The Crime of Aggression, Selectivity, and the International Rule of Law: Reflections from Vienna and a Conversation to Continue – Vienna 2026 · Day 5
The final day of the International Law Association Conference in Vienna offered an especially thought-provoking discussion on one of the most consequential unresolved questions in contemporary international criminal law: how to reconcile the recognition of aggression as one of the four core crimes under the Rome Statute with the exceptionally restrictive jurisdictional regime governing the International Criminal Court’s ability to prosecute it.
On 21 August 2026, at the University of Vienna Faculty of Law, I attended the panel “The Way Ahead for the Crime of Aggression: Harmonizing the Exercise of the International Criminal Court’s Jurisdiction Over All Core Crimes.”
The discussion brought together Claus Kreß, Professor of International Law and Criminal Law at the University of Cologne; Patrycja Grzebyk, Associate Professor of International Law at the University of Warsaw; Jennifer Trahan, Professor at the Center for Global Affairs at New York University; and Patryk I. Labuda, Assistant Professor of International Law and International Relations at Central European University. The panel was moderated by Astrid Reisinger Coracini, Senior Scientist in International Law at the University of Salzburg, with welcoming remarks by Stephan Wittich, Professor of International Law at the University of Vienna.
I did not intervene in the panel discussion. I listened. What follows therefore is not an account of arguments that I presented in Vienna, but a set of reflections prompted by a discussion that raised questions extending well beyond the technical design of Article 15 bis of the Rome Statute.
A Core Crime with an Exceptional Jurisdictional Regime
At the center of the panel was a striking institutional paradox.
Aggression occupies a foundational position in the architecture of international law. The prohibition of the unlawful use of force is central to the post-1945 international legal order, and the Rome Statute recognizes the crime of aggression alongside genocide, crimes against humanity, and war crimes as one of the four core international crimes.
But the jurisdictional architecture applicable to aggression differs substantially from that governing the other three crimes.
The Kampala amendments represented an extraordinary achievement after decades of political and legal negotiation. At the same time, the compromises necessary to secure that achievement created significant limitations on the ICC’s exercise of jurisdiction over aggression. Those limitations became dramatically visible following Russia’s full-scale invasion of Ukraine in 2022.
The difficulty was therefore not created by Ukraine. Ukraine exposed a structural problem that was already embedded in the institutional architecture.
This distinction matters.
A jurisdictional arrangement that could once be treated as an intricate problem of Rome Statute design suddenly confronted one of the clearest and most consequential cases in which the contemporary international legal order was being asked to respond to an alleged act of aggression.
The question consequently became unavoidable: what does it mean for an international legal system to identify aggression as one of its gravest crimes while simultaneously maintaining an institutional structure that can prevent its principal permanent criminal court from exercising jurisdiction over precisely the situations in which accountability may be most important?
That is not simply a question of criminal jurisdiction. It is a question about the international Rule of Law.
From Nuremberg to Generality
One of the most important themes running through the discussion concerned the legacy of Nuremberg.
The historical significance of Nuremberg does not lie only in the prosecution of particular individuals after a particular war. Its deeper claim was that the principles being articulated should become law rather than remain an exceptional form of justice applicable only to defeated enemies.
That aspiration toward generality remains fundamental.
International law cannot ultimately rest upon a principle according to which aggression is criminal when committed by certain states but insulated from meaningful accountability when committed by others.
This does not mean that every situation is legally identical, that every use of force satisfies the Rome Statute definition of aggression, or that differences between conflicts should be erased. The crime itself contains demanding thresholds, including the requirement of a manifest violation assessed through its character, gravity, and scale.
But once the legal conditions are satisfied, the identity, geopolitical alignment, military power, or institutional position of the state concerned should not determine whether the relevant legal norm exists in practice.
The deeper issue is therefore equality before international law.
A legal prohibition whose enforceability changes fundamentally according to the political position of those subject to it risks becoming something different from the Rule of Law.
Ukraine and the Transformation of a Theoretical Problem
The Vienna discussion repeatedly returned to the significance of 2022.
Before Russia’s full-scale invasion, limitations on ICC jurisdiction over aggression could remain, for many observers, an abstract or highly technical feature of the Rome Statute system.
Ukraine transformed that theoretical difficulty into an immediate institutional problem.
The ICC could exercise jurisdiction over other international crimes arising from the situation while its ability to address the crime that concerns the initial resort to unlawful force remained constrained.
This is one reason why proposals for a Special Tribunal for the Crime of Aggression against Ukraine assumed such importance.
The subsequent Council of Europe framework represents an attempt to respond to that accountability gap. The panel also demonstrated why the creation of a special tribunal cannot by itself resolve the underlying structural question.
If the jurisdictional architecture of the ICC remains unchanged, another future act of aggression may reproduce essentially the same dilemma.
This makes the debate over harmonization particularly important.
The question is not simply how to respond to Ukraine. It is whether the response to Ukraine can contribute to a legal architecture capable of responding more consistently to future situations as well.
Special Justice and General Reform
This produces an important tension between ad hoc institutional responses and universal legal reform.
International criminal law has repeatedly developed through institutions created in response to particular historical circumstances. Nuremberg itself was exceptional. The tribunals for the former Yugoslavia and Rwanda were situation-specific. Hybrid and internationalized criminal mechanisms have subsequently developed in different forms across jurisdictions.
Selectivity is therefore not new to international criminal justice.
The more difficult question is what should follow from that fact.
One possible response is to regard selectivity as discrediting any new situation-specific institution.
Another is to ask whether an imperfect or selective institution can nevertheless perform a transitional function: filling an accountability gap today while helping create conditions for more general rules tomorrow.
The discussion in Vienna strongly suggested the importance of the second question.
A Special Tribunal for Ukraine and reform of the Rome Statute need not be understood as competing projects. The first addresses an immediate institutional deficiency. The second addresses the systemic conditions that produced that deficiency.
Indeed, the strongest justification for exceptional institutional action may ultimately depend upon whether it contributes to less exceptional justice in the future.
That is where the debate becomes particularly relevant to the Rule of Law.
Selectivity, Double Standards, and Institutional Legitimacy
Perhaps the most difficult dimension of the panel concerned the accusation of double standards.
The response to Ukraine has inevitably generated comparisons with other conflicts and other uses of force. Iraq, Libya and other situations were invoked during the discussion. More broadly, perceptions of unequal treatment have profoundly shaped debates about the legitimacy of international criminal justice, particularly outside Europe and North America.
These objections cannot simply be dismissed.
At the same time, the existence of previous inconsistency cannot itself become an argument for permanent inconsistency.
There is an important distinction between saying:
justice has been selective in the past, therefore justice should not be pursued now
and saying:
justice has been selective in the past, therefore the institutions created now should contribute to reducing selectivity in the future.
The second proposition seems to me far more consistent with the Rule of Law.
The challenge is therefore not to deny selectivity but to prevent selectivity from becoming structural permanence.
This requires attention not only to formal jurisdiction but also to institutional legitimacy.
If mechanisms of international criminal justice are perceived as projects belonging primarily to a particular geopolitical region, their capacity to articulate genuinely international norms will inevitably be weakened.
The discussion concerning support for the Special Tribunal beyond Europe was therefore particularly important. Participation by states and legal communities from different regions is not merely symbolically desirable. It affects the legitimacy, precedential significance, and future development of the institution itself.
Aggression Is Not Merely a European Question
Another point that stayed with me after the panel was the insistence that the law of aggression should not be understood as primarily a European concern.
Russia’s invasion is occurring in Europe, but the normative issue is universal.
The prohibition of aggression matters especially for states that cannot rely upon military power to guarantee their territorial integrity. It matters for small and middle powers. It matters for societies with histories of invasion, imperial domination, and colonialism. It matters wherever sovereign equality is intended to operate as something more than a formal proposition.
Seen from this perspective, the crime of aggression is intimately connected to the basic architecture of international order.
The prohibition of force is not fundamentally a mechanism for protecting governments simply because they govern. Nor should international law be interpreted as conferring immunity from criticism upon authoritarian regimes.
The principle protects states and populations from the catastrophic consequences that follow when military power becomes an accepted instrument for changing borders, governments, or political outcomes.
The Vienna discussion also underscored the importance of distinguishing legal analysis from political approval or disapproval of particular governments. The illegality of an unlawful use of force cannot depend upon whether the government of the targeted state is politically attractive to outside observers.
That distinction is fundamental to any legal order that aspires to constrain power.
The Human Consequences of Institutional Design
The discussion also reminded me why apparently technical jurisdictional questions should not be isolated from their human consequences.
Aggression concerns more than the formal violation of territorial sovereignty.
The decision to initiate an unlawful war can generate consequences on an enormous scale: deaths and injuries, displacement, destruction of infrastructure, environmental devastation, economic dislocation, cultural loss, and the multiplication of situations in which war crimes and other international crimes may subsequently occur.
This does not collapse aggression into those other crimes. Each has its own legal elements and its own basis of responsibility.
But it does demonstrate why jurisdictional design is never merely technical.
Rules determining when an institution may exercise authority ultimately determine which harms can enter the field of legal accountability, whose conduct becomes legally visible, and which violations remain beyond the reach of existing institutions.
In this respect, the debate over Article 15 bis is also a debate over the distribution of institutional authority within the international legal system.
Regionalism, Universality, and Institutional Pluralism
The discussion of the proposed tribunal also raised another question that deserves greater attention: the relationship between regional and universal institutions.
International criminal law was at one time imagined as moving progressively toward a centralized architecture around the ICC. Contemporary developments may instead suggest a more plural institutional landscape.
National courts, the ICC, internationalized tribunals, regional organizations, investigative mechanisms, and specialized accountability institutions may increasingly coexist.
Institutional plurality is not necessarily institutional weakness.
The question is whether these different mechanisms can operate within a sufficiently coherent legal framework, reinforce rather than undermine common standards, and ultimately contribute to the universalization of accountability.
The Ukraine tribunal may therefore become significant not only for Ukraine but also as an experiment in the evolving relationship between regional institutional capacity and universal international criminal law.
Its legitimacy will depend in part on how that relationship develops.
From the Vienna Panel to a New ESIL Conversation
The discussion did not end for me when the panel concluded.
Precisely because the questions raised in Vienna extend beyond the immediate negotiations over the Rome Statute, I spoke afterwards with Jennifer Trahan and Patrycja Grzebyk about continuing the conversation within the European Society of International Law Interest Group on European and International Rule of Law Webinar Series.
I was delighted that both responded very positively to the idea.
The webinar is provisionally entitled:
Aggression, Accountability, and the International Rule of Law: Historical Foundations and the Challenge of Ukraine
The purpose will not be simply to organize another discussion about the war in Ukraine.
Instead, I hope to use Ukraine as the contemporary point of departure for a broader conversation about the historical foundations, institutional architecture, legitimacy, consistency, and future development of international criminal law.
Jennifer Trahan’s new book, The Crime of Aggression and Russia’s Invasion of Ukraine, provides an especially valuable foundation for examining the historical development of the crime of aggression, the Rome Statute and Kampala architecture, the jurisdictional limitations revealed by Ukraine, the negotiations surrounding a special tribunal, and the wider question of how aggression can become subject to a more coherent and general system of international criminal accountability.
Patrycja Grzebyk’s recent scholarship opens complementary historical and contemporary dimensions.
Her book Polish Contributions to International Criminal Law, co-authored with Bartłomiej Krzan and Karolina Wierczyńska, recovers an important intellectual, judicial, and institutional genealogy in the development of international criminal law, including Polish contributions to the conceptualization and prosecution of genocide, crimes against humanity, war crimes, and crimes against peace and aggression.
Her work with Dominika Uczkiewicz on The Russian-Ukrainian Conflict and War Crimes: Challenges for Documentation and International Prosecution broadens the inquiry toward documentation, evidence, domestic and international prosecution, war crimes, genocide, jurisdiction, and the relationship among multiple accountability mechanisms.
Patrycja also drew my attention after Vienna to her forthcoming chapter, “The Law as the Sharpest Sword in the Russian Arsenal: The Instrumentalization of International Law in Russia’s Wars.” That work introduces an additional and particularly important dimension: international law must be studied not only as an instrument capable of constraining power, but also as a language and institutional resource that political actors may attempt to appropriate, reinterpret, and instrumentalize in pursuit of objectives incompatible with the legal order itself.
This raises another fundamental Rule of Law question: how can legal institutions preserve the authority of law when law itself becomes part of the strategy through which unlawful conduct is justified?
From Accountability for Ukraine to the Future of International Criminal Justice
The future ESIL conversation will therefore allow us to move between history and the present, between doctrine and institutional design, and between the demands of accountability in one conflict and the legitimacy of the international criminal justice system as a whole.
Several questions seem especially important:
How did the contemporary legal architecture of individual criminal responsibility develop, and whose intellectual and institutional contributions shaped it?
What has Russia’s aggression against Ukraine revealed about the structural limits of that architecture?
Can a Special Tribunal provide meaningful accountability while also becoming a catalyst for broader reform?
How should international criminal law respond to concerns about selectivity and double standards without allowing past failures to become reasons for future inaction?
What relationship should exist among the ICC, domestic jurisdictions, regional institutions, and special or internationalized mechanisms?
And perhaps most importantly: can the response to Ukraine help strengthen a general architecture of accountability applicable beyond Ukraine itself?
The Vienna panel left me thinking that this last question may ultimately be the most consequential.
The challenge is not only to determine whether international law can respond to one manifest crisis.
The deeper challenge is whether the institutions created or reformed in response to that crisis can move international law closer to a system in which the prohibition of aggression is applied with greater consistency, legitimacy, and equality.
The distance between those aspirations and existing institutional realities remains considerable.
But that distance is precisely why the conversation must continue.
Paolo Davide Farah, Paolo Farah