Very belatedly, in December 1974, the UN General Assembly attempted to fill this normative vacuum by issuing Resolution 3314 (XXIX), giving an elaborate definition of ‘aggression’. Aggression is referred to in the preamble as ‘the most serious and dangerous form of the illegal use of force’,39 while Article 1 describes it in lapidary fashion as ‘the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State’.40 Article 3 gives a wide-ranging analytical specification of possible modes of aggression.41 However, an important caveat is introduced in Article 2, where it is specified that aggression is not to be automatically imputed to the state that is the first to use military force against another state. The Security Council, taking into account the circumstances, and whether the consequences of the attack are of sufficient gravity, may decide that the first to use force is not in fact guilty of the crime of aggression.42
As has been observed, this definition—produced by the General Assembly and not the Security Council, and hence not mandatory—is incomplete, and was always intended to be so.43 Article 4 actually states that it is not to be regarded as exhaustive, since the crime of aggression can be combined with other alleged transgressions in war at the discretion of the Security Council.44 Moreover—and this is a particularly grave omission—the resolution does not contemplate any sanction against those committing a crime of aggression. Article 5 limits itself to two utterly banal considerations in the wake of the sentence pronounced by the Nuremberg Tribunal, namely that ‘a war of aggression is a crime against international peace’ and that it incurs ‘international responsibility’.45
Antonio Cassese has argued that, in this and other cases, one can see the tendency of the major powers to ensure that the notions of ‘act of aggression’ and ‘war of aggression’ should not be clearly defined, thereby making it impossible to enforce the provision laid down in Article 2, section 4 of the UN Charter forbidding nations to use force. In his view, this tendency prevailed because the major powers,
when it comes to actively applying this measure, aim to keep a large degree of freedom of action both at the individual level, and collectively through the United Nations Security Council. The definition of ‘aggression’ has been left in a sort of limbo with respect to its characterization both as a state crime and as an international crime imputable to individuals.46
More generally, Giorgio Gaja has pointed out the existence of an ‘evident paradox’: while the prohibition of the use of force is a fundamental principle of international law, and violation of this principle is considered one of the most serious examples of ‘international crime’, it is practically unknown for one state to ask for sanctions to be imposed on another state or its citizens on the basis of an accusation of aggression.47 The criminalization of aggressive war has thus failed to produce any significant developments—not in normative terms, not in the structure of the international legal system, and still less in dissuading nations from having recourse to the arbitrary use of force.
War as a crime for which individuals are indictable
The new concept of war that gained ground in the first decades of the twentieth century did not just postulate a war of aggression as an international crime imputable to states: it also introduced, as we have seen, the possibility of individuals being indicted for this and other international crimes. And this has played a fundamental role in moulding international criminal justice.
Up to the end of the Second World War, international institutions had never prosecuted individuals in law. Indeed, from the dawn of modern international law and its founding fathers Hugo Grotius, Alberico Gentili and Baltasar Ayala, internationalist doctrine had excluded the possibility of individuals being considered subjects in the international legal order, whether in conjunction with, or in place of, the nation-states. International courts of justice had never been invested with obligatory jurisdiction, and had in fact been confined to relatively marginal issues. To maintain a world order, the major powers had had recourse to political and military force, treaties and diplomacy, rather than to legal instruments. This was the case, in particular, for the Holy Alliance, the League of Nations and the United Nations.
From the first decades of the twentieth century, commentators in the West began to argue the case for extending the judicial function to the international arena. As we have noted—and as Carl Schmitt insisted—the dress rehearsal for this radical doctrinal departure was the incrimination of Kaiser Wilhelm II of Hohenzollern as a war criminal at the end of the First World War. In its indictment of the venerable emperor, Article 227 of the Versailles Treaty accused him of ‘supreme offence against international morality and the sanctity of treaties’, a formula dictated by Wilson.48 This accusation, with its rhetorical and scrupulously non-legal language, did not refer to the traditional notion of ‘war crimes’, involving violation of the so-called jus in bello. Such a violation concerned the conduct of belligerents that infringed the norms of the ‘law of warfare’, meaning the proper conduct of war on land or at sea, the rights of prisoners, and so on.
Having achieved victory in the First World War, the Entente allies were determined to criminalize and punish warfare as such—specifically as an act or series of acts of aggression—and insisted that individuals, cited by name, be held responsible for this crime.49 Article 227 required that the kaiser should stand trial, together with high-level German political and military figures, before an international court composed of judges representing the five victorious powers (Britain, France, Italy, Japan and the United States). Furthermore, other articles in the Treaty required Germany—although no such obligation was to be found anywhere in either international treaties or customary law—to hand over some 900 named individuals accused of violating laws and war customs, so that they too could be tried.50
As is well known, the trial of Kaiser Wilhelm II and his collaborators never took place. He had taken refuge in Holland, and that country refused to extradite him, rightly arguing that there was nothing in international law that could countenance the incrimination of a head of state as being personally responsible for an international misdemeanour. According to the international legal order as it stood, the only subject in law was the state: only a state could be indicted for international wrongdoing, and any sanctions imposed, whether economic, financial, territorial, military or other, had nothing to do with criminal law. For its part, the German government refused to hand over the 900 individuals, but did consent to having them tried on German soil, in front of the Supreme Court in Leipzig, which was agreed to by the victorious powers. But only a few of the accused actually stood trial, and only light sentences were handed down. Yet, in spite of this paltry result, the whole affair had significant consequences in normative terms. Article 3 of the Fourth Hague Convention, 1907, stated that only states—not individuals—could be called to answer for violations of the law of warfare, whereas the precedent set by the Versailles Treaty made this principle partially inoperative.51
The real origin of international criminal jurisdiction can be identified with the institution, in 1945 and 1946 respectively, of the Nuremberg and Tokyo Tribunals. The creation of these two criminal courts had been anticipated, in the theoretical sphere, by Hans Kelsen’s essay Peace through Law, which appeared in 1944. Kelsen set out an institutional strategy for attaining peace, borrowing from Kant (notably the celebrated pages in Zum ewigen Frieden) the ideal of perpetual peace, with federalism as a model, and indeed the idea of making individuals, as well as states, subjects in international law.52 Kelsen maintained that, in the aftermath of the Second World War, his project for a ‘permanent league