ or ‘unable’ genuinely to do so.Footnote ⁶⁷ To enable States to exercise primary jurisdiction over core international crimes, and investigate and prosecute these crimes domestically, enacting legislation implementing the provisions of the Rome Statute in their respective national legal systems is key. National implementation is indispensable for meeting the ‘duty of every State to exercise its criminal jurisdiction over those responsible for international crimes’ – as stated in the Preamble of the Rome Statute.Footnote ⁶⁸ Whilst the Rome Statute does not expressly mandate the enactment of national implementing legislation with regard to the Statute crimes, and the decision to implement is left at the discretion of States, over half of the State Parties have so far at least partially adopted domestic implementing legislation with respect to the four core crimes listed under Article 5 of the Statute.Footnote ⁶⁹

Importantly, the Rome Statute also does not expressly require States to enact domestic implementing legislation incorporating the core crimes in a uniform manner. As such, States are afforded wide discretion when carrying out the implementation task. Substantive differences, therefore, exist in how core international crimes have been interpreted when transposed into national systems. A number of States have replicated the exact wording of the Rome Statute or made explicit reference to its relevant provisions.Footnote ⁷⁰ Other States have instead reformulated, rephrased, and redrafted the conducts criminalised in the Rome Statute, using different terms from those employed by Articles 6–8bis of the Rome Statute in both retrograde or progressive ways.Footnote ⁷¹

Some States have adopted narrower definitions of core international crimes – for instance, omitting racial groups or ethnic groups in the domestic definition of genocide,Footnote ⁷² or adopting restrictive definitions of torture or enforced disappearance of persons.Footnote ⁷³ Conversely, other States have employed broader definitions based on general international law, other applicable treaties, or new developments in customary international law. Indeed, the implementation task carried out by States is ‘not performed in a vacuum’ but is necessarily influenced by developments in the field of international criminal law.Footnote ⁷⁴ For instance, several State Parties – such as some Latin American and ex-Soviet countries – have expanded the scope of application of genocide by increasing the number of protected groupsFootnote ⁷⁵ or broadening the list of punishable acts to include displacement or deportation.Footnote ⁷⁶ Other States, like Germany, have also included conducts that have been omitted during the negotiations of the Rome Statute – for example, the prohibition on the use of chemical and biological weapons among their war crimes provision.Footnote ⁷⁷ Moreover, while some States have maintained in their legislation the distinction between international and non-international armed conflicts,Footnote ⁷⁸ this has been blurred by others.Footnote ⁷⁹ A number of States consider direct attacks against civilian objects, disproportionate attacks, and human shields to be war crimes also in non-international armed conflicts, even though the Statute only recognises these as war crimes solely when committed in the course of an international armed conflict.Footnote ⁸⁰

Whilst provisions of national legislation do not always correspond to the definitions enshrined in the Rome Statute, they purport to criminalise the same conduct. It is, therefore, crucial to consider what happens if national core crimes provisions differ from those found in the Statute – for instance, in a situation case where a State Party conflates international and non-international armed conflicts. Where the ICC is required to step in owing to the national legal system being unwilling or unable genuinely to investigate or prosecute in line with the principle of complementarity, it is at that interface that the interpretation of the national core crime provision in relation to the provision found in the Rome Statute becomes relevant.

11.6 Rules of Interpretation at the ICC

Since its coming into force, ICC judges have been repeatedly tasked with interpreting and applying the Rome Statute.Footnote ⁸¹ The applicable sources of law to be employed by the ICC are clearly laid down in Article 21 of the Rome Statute, which places primary importance on the text of the Statute, its Elements of Crimes, and the Rules of Procedure and Evidence.Footnote ⁸² Only residually, and ‘where appropriate’, judges should apply applicable treaties and principles and rules of international law,Footnote ⁸³ as well as general principles of law derived by the Court from national laws, including the national laws of States that would normally exercise jurisdiction over the crime.Footnote ⁸⁴

While Article 21 of the Rome Statute does not explicitly address the issue of interpretation, the inclusion of a list of applicable sources therein creates a de facto framework that guides ICC judges in their interpretative process. In this regard, paragraph 3 of Article 21 explicitly refers to both ‘application’ and ‘interpretation’ of the law, acknowledging the intertwined nature of the two processes. Although a distinction exists between application and interpretation,Footnote ⁸⁵ interpretation is the first step in the process of applying the law. Therefore, Article 21 sets out a hierarchy between sources that ICC judges are to apply when interpreting core international crimesFootnote ⁸⁶ and places the definitions contained in the Statute at the top to increase legal certainty and ensure better respect for the principle of legality.Footnote ⁸⁷ Whilst the other sources of law, including custom,Footnote ⁸⁸ occupy a subsidiary position,Footnote ⁸⁹ they still have an acknowledged role to filling gaps in the text of the Statute or as an aid for the purposes of interpretation.Footnote ⁹⁰ In any case, these rules of interpretation seem to provide an imperfect solution. They only apply to the Court itself and fail to take into account how States behave within the Rome system of justice created by the Statute, outside it but, in any case, not before the Court.

However, as an international treaty, the Rome Statute is also subject to rules and principles of interpretation codified by the 1969 Vienna Convention on the Law of Treaties (VCLT) – in particular, Articles 31 and 32.Footnote ⁹¹ As a general principle of interpretation, a treaty shall be interpreted ‘in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose’.Footnote ⁹² Moreover, according to Article 31(3) of the Convention, judges are required to take into account subsequent agreements, practice, and law when interpreting a treaty.Footnote ⁹³

The provisions of the ICC Statute on applicable law and the rules set forth by the 1969 VCLT share common ground, yet they are also distinguishable. Both the ICC Statute and the VCLT emphasise the importance of interpreting treaties in accordance with the ordinary meaning of terms, the context, and in light of their object and purpose. Furthermore, Article 31(3) of the VCLT, on the consideration of subsequent agreements, practice, and relevant rules of international law, aligns with the ICC’s approach outlined in Article 21. By virtue of this provision, judges are expected to take into account applicable treaties and principles and rules of international law. It could therefore be argued that the ICC Statute is a specialised regime of the VCLT’s general principles for treaty interpretation able to meet the specific needs, complexities, and criminal nature of the ICC.

In any case, the interplay between Article 21 ICC Statute and Article 31(3) of the 1969 VCLT, puts ICC judges at the interface: taking into account subsequent agreements, practice and other sources of international law, including custom, when interpreting the Rome Statute. As customary law and general principles of law will continue to evolve beyond the Statute, and new relevant treaties between States will continue to be concluded, the definitions of crimes cannot be considered frozen in time but need to be interpreted in the light of the law existing when the crime is alleged to have been committed.Footnote ⁹⁴

11.7 The Role of Domestic Implementing Legislation in Interpreting the Rome Statute Crimes

The idea of international criminal law not being static raises questions of how domestic implementing legislation can influence the interpretation of the definitions of core international crimes crystallised in the Rome Statute. This is particularly the case when the Court steps in in light of the principle of complementarity, that is, when national authorities are unwilling or unable to genuinely investigate or prosecute the case. It is, therefore, crucial to understand how potential new State practice and emerging customary international law would interact with the treaty provisions and what this means for their interpretation. The adoption of divergent national legislation on core international crimes does not necessarily have an impact on the interpretation of such crimes.Footnote ⁹⁵ However, a ‘systematic, repetitive and concordant’ practice of StatesFootnote ⁹⁶ interpreting and applying the definitions of core international crimes at the domestic level might contribute to the evolution of international customary law alongside the definitions of crimes contained in the Rome Statute.Footnote ⁹⁷ If a considerable number of States, both parties and non-parties to the Rome Statute, enacted a particular definition for a crime that departs in some way from Articles 6–8bis of the Statute, it might be argued that a new State practice and possibly opinio juris emerges on the content of the elements of that crime under customary international law. If several States consistently interpret and apply definitions of core international crimes at the domestic level, in a way that departs from the Rome Statute but aligns with one another, this can be perceived as subsequent practice that reflects a common understanding among those States. In this context, the consistent domestic practices might qualify as evidence of the parties’ agreement regarding the interpretation of the relevant treaty provisions, falling under Article 31(3)(b) of the 1969 VCLT.Footnote ⁹⁸ Furthermore, even when such consistent State practice does not fully align with the common intention of the parties but reflects general practice among a significant number of States, it could still fall under Article 32 of VCLT, as a supplementary means of interpretation. It could help clarify the treaty provisions in question and confirm the meaning resulting from the application of Article 31.Footnote ⁹⁹

For instance, with regard to the definition of genocide, a clear trend in State practice towards expanding the list of protected groups to include political and social groups can be identified. To the extent that this evidence of State practice is supported by opinio juris, it could be argued that a larger number of protected groups has crystallised into customary law.Footnote ¹⁰⁰ Moreover, there is also a growing trend to abandon the distinction between international and non-international armed conflicts among war crimes, which appears in line with customary international law.Footnote ¹⁰¹ But does this mean that where a State Party has blurred the distinction between international and non-international armed conflicts – and the ICC steps in owing to complementarity – ICC judges could extend their subject matter jurisdiction to war crimes that constitute customary international law but have been excluded from the list of Articles 8(2)(e) of the Rome Statute?

In this regard, it is key to emphasise that the International Criminal Court is bound by the principle of legality. Article 22 provides that no one can be held responsible under the Rome Statute ‘unless the conduct in question constitutes, at the time it takes place, a crime within the jurisdiction of the Court’.Footnote ¹⁰² Moreover, the definition of a crime shall be ‘strictly construed’.Footnote ¹⁰³ The goal of the drafters in Rome was to set out all crimes falling under the jurisdiction of the ICC, in as specific and detailed a manner as possible.Footnote ¹⁰⁴ Therefore, ICC judges are strictly bound by the limits of the Statute’s wording when undertaking their interpretative task.Footnote ¹⁰⁵

A certain level of flexibility within the Statute’s framework is still acceptable ‘to avoid excessive rigidity and to keep pace with changing circumstances’.Footnote ¹⁰⁶ However, whilst the rule of strict construction shall not be intended as curbing the progressive juridical clarification or specification of the content of an offence,Footnote ¹⁰⁷ the Rome Statute cannot be stretched to create new criminal prohibitions that would then be applied retrospectively. An interpretation based on the object and purpose of the Statute could not be used to create a body of law extraneous to the terms of the treaty or incompatible with the literal reading of the text.Footnote ¹⁰⁸ It is therefore necessary to distinguish between the creation of new substantive law and an interpretation within the scope of existing law. Whilst a reinterpretation based on subsequent practice or customary international law can be considered acceptable, if it can be reasonably accommodated by the actual text of the Statute, modification of the text under the guise of interpretation is not permissible. A significant departure from the intention of the drafters in order to give effect to a creative interpretation of the Statute would appear to be ‘an inappropriate arrogation of the legislative function by the judiciary’.Footnote ¹⁰⁹

The principle of legality, and the primacy of the Statute’s text, therefore, prevents judges from expanding the Court’s jurisdiction to prosecute crimes that are merely based on subsequent practice or custom but are not clearly defined in the Statute.Footnote ¹¹⁰ For instance, ICC judges could not extend their subject matter jurisdiction to the war crime of intentionally directing attacks against civilian objects, as this would broaden the punishable acts beyond Article 8(2)(e) of the Rome Statute. However desirable this may be, and even though a trend is beginning to develop in national legislation to conflate the two, the Statute provision remains abundantly clear. Similarly, finding an accused guilty of political or social genocide before the ICC would clearly constitute a violation of the principle of legality.Footnote ¹¹¹

Such an approach may seem unduly restrictive, if not conservative. A rigid textual interpretation of the Rome Statute promotes a narrower scope for the definitions of core international crimes.Footnote ¹¹² Brave acts of judicial activism have also occurred in a number of instances in the past, including before international tribunals. For instance, in Tadić, the ICTY Appeals Chamber held that the nexus with an armed conflict in crimes against humanity was no longer required under customary law.Footnote ¹¹³ In the Akayesu judgment, the ICTR Tribunal expanded the traditionally protected groups under the definition of genocide to include ‘any stable and permanent group’.Footnote ¹¹⁴ However, although the Rome Statute’s provisions offer some flexibility, they are not fully flexible. It is important not to try and stretch them beyond their limits and intended scope.

ICC judges need to be cautious when taking into account subsequent practice and customary international law that significantly narrow or broaden the scope of a crime. Whilst they should be mindful of developments both in State practice and customary law, they must also recognise that the Statute is a treaty and that the interpretation and application role of the judiciary must be kept distinct from the law-making role.Footnote ¹¹⁵ It is the Assembly of States Parties, as the ‘management oversight and legislative body’ of the ICC,Footnote ¹¹⁶ that can initiate amendments to the Statute following the Article 121 Rome Statute process. For example, a unified approach to war crimes committed in non-international and international armed conflicts could only be taken at the ICC if an amendment were to be made to Article 8(2)(e) of the Rome Statute to incorporate analogous provisions to those found in Article 8(2)(b).Footnote ¹¹⁷

11.8 Concluding Remarks

The chapter has explored what happens at the interface of domestic legal systems having interpreted core international crimes differently from the Rome Statute and the International Criminal Court. Addressing the question of how national implementing legislation can influence the interpretation of the definitions of core crimes crystallised in Articles 6–8bis of the Rome Statute provides useful insights into the progressive evolution of international criminal law and sheds some light on the under-explored interaction between implementation and interpretation of the Statute. However, the rules of interpretation laid down in Article 21 of the Rome Statute do not represent an adequate framework to fully unpick this interaction. They only apply to the Court itself and fail to take into account the interpretation of core international crimes provided at the domestic level.

ICC judges ought to be cognisant of how State Parties have implemented core international crimes in their domestic law when engaging in the interpretation of the Rome Statute. Under the 1969 VCLT, judges are required to take into account subsequent agreements, practice, and other sources of international law when interpreting a treaty. It may be that some flexibility within the Statute exists, and judicial activism and creativity may be inevitable or even desirable to keep pace with progressive developments in the field. However, it remains necessary to distinguish between an interpretation within the scope of the existing law and modification of the text of a treaty. ICC judges cannot expand the scope of core international crimes’ definitions to include new types of conduct or contextual circumstances that are not clearly supported by the text of the Statute. Therefore, where (re)-interpretation based on subsequent State practice cannot be reconciled with the text of a specific provision, formally amending the Statute – following the procedure set forth in Article 121 of the Rome Statute – remains the only available route. Keeping the Rome Statute under review and ensuring its continued evolution in accordance with customary international law would maintain the unity of the international criminal law system and avoid (further) fragmentation.