By Marco Vöhringer, Research and Teaching Associate at Leipzig University, Germany.

November 20, 2025 marked 80 years since the commencement of the trials before the International Military Tribunal (IMT) in Nuremberg. These prosecutorial efforts against key Nazi criminals are considered the founding moment of modern international criminal law (ICL). On this occasion, the present piece will analyze the development of ICL through the prism of the positivist-naturalist divide in legal theory. At its core, this post will make three arguments. The first is that in the origins of ICL, notions of a universal sense of morality – typically formulated as the “conscience of humankind” – served as a justification for deviating from the principle of legality, i.e. to prosecute acts that had not previously been criminalized. Secondly, it will be argued that a tendency to invoke a common human morality to justify criminal prosecutions beyond the positive legal framework persists in modern ICL. Finally, this piece will offer a positivist critique of thus drawing on moral criteria to bypass the principle of legality.

Moral Language in the Origins of ICL

The positivist objection that there is no basis in positive law for a given criminal proceeding has been a theme throughout the development of ICL, even in the pre-Nuremberg era. During the discussions at the Commission of Responsibility of the Authors of the War set up after World War I, the US voiced this criticism as part of its opposition to the planned trying of German officials. Its representatives stated that “if […] there is no law making them crimes […], they are moral, not legal, crimes” and should therefore not be subject to prosecution (Commission report, p. 139).

Discussions about the nullum crimen sine lege-principle were also at the heart of the legal controversies surrounding the trials in Tokyo and Nuremberg after World War II. It was widely held that, in particular, crimes against the peace (today known as the crime of aggression) and crimes against humanity did not exist under international law prior to 1945. Perhaps the most extensive such critique was presented by dissenting Justice Radhabinod Pal at the Tokyo War Crimes trial who spoke of a non-justifiable “revolutionary creation of ex post facto international law” (Dissent, p. 30).

Only rarely were the responses to these lines of criticism of a legal nature. Indeed, following WW I, the Commission itself noted that “a war of aggression may not be considered as an act directly contrary to positive law.” Still, it left open the possibility of taking action against individuals who were guilty of aggression. It defended this conclusion by pointing to the gravity of the acts in question, which, in the Commission’s eyes, constituted “conduct which the public conscience reproves” (Commission report, p. 118-120). Similarly, when turning to the question of immunity, the Commission did not offer precedents or other legal arguments to support its case that the immunity of heads of state could not be absolute. Instead, it simply held that the opposing conclusion “would shock the conscience of civilized mankind” (p. 116).

At the Nuremberg trials, the phraseology was similar. In its discussion of the principle of legality, the IMT considered that the defendants must have known that what they were doing was “wrong” and that it would therefore be “unjust if [their] wrong were allowed to go unpunished” (IMT Judgment, p. 219). Accordingly, Kreß concludes that “the Nuremberg Judgment let the doctrine of substantive justice trump the idea of legality”. American chief prosecutor Robert Jackson can be understood as having admitted as much when he stated in his opening remarks that the IMT was to prosecute not just legal, but also “moral wrongs“. Jackson also sought to avoid the “pitfalls” of doctrinal discussions, proposing instead to focus on “those things which fundamentally outrage the conscience of the American people”. All of this goes to show how notions of morality were relied upon to justify sidestepping the nullum crimen sine lege-maxim. 

Morality and the Principle of Legality in Modern ICL

In the further stages of development of ICL, respect for the principle of legality became a greater and more explicit concern. Though it was not expressly included in their final statutes, upholding the legality principle was a priority in the creation of the ad hoc tribunals in the 1990s. The UN Secretary General’s report on the establishment of the International Criminal Tribunal for the Former Yugoslavia (ICTY) reads: “[T]he application of the principle nullum crimen sine lege requires that the international tribunal should apply rules […] which are beyond any doubt part of customary law.” This reflected an effort to link the ICTY’s jurisdiction to the generally recognized sources of public international law laid down in Article 38(1) of the ICJ Statute and thus to provide a positive legal basis for its prosecutions. In contrast, the report avoids the kind of moral language that had been used to define the criminal tribunals’ mandates in Versailles and Nuremberg.

This process of committing to and strengthening respect for the principle of legality was seemingly completed with the establishment of the International Criminal Court (ICC). Articles 22-24 of the Rome Statute explicitly recognize the nullum crimen sine lege-guarantee and give binding power to its different subcomponents, such as the principle of non-retroactivity or the in dubio pro reo-rule. This is complemented by the extensive definitions of the crimes over which the ICC has jurisdiction (Articles 6-8bis and 70). For instance, through the comprehensive legal definition of crimes against humanity in Article 7, it would seem that “humanity” has become a concept of positive law and that there was no further need to rely on its “conscience” as a supra-legal category.

However, despite these developments, a propensity to rely on a proclaimed universal morality or “conscience of humankind” to overcome the requirements of legality remains discernible at the ad hoc tribunals and, to a lesser extent, at the ICC. 

Turning first to the ICTY, the way in which the tribunal set out to determine which principles were part of customary international law – and thus of its jurisdiction – was often a far cry from a strict application of the state practice- and opinio iuris-criteria (see Jacobs, p. 24 et seq.). For instance, in the Tadić case, the Tribunal held that “elementary considerations of humanity” demanded that those methods of warfare which are prohibited in international armed conflict ought to be equally illegal in non-international strife (para. 119). In Kupreskic, the ICTY went on to claim that “principles of humanitarian law may emerge […] under the pressure of the demands of humanity or the dictates of public conscience, even where State practice is scant or inconsistent” (para. 527). Through this questionable appraisal of customary law, the ICTY, while not formally challenging the applicability of the legality principle, did display some expansionist tendencies in fulfilling its mandate, and it did so using the by now familiar language of morality and conscience. 

As regards the Rome Statute, there is no denying that it leaves a lot less room for the kind of judicial activism in which the ICTY was arguably engaged. Nevertheless, the ICC Statute still bears at least some potential for sidestepping the principle of legality. For one thing, the definitions of the core crimes in Articles 6-8bis, though comprehensive, still occasionally rely on normative concepts – such as “outrages upon personal dignity” (Article 8(2)(b)(xxi),(c)(ii))   – that seem to lack the precision that is required under the nulla poena sine lege certa-maxim (see Olásolo, p. 309).  Further, it has been suggested that using the object and purpose of the Statute to deduce the meaning of its provisions could prove to be another way of circumventing the principle of legality (see Jacobs, p. 33 et seq.): The purpose of the Rome Statute being to end impunity, this could warrant an expansive application of the crimes included therein, inviting the ICC to base its judgments on considerations of morality at the expense of a strict application of the principle of legality. To what extent it will indeed do so remains largely to be seen. Its reasoning in Lubanga, however, leaves doubts about the Court’s commitment to upholding the nullum crimen sine lege-principle: The Appeals Chamber spoke of a “risk of acquittals” which “would be contrary to the aim of the Statute to ‘put an end to impunity’” (para. 77).

Whose Morality is it, Anyway? A Positivist Critique

So far, it has been argued that references to humanity’s common sense morality have been used to defend infringements of the legality principle in ICL. We now turn to the normative assessment of this practice.

There certainly is an intuitive sense that in the face of massive war crimes or genocide, a strict insistence on the principle of legalitywould be misplaced. Indeed, upholding the abstract legal principle of nullum crimen sine lege at the cost of allowing mass atrocities to go unpunished may prima facie seem like a form of legalistic parochialism. Recently, attempts have also been made to invoke the so-called Radbruch formula to justify activist tendencies at international criminal courts. This concept, coined by German legal philosopher Gustav Radbruch, holds that positive law must step aside where it enters into an intolerable conflict with the demands of material justice.

Yet, even against this backdrop, this author remains deeply skeptical about the practice of relying on supra-legal notions like “humanity” and its conscience in international criminal proceedings. These reservations stem from a simple question: If certain types of acts are indeed condemned by all of humankind, why would states fail to criminalize those acts? Where there is a genuinely universal consensus that certain deeds should be punished, it would seem like the most natural step to agree on a legal rule stipulating just that. If, in a given case, there is no such international legal rule, this is an indicator that there may in fact be no such universal agreement on the desirability of criminal prosecutions. Therein also lies the Achilles’ heel of the Radbruch formula when invoked in an international criminal context: It presupposes a shared international conviction that certain acts are not only morally reprehensible, but also that criminal prosecutions at an international level are an ethically imperative response to such acts. Yet where states have refrained from creating a legal rule to this effect, such an internationally shared understanding cannot simply be assumed. Accordingly, in the absence of an appropriate legal framework, references to morality and to the “conscience of humankind” give rise to the suspicion that the moral views of a particular group of states or judges are cloaked in the universal language of humanity. But one group’s very strongly held moral beliefs are not necessarily also the moral beliefs shared by all other groups. The most reliable test of whether certain deeds are universally condemned is to leave it up to the states to criminalize those deeds. Where this has not occurred, i.e. where there is no sufficient positive legal basis, international courts should think twice before prosecuting individuals in the name of a “conscience of humankind”. Otherwise, they risk consolidating the impression that ICL is not actually about ending impunity but about imposing the views of victors on others in the ironic name of “humanity”.

Photo credit: United States Holocaust Memorial Museum, courtesy of National Archives and Records Administration, College Park