The crime of aggression is one of the most controversial topics of international law: ever since the proposal was made during the London Conference to hold Kaiser Wilhelm II responsible for the initiation of World War I, states and scholars have devoted time and attention (and many thousands of pages!) to this crime. Patrycja Grzebyk's monograph is a comprehensive review of the most salient points in the long drawn-out process culminating in the adoption at Kampala of a definition of aggression for the purposes of the International Criminal Court (ICC). In her own words, the monograph seeks: “first, to identify the legal norms that define and delegalise aggression; secondly, to determine the basis for criminalisation of aggression and the scope of this delegalisation (and thus necessarily to establish the elements of the crime of aggression); and, thirdly, to identify the key risks and difficulties inherent in trials of aggression” (p. 2). The book is correspondingly arranged in three Parts, dealing with each of these aims.

Part I of the monograph provides an overview of the developments that led to the prohibition of the use of force in international law. Grzebyk discusses how, sometime in the late nineteenth and early twentieth centuries, the conceptualisation of war underwent a radical change: from an “extra-legal” event the initiation of which was beyond the realm of law, war became a phenomenon the commencement of which was capable of (and needed) legal regulation. From this realisation onwards, multiple efforts followed to constrain and, ultimately, to prohibit the use of force in international relations. The author thus discusses the (procedural) limits imposed on recourse to war by the League of Nations Covenant, the subsequent attempts to close the Covenant's “gaps” in bilateral and multilateral instruments including the Locarno Treaties and the Kellogg–Briand Pact, and finally the prohibition on the use of force in the UN Charter and the establishment of the current collective security system. Grzebyk's evaluation of these efforts is both hopeful and sceptical, as it is “hard to miss the radical dissonance between the declarations to abstain from the use of force, and the actual use of force” (p. 37).

Alongside efforts to prohibit the use of force, Grzebyk chronicles the multiple and abortive attempts undertaken by states and scholars alike to provide a definition of (state) “aggression”. Grzebyk critically discusses the various efforts undertaken both within the General Assembly and in the International Law Commission to define this term. The result of these efforts was the Definition of Aggression annexed to Resolution 3314, adopted by consensus in the General Assembly in 1974. Grzebyk is rightly doubtful about the value of this resolution. While she notes that the Definition of Aggression provided useful guidance to the International Court of Justice in the Nicaragua case, she also states that it was, ultimately, an “[un]satisfactory solution” (p. 74). Indeed, the resolution was intended to guide the Security Council in making determinations pursuant to Article 39 of the UN Charter (and not, as the author says at p. 50, to clarify the uses of force against which self-defence is permitted); it is not in itself binding (not even on the General Assembly, as the author inaccurately states at pp. 51, 74); and it was not intended as a statement of (existing) international law. These misgivings notwithstanding, the author affirms that Resolution 3314 may represent the agreed “core” of the definition of aggression (p. 75).

Grzebyk then charts the developments culminating in the compromise package on aggression adopted at the Kampala Conference in 2010 (the entry into force of which is postponed until, at least, 2017) in Part II of the monograph. Her discussion of the Nuremberg and Tokyo judgments is extensive and critical: the Tribunals applied the law retroactively as no crime of aggression existed before 1945 (p. 97, see also pp. 127, 240–41). She is equally critical of the aggression amendments to the ICC Statute adopted in Kampala. Writing a few years after the hype created by this “historic” event, Grzebyk's appraisal of the Conference's achievements is ambivalent. While she praises the acceptance of a definition as an important milestone, this definition is, regrettably, beset by ambiguities both substantive (e.g., what is the role of the reference to Resolution 3314?) and procedural (e.g., how will the amendments enter into force?) (pp. 120–28, see further pp. 219–23).

Most importantly, however, Grzebyk maintains that the crime of aggression is not recognised in contemporary customary international law: the not-yet-in-force Kampala definition was an exercise in “legislation” and not “codification” (pp. 130, 191). In contrast to the majority view, Grzebyk asserts that the “legal basis for criminalising aggression is rather frail” (p. 217). She backs her assertion with a review of international and domestic case law on the crime of aggression (pp. 131–89). While an exhaustive review – she summarises the charges and findings with respect to each individual defendant in all cases – it is fair to query whether the author would have reached the same conclusion had she broadened the scope of her investigation. Indeed, the narrow focus (only on cases in which individuals have been charged with aggression) entailed the exclusion from the analysis of other relevant instances of practice, including the views and opinions expressed by states during the drafting of the ICC Statute and during the negotiations leading to the aggression amendments adopted at Kampala.

The reader can glean from Parts I and II of the monograph that, despite attempts to remain an objective observer of these developments, Grzebyk is au fond unsympathetic towards the criminalisation of aggression. This view becomes very clear as she embarks on the analysis of the many procedural difficulties and the risks facing aggression trials in Part III of the book. Aggression trials, she argues, may be so detrimental to international peace and security and for the international criminal justice system that she wonders whether criminalising aggression serves any purpose at all. In the concluding sentence of the monograph, she states that: “Given the potential negative impact of aggression trials, one question is worthy of serious consideration: at this stage of advancement of international criminal law and its enforcement, is it really a good idea to prosecute and try aggression?” (p. 265).

Grzebyk's doubts in respect of aggression prosecutions are founded on two sets of arguments. First, (domestic and international) trials of aggression face many and serious procedural difficulties: few states have criminalised aggression domestically, and it is unclear whether international law allows universal jurisdiction for this crime; immunities may obstruct efforts actually to bring individuals charged with aggression to trial; statutes of limitations and amnesties may apply, and so on. The requirement of prior determination of state aggression and, specifically in the context of the ICC, the powers vested in the Security Council to both make that determination or defer prosecutions may produce further complications. Many of these difficulties are, in the author's view, “insurmountable” (p. 243).

Second, the effects of aggression prosecutions for international law generally are, on the whole, negative. The author discusses separately the impact of aggression trials for the maintenance of international peace and security, and for the international criminal justice system, though her arguments are frequently intertwined. Roughly three main concerns underpin Grzebyk's views: aggression trials are not conducive to peace, rather aggression indictments have the potential to exacerbate armed conflicts; there is no evidence that aggression prosecutions have a deterrent effect, and deterrence as a goal is “only useful insofar as it contributes to the general happiness of the population” (p. 254, citing Jeremy Bentham); and, finally, the political character of aggression prosecutions may open such trials to challenges of lack of impartiality thereby undermining their legitimacy.

What's more, individuals who could be charged with aggression (the “leadership” of the aggressor state) can also be charged with other international crimes – and historically, they have been charged and found guilty of these other crimes. So, to argue against aggression trials is not, really, to uphold the impunity of these individuals: international law already has set in place mechanisms (substantive and procedural) to punish these leaders. In Grzebyk's words: “today, with the new broader interpretation of joint criminal enterprise and command responsibility, convicting leaders guilty of aggression on charges of the traditional crimes of international humanitarian law would present no significant problems” (p. 260). The implication of this argument is that it is not so much what leaders engaging in aggression are punished for that matters so long as they are punished – even on the basis of theories, such as joint criminal enterprise, which remain controversial. The concern is that these leaders be punished because aggression is wrong (but not illegal, as Grzebyk argues that it is not a customary crime), so we will punish them for something. This retributivist solution unfortunately ignores the important role of fair labelling and just deserts of the criminal law.

This seemingly pragmatic approach also betrays some naivety on the part of the author. She assumes that prosecutions for these other international crimes are less procedurally and substantively difficult and less subject to political pressures than aggression trials have been and will be. But one need only think of the complex history of the prosecution of leaders such as Slobodan Milosević, Saddam Hussein and Charles Taylor, and also of high ranking military officials such as Ratko Mladić and Radovan Karadžić, charged with a combination of war crimes, crimes against humanity and genocide, to realise that this is not the case. Likewise, the extradition and the transfer of individuals charged with these crimes to domestic courts and the ICC may be obstructed by claims of immunity as well. Some of the “negative impacts” that the author attributes to aggression trials are, moreover, just as applicable to these crimes. For one, the author's argument that indictments for aggression may prolong ongoing conflicts is grounded on the (arguable) effect that the indictment of Karadžić, Mladić and Taylor may have had on the continuation of hostilities (p. 250). Yet, those indictments were not for the crime of aggression, but precisely for war crimes and crimes against humanity.

This said, the author is correct in that the challenges posed by aggression trials deserve serious consideration. Primary among these are the conditions and requirements for the prior determination of state aggression, the interaction between law and politics in the making of these determinations, and the relationship between individual and state responsibility. Aggression trials also draw attention to the broader challenges that international criminal law faces, including the aims of the international criminal justice system (e.g., some form of utilitarianism, retributivism, something else?) and the role, if any, of international criminal law in the maintenance of international peace and security. Grzebyk's study touches on all these issues, some directly and some implicitly. While some of these questions may require more profound exploration, this monograph has the merit of comprehensively addressing them.

Criminal Responsibility for the Crime of Aggression is a well-researched study, with sources in as many as four different languages (though the English translation does the author a disservice). Notwithstanding, this work is a welcome addition to the literature on aggression, a topic sprawling again in the aftermath of the adoption of the Kampala amendments. The work attests, both in its account of the development of this crime as well as in its argument, to the many fundamental disagreements that still permeate international law's project to criminalise aggression a century after the idea was first formulated.