The legal philosophy of the 20th century has contributed to the development of international criminal law by rethinking fundamental legal concepts and theories concerning the nature of international law, its relation with national laws, the connection between the law and the State, and the very idea of responsibility. This was achieved, in the first place, through the reflection of Hans Kelsen, who put forward the idea of a system of enforceable criminal norms at the international level, directed at individuals and having a positive legal foundation. In the years immediately following the Second World War, a number of legal theorists and, in particular, Gustav Radbruch, argued in favour of a necessary connection between law and morality, on whose basis it could be claimed that the worst atrocities were punishable even when allowed by state norms, and even in the absence of positive international norms. In the last decade, the practice of international criminal law, through ad hoc tribunals and the International Criminal Court, has stimulated theoretical reflections on a variety of further fundamental issues, like impartiality, judicial truth, justification of punishment, side-effects of prosecution and transitional justice.
The request of 16 January1920, drew attention, among other things, ‘to the cynical violation of the neutrality of Belgium and Luxembourg, the barbarous and merciless hostage system, the mass deportations of populations, the carrying away of the young women of Lille, torn from their families and thrown defenceless into the most promiscuous environment, the systematic devastation without military justification of entire territories, the unrestricted submarine warfare including the inhuman abandon of victims on the high seas, the innumerable acts committed by the German authorities against non-combatants in contempt of the laws of war, etc. The responsibility for all these acts, at least the moral responsibility, lies with the supreme chief who commanded and who took advantage of his power to break, or to permit to be broken the most sacred rules of the human conscience’.
Bassiouni, supra note 8, p. 35, notes that ‘individual criminal responsibility’ can be considered ‘an axiomatic principle of ICL’.
Bassiouni, ibid., p. 327,following Stefan Glaser, ‘La méthode d’interpretation en droit international pénal’, 9 Rivista Italiana di Diritto e Procedura Penale (1966), argues that, in international criminal law, the principle ‘nullum crimen sine lege’ stands as ‘nullum crimen sine iure.’
Kelsen, supra note 15, pp. 204–205. On Kelsen’s analysis of sovereignty, see Jean L. Cohen, Sovereignty in the Context of Globalization: A Constitutional Pluralist Perspective, in Besson and Tasioulas, supra note 6, pp. 264–268.
Kelsen, supra note 15, pp. 187–88.
See Kelsen, ibid., pp. 153, 154,where it is emphasized that recognition theory is a hypothesis necessary to establish the relationship between the state and international law: ‘Through ‘recognition’, international law becomes part of each state’s legal order, which in this way – and this is the decisive element – also preserves its sovereignty over international law’.
Kelsen, supra note 15, p. 265, states that war has to be a ‘sanction’: otherwise, if it is not, it must be characterized as ‘a crime, a violation of international law’.
Kelsen, supra note 32, p. 18.
Kelsen, supra note 36, pp. 325, 327.
Kelsen, supra note 32, p. 16; Kelsen, supra note 15, p. 266; Hans Kelsen, Introductions to the Problems of Legal Theory (Clarendon, Oxford,  2001), pp. 108–109.
Kelsen, supra note 36, pp. 327–328.
Kelsen, supra note 40, pp. 88 ff., argues that such crimes are linked to the violation of international treaties (and so are crimes against peace) and to the laws of war.
Kelsen, supra note 40, pp. 87–88. This position, which constitutes a major exception to his theory, it is supported by Kelsen only in matters pertaining to international law.
Kelsen, ibid., pp. 110ff.
Kelsen, supra note 32, p. 238. See also H. Kelsen, ‘Will The Judgement in the Nuremberg Trial Constitute a Precedent in International Law?’, 1 The International Law Quarterly (1947) 153–171.
Paulson, supra note 53, p. 499. According to Alexy, supra note 53, pp. 32–33, ‘Radbruch had to make only a minor adjustment in the system. It establishes a ‘hierarchy’, which corresponds to Radbruch’s older positivist understanding, in which purposiveness was at ‘the bottom’ and legal certainty generally preceded justice. Only in the extreme case of intolerable injustice does the relationship reverse. When there exists such a thing as extreme injustice then this way of conceiving the relationship of legal certainty and justice is not only acceptable but mandatory.’
Alexy, supra note 53, p. 32. According to Radbruch, supra note 54, p. 6: ‘To be sure, one value comes with every positive-law statute without reference to its content: Any statute is always better than no statute at all, since it at least creates legal certainty. But legal certainty is not the only value that law must effectuate, nor is it the decisive value. Alongside legal certainty, there are two other values: purposiveness and justice.’
Radbruch, supra note 54, p. 7.
Alexy, supra note 53, p. 16.
Radbruch, supra note 54, p. 7: ‘It is impossible to draw a sharper line between cases of statutory lawlessness and statutes that are valid despite their flaws. One line of distinction, however, can be drawn with utmost clarity: Where there is not even an attempt at justice, where equality, the core of justice, is deliberately betrayed in the issuance of positive law, then the statute is not merely ‘flawed law’, it lacks completely the very nature of law.’
Alexy, supra note 53, p. 16. From this point of view, Radbruch’s formula connects positive law to a higher law establishing the protection of human rights (on this aspect, see Gustav Radbruch, ‘Five Minutes of Legal Philosophy’, 26 Oxford Journal of Legal Studies ( 2006), pp. 14–15: ‘[t]here are principles of law, therefore, that are weightier than any legal enactment, so that a law in conflict with them is devoid of validity. These principles are known as natural law or the law of reason. To be sure, their details remain open to question, but the work of centuries has in fact established a solid core of them, and they have come to enjoy such far-reaching consensus in the so-called declarations of human and civil rights that only the dogmatic sceptic could still entertain doubts about some of them’).
Bix, supra note 52, p. 50.
Kelsen, supra note 49, p. 165.
Bassiouni, supra note 8, pp. 72, 300.
Kelsen, supra note 49.
Fuller, supra note 51, p. 655; Alexy, supra note 57, p. 36.
Hart, supra note 66, p. 76.
Fuller, supra note 51, p. 657.
According to Fuller, supra note 51, p. 660, ‘a dictatorship which clothes itself with a tinsel of legal form can so far depart from the morality of order, from the inner morality of law itself, that it ceases to be a legal system’.