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The Oxford Companion to International Criminal Justice edited by Cassese, Antonio (22nd January 2009)

Part B Issues, Institutions, and Personalities, A, Aggression

Jens Ohlin

From: The Oxford Companion to International Criminal Justice

Edited By: Antonio Cassese (Editor-in-Chief)


In many ways, the concept of aggression belongs as much to international law (as a question of state responsibility) as it does to ICL (as an issue of individual criminal liability). On a conceptual level it would seem that the crime requires both individual conduct (on the part of the accused) and state conduct (on the part of the state on whose behalf the a ggressive war is planned or initiated). The combination of these two elements raises deep questions as to the degree to which aggression is to be found by criminal courts prosecuting individuals, or in the alternative, political institutions such as the SC categorizing the actions of states. The result is a crime whose exact definition is elusive, and whose application in concrete instances remains controversial, although its legal contours are in other ways well defined.

The historical root of aggression as an international crime begins in Nuremberg (see Nuremberg International Military Tribunal) with the prosecution of Nazi leaders for conspiracy to wage aggressive war. The crime itself was established by the London Agreement which included waging a war of aggression as one crime among the more general category of ‘crimes against peace’, including the ‘planning, preparation, initiation or waging’ of a war of aggression or a war in violation of international treaties (Art. 6(a), IMT Charter). The latter description is important because it reveals that international law—not criminal law—provides the baseline against which an aggressive war will be defined. In criminal terms, this requirement of aggressive war can be thought of as a predicate requirement imported from international law. Today the predicate requirement is fulfilled by the UN Charter itself which outlaws aggressive war (Art. 2(4), UN Charter, outlawing the ‘use or threat of force’). The UN Charter’s prohibition on aggressive war as an international wrongful act is also assumed to codify existing customary international law.

Although the Nuremberg trials received critical scholarly attention—the nulla poena sine lege problem is still hotly contested—the UN GA nonetheless retrospectively recognized the basic principles of the Nuremberg Charter (UN GA Res. 95(I), 11 December 1946). There is consequently widespread agreement over the unlawfulness of aggression as an international wrongful act, even if its application by the IMTFE and IMT was controversial.

Despite these historical precedents, the crime of aggression was not included within the SC’s authorizing statutes for the ICTY and ICTR. This is notable since both wars in the former Yugoslavia and Rwanda arguably contained elements of aggression. The crime was included in the ICCSt., although the ICC has no jurisdiction to prosecute the crime, according to Art. 5(2), until the parties of the Statute agree to a definition of the crime. This creates the somewhat unstable logical situation where the concept itself is recognized as a crime but there is no binding definition of it. It can therefore be said that aggression is a crime in search of a definition.

Nonetheless, there have been extensive attempts at defining it which provide some content and from which some agreement on the crime’s definition can be inferred, including a 1974 definition by the GA, a 1996 draft code by the ILC, and individual SC decisions defining particular military situations as acts of aggression. These attempts all indicate the difficulty of constructing a non-circular definition that state parties can agree to.

The 1974 definition includes a predictable recitation of the standard military forms of aggression: invasion, bombardment, blockade, and attacks by land, sea, or air. However, the contentious element of aggression is not the military means by which it is realized, but the criteria by which justifiable (p. 237) force is distinguished from military aggression. On this point the 1974 definition merely parrots the Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States and the UN Charter itself. The 1974 definition also prohibits ‘[t]he sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to the acts listed above, or its substantial involvement therein’ (UN GA Res. 3314 (XXIX), 14 December 1974). The ICJ in Nicaragua interpreted this provision to reflect customary international law (see Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States of America), ICJ Reports (1986), 14 at § 195).

The ILC’s attempts at definition are equally problematic. The 1996 Draft Code of Crimes against the Peace and Security of Mankind simply states in Art. 16 that an ‘individual who, as leader or organizer, actively participates in or orders the planning, preparation, initiation or waging of aggression committed by a State shall be responsible for a crime of aggression’. Of course, the fact that Art. 16 refers to aggression indicates that the provision cannot serve as a non-circular definition of the crime. At its best, Art. 16 of the Draft Code can only establish individual criminal liability for the crime. But the definition remains elusive.

The SC has on a few occasions labelled individual military incursions as acts of aggression, and one might infer a definition of the crime by a close examination of these particular cases. SC Res. 573 vigorously condemned Israel for a ‘flagrant violation of the Charter of the United Nations, international law, and norms of conduct’ after air strikes against Palestine Liberation Organization (PLO) targets in a suburb of Tunis in 1985 violated the territorial integrity of Tunisia (UN SC Res. 573, 4 October 1985). SC Res. 573 also demanded that Israel refrain from similar acts of aggression in the future. Similarly, SC Res. 577 condemned South Africa for military incursions into Angola, calling these ‘flagrant’ violations ‘hostile and unprovoked acts of aggression’ committed by a ‘racist regime’ (UN SC Res. 577, 6 December 1985). The Resolution also demanded that South Africa withdraw all troops from Angolan territory. SC Res. 660 condemned the Iraqi invasion of Kuwait and ordered a military withdrawal, but strangely failed to use the term ‘aggression’ at all (UN SC Res. 660, 2 August 1990). Even SC Res. 662, passed by the SC one week later, declared the Iraqi annexation null and void and concluded that it had no legal validity, but once again failed to label the incursion as an ‘aggression’ (UN SC Res. 662, 9 August 1990). This is curious since the Iraqi invasion of Kuwait, a sovereign neighbour, appears to be a paradigmatic and uncontroversial case of unprovoked aggression. This exposes the difficulty of inferring, through induction, a general definition of aggression based on individual instances where SC decisions have provided the world community with definitive legal answers as to what constitutes aggression in a few isolated cases. Although all of the Resolutions refer to the violation of territorial integrity, a more searching definition of aggression is difficult to construct based on isolated pronouncements.

It is nonetheless possible to construct the basic elements of the crime of aggression by drawing together the common characteristics of each source of law. Aggression includes traditional forms of military attack against the territorial integrity of another state, when not in self defence, such as invasions or bombardment by land, sea, or air. The sending of armed militia across the border may also constitute aggression, provided the incursion is significant enough to meet some threshold requirements. Where to locate that threshold is a difficult question of law application to fact. Furthermore, allowing the use of your territory for an armed attack by a militia against a third state may constitute aggression if there is reason to impute the militia’s activity to the state. In such a situation it is possible that a campaign of terrorism could be defined as aggression. It is unlikely that a lone individual engaging in an attack without state involvement could be guilty of aggression, since such an attack would be more likely defined simply as a crime of terrorism. Purely economic forms of power, such as an embargo, are not likely to be covered by any sensible definition of aggression, although a blockade may be included insofar as it is implemented with military assets and indirectly threatens territorial integrity.

For individual criminal liability, it is clear that general intent is required: the charged individual must have the intent to participate in the military aggression in some capacity (Göring and others, 1 Trial of the Major War Criminals, at 224–226). It is not necessary to demonstrate that the individual was aware or approved of all aspects of the aggressive war, but simply that the individual knew that he was participating in a war of aggression. The IMT convicted Nazi leaders on a charge of conspiracy to wage aggressive war on the theory that crimes against the peace are collective crimes that are, by definition, conspiratorial. However, the level of participation required remains unclear. When a state engages in a war of aggression, many individuals are ‘involved’ or (p. 238) ‘participate’ in some way in the aggression. Are they all criminally liable? Both military commanders and civilian leaders engage in planning or preparation for war and are clearly liable under the Nuremberg principles. However, Art. 6(a), IMT Charter criminalized not just planning and preparation but also waging aggressive war. Is every soldier in the armed forces therefore guilty of aggression as a bit player in the grand conspiracy? Probably not, but the bare language of the IMT Charter nonetheless suggests this possibility. Another intriguing possibility is whether a national court exercising universal jurisdiction could prosecute another state’s political leaders for the international crime of aggression. However, national courts have been extremely wary of wading into such political waters. A 2006 House of Lords decision in England recognized the crime of aggression in customary international law but concluded that it was not a crime under British law absent statutory enactment (R v. Jones, citing possible interference with the conduct of foreign affairs). Similarly, the German chief federal prosecutor declined to investigate US defence officials for the crime of preparing an aggressive war against Iraq. A few states have enacted aggression as a domestic crime, but none have ever prosecuted it.

There is also some debate in the scholarly literature over whether special intent is required for individual criminal liability. Glaser argued that the defendant must have the intent to pursue some aggressive purpose, such as territorial gain, occupation, or some other specific advantage. Under this view, a military incursion for the sole purpose of humanitarian intervention would not constitute aggression, nor would a military attack taken with the sincere but mistaken belief that the attack was required by the doctrine of self defence. In contrast, Cassese argued in 2003 that customary international law prohibits aggression regardless of motive, altruistic or otherwise, and that anything less would threaten international law’s fundamental value of preserving collective peace and security (Cassese, Int. Crim. Law at 115–116; he later changed his view, eventually sharing Glaser’s opinion: see A. Cassese, ‘On some Problematical Aspects of the Crime of Aggression’, 20 LJIL (2007) 841–849). However, it is important to note that aggression need not encompass the entire spectrum of unlawful use of force by states. It is, for example, logically consistent to determine that the military action of a state is unlawful according to the principles of international law, yet does not rise to the level of aggression, nor meet its specific criteria. This determination would be made without prejudice as to what sanctions international law might impose against a rogue state’s unlawful—yet not aggressive—actions. Individual criminal liability could then be reserved for cases of individual complicity with state action that pursued distinctively aggressive aims, as was clearly the case with the Nazis. This area of the law remains unsettled because there are no recent prosecutions of the crime, and the ICCSt. is silent on the question of special intent because the crime is not included in the ICC’s jurisdiction pending adoption of a definition in accordance with Art. 5(2).

The interplay between the collective and individual aspects of the crime (the former relating to state responsibility, the later to individual criminal liability), and who determines them, is similarly unsettled. The SC can label state actions ‘aggressive’ and order remedial measures in accordance with its Chapter VII powers (see Chapter VII Measures). If an international criminal tribunal is adjudicating a criminal charge of aggression against a particular individual, the first question is whether the action at the state level constituted aggression under the body of international law concerning state responsibility. Since the SC carries the highest level of enforcement authority in the modern system of international law, their findings on the matter would seem to be binding. Does this mean that an international criminal tribunal such as the ICC would be required to accept the SC finding without independent review? This suggests the unfortunate possibility that decisions as to an element of a criminal offence are decided in a non-judicial proceeding during which the accused enjoys none of the procedural protections of a legitimate criminal trial. This offends traditional principles of fairness and due process. However, there may be some rationale for this division of labour: the SC has greater institutional competence to decide questions of international law, while criminal tribunals have greater competence to judge individual conduct. The alternatives are that the tribunals show great deference to the SC determination or that they show no deference at all and engage in de novo review of matters pertaining to international law. The issue remains unresolved.

The dilemma brings to the forefront the distinctive nature of aggression as a hybrid notion of international and criminal law. Like genocide, aggression includes an irreducibly collective aspect, pursued at the level of states, defined and governed by the general principles of international law, and adjudicated by political institutions such as the SC. Yet aggression is a crime yielding not only ‘collective culpability’ but individual criminal responsibility as well, defined and governed by general principles of (p. 239) criminal law, and tried by criminal tribunals devoted to individual punishment.

jens ohlin

  • S. Glaser, ‘Quelques remarques sur la définition de l’agression en droit international pénal’, Festschrift für Theodor Rittler (1957) 388–393.
  • I. Brownlie, International Law and the Use of Force by States (Oxford: Clarendon Press, 1963) 185–194.
  • J.A. Bush, ‘“The Supreme … Crime” and Its Origins: The Lost Legislative History of the Crime of Aggressive War’, 102 Columbia Law Review (2002) 2324.
  • Cassese, Int. Crim. Law, 110–117.
  • G. Gaja, ‘The Long Journey towards Repressing Aggression’, Cassese et al., ICC Commentary, I, 427–441.
  • C. Kress, ‘The German Chief Federal Prosecutor’s Decision not to Investigate the Alleged Crime of Preparing Aggression Against Iraq’, 2 JICJ (2004) 245.
  • C. Villarino Villa, ‘The Crime of Aggression before the House of Lords’, 4 JICJ (2006) 866.