Case Explained: Building a Special Tribunal for the Crime of Aggression against Ukraine – EJIL: Talk! – Legal Perspective

On June 25, 2025, Ukraine and the Council of Europe signed an agreement in Strasbourg to establish a Special Tribunal for the Crime of Aggression against Ukraine (“Tribunal”). The Tribunal is intended to create an internationally supported forum to prosecute the crime of aggression against Ukraine, widely recognized as the foundational illegality underpinning Russia’s full-scale invasion of Ukraine.

The legal foundation for the Tribunal was the product of efforts by Ukraine, together with a coalition of partner states known as the “Core Group.” We use this brief piece to discuss core elements of the Tribunal’s design, with particular attention to four issues: immunities, jurisdiction, the definition of the crime of aggression, and proceedings in absentia. While we were U.S. delegates involved in almost all Core Group meetings, we write this piece in our personal capacities, and it does not necessarily represent the views of the U.S. Department of State or the U.S. government.

Immunities and Jurisdiction

Because the crime of aggression is inherently a leadership crime, any meaningful tribunal must contemplate the possibility of holding heads of state, heads of government, and foreign ministers (the “troika”) accountable. Yet, under customary international law, such officials enjoy personal immunity (immunity ratione personae) in foreign courts while they are in office. These protections remain intact unless the court is established under a clear and accepted exception, such as by the Security Council exercising coercive powers under Chapter VII or via waiver by the relevant State. 

Existing jurisprudence has offered only limited and, at times, internally inconsistent guidance on the interaction between international or hybrid tribunals and the personal immunities of sitting heads of state. Most notably, the International Criminal Court Appeals Chamber in Al-Bashir and the Special Court for Sierra Leone Appeals Chamber in Taylor have asserted that sitting heads of state do not enjoy immunity before “international courts.” However, both decisions have drawn criticism for resting on circular or underdeveloped reasoning, asserting the inapplicability of immunity by reference to the tribunal’s “international” character, without sufficiently identifying the legal source that authorizes the displacement of customary immunities ratione personae or persuasively addressing, as a threshold matter, the inapplicability of those rules before “international courts.”

As recent scholarship has observed, including a piece authored for Just Security by one of the authors, these rulings often fail to engage in a sustained analysis of the constitutive elements of customary international law, namely, consistent state practice and opinio juris, and instead rely on institutional status to justify substantive conclusions. This doctrinal ambiguity complicates policymakers’ ability to determine with clarity the legal limits and scope of immunities in future tribunal contexts, which in turn limits the utility of existing jurisprudence as a reliable foundation for institutional design.

The Tribunal’s Statute presents a disciplined legal approach to these issues. Rather than asserting that the Tribunal is international in character in a way that might trigger an exception to immunity, or invoking contested theories such as pooled delegation of jurisdiction or universal jurisdiction, Article 1 of the Statute grounds jurisdiction in Ukraine’s sovereign right to prosecute the crime of aggression committed on its territory. Territorial jurisdiction is a well-established jurisdictional basis under international law. The Tribunal operationalizes that right through an agreement between Ukraine and the Council of Europe, delegating Ukraine’s authority to a specially constituted body by reinforcing that “the jurisdiction of the Special Tribunal shall be based on the territorial jurisdiction of Ukraine.” Other states may support the effort through participation in an Enlarged Partial Agreement, but their participation does not alter the Tribunal’s underlying jurisdictional foundation.

This construction serves several important purposes. First, it provides a sound legal foundation for long-term sustainability and legitimacy, rooted in existing international law that recognizes a state may prosecute crimes committed on its territory and delegate that authority to other states or judicial bodies. Second, it guards against the Tribunal being judicially recharacterized as an “international court” possessing authority to displace immunities. Rather, the Tribunal exercises authority that Ukraine as a sovereign state has: no more and no less. A recharacterization, even if unintended, could undermine the legal and political legitimacy of the Tribunal’s proceedings by exposing it to claims of overreach or noncompliance with international law. Finally, it avoids ambiguity that could otherwise allow national or international courts to reinterpret the Tribunal’s legal nature beyond what was agreed.

While Article 4(2) of the Statute sets out that the official position of an individual shall not relieve him or her of criminal responsibility nor mitigate punishment, a formulation traceable to the Nuremberg Charter (see Article 7), this speaks to the unavailability of certain defenses rather than the Tribunal’s ability to exercise jurisdiction.  Article 23(5) of the Statute makes clear that:

“where the indictment concerns a head of State, head of government or minister of foreign affairs, the Pre-Trial Judge shall not confirm the indictment and shall order the proceedings be suspended until that person no longer holds that office or an appropriate waiver has been presented to the Special Tribunal. During the suspension, the Pre-Trial Judge shall not otherwise act upon the indictment.” (emphasis added).

In this way, the Statute affirms the principle of individual accountability while respecting the jurisdictional limits imposed by international law on the prosecution of sitting troika members. As the International Court of Justice explained in the Arrest Warrants case:

Immunity from criminal jurisdiction and individual criminal responsibility are quite separate concepts. While jurisdictional immunity is procedural in nature, criminal responsibility is a question of substantive law. Jurisdictional immunity may well bar prosecution for a certain period or for certain offences; it cannot exonerate the person to whom it applies from all criminal responsibility. 

The Statute’s approach in this regard ensures legal clarity. It strengthens the credibility of the Tribunal by signaling that it is designed to work within, not around, the framework of international law. And it preserves the possibility of future accountability without undermining foundational legal principles in the present.

The Definition of the Crime of Aggression

The definition of the crime of aggression in the Tribunal’s Statute, as provided under Article 2, reflects a conceptually complex aspect of its design given contrasting views on the definition of the crime of aggression under Article 8 bis of the Rome Statute and its status under international law. 

Although the Kampala Amendments have not been widely ratified (only 49 of the 125 Rome Statute states parties have done so, and not all have incorporated the definition domestically), the Core Group negotiating the Tribunal’s Statute included a significant number of Kampala ratifiers. Approximately 27 of the Kampala ratifying states participated in the Core Group, a number that grew to include Ukraine as well when it ratified both the Rome Statute and the Kampala Amendments on October 25, 2024 (notwithstanding its decision not to domesticate the 8 bis definition in its criminal code). Thus, a significant number of Core Group states regarded Article 8bis of the Rome Statute as the international definition of the crime of aggression.  Replicating that definition in the Tribunal’s Statute could be viewed as cementing that status and not stepping back from efforts at Kampala in developing a fit-for-purpose definition for the UN Charter era. It could also be seen as bolstering the Tribunal’s status and/or legitimacy as an “international court.”

Other states maintained lingering concerns with or simply opposed the definition adopted at Kampala, which in turn impacted their ability to support the definition’s application in this context. The concerns related to the content and formulation of the definition in Article 8 bis are well documented elsewhere (see, e.g., Koh and Buchwald). Applying this definition before a new tribunal when the definition in Article 8 bis was expressly adopted “[f]or the purpose” of the Rome Statute within a tightly constrained treaty-based jurisdictional framework would raise the prospect that the Kampala definition would be seen as applying by force of customary international law. Such a prospect is contested as inconsistent with the formation of customary international law, which requires widespread state practice and opinio juris.  These requirements were not met by the consensus adoption of a resolution that defined the term solely for purposes of a specific statute that binds only a limited number of states that consent to its application, and that was qualified by various understandings testifying to the limited impact of its adoption.    

Moreover, the application of the Article 8 bis definition outside its original treaty context would raise potential tensions with the principle of nullum crimen sine lege, which prohibits retroactive criminalization and demands that criminal law be clearly defined and accessible at the time of the alleged criminal conduct. Because the Kampala Amendments are neither universally ratified nor generally accepted as customary international law, applying them in this new context, especially when Ukrainian and Russian law apply a different definition of the crime, would risk undermining the foreseeability and legality requirements fundamental to international criminal law.

Given these tensions, Article 2 of the Statute is a bit of a muddle. The definition draws extensively from Article 8 bis of the Rome Statute but does not replicate it in whole. Article 2(1) of the Tribunal’s Statute reproduces Article 8 bis(1) of the Rome Statute. However, Article 2 of the Tribunal’s Statute omits the detailed list of constitutive acts enumerated in Article 8 bis(2) of the Rome Statute, instead directing the Tribunal to take into account UN General Assembly Resolution 3314, which contains that list and other guidance on assessing aggression. Article 2 also directs the Tribunal to take into account political assessments made by the UN General Assembly in assessing the state act of aggression specifically with respect to Ukraine. And finally, it nods toward the customary definition articulated at Nuremberg and Tokyo criminalizing a “war of aggression,” albeit in a fairly convoluted fashion. 

In the end, the definition can be seen as creature of compromise on an issue that is possibly least contested as a factual or legal matter in this context: Russia’s invasion of Ukraine plainly constitutes aggression and there are certainly individuals responsible for the crime of aggression under any definition. Yet, beyond this case, the definition remains politically contested, underscoring continuing divisions over how the crime of aggression should be defined and prosecuted under international law more generally.

In Absentia Proceedings

Article 28 of the Tribunal’s Statute permits trials and the issuance of judgments in absentia, subject to stringent procedural safeguards consistent with established principles of international law. The inclusion of in absentia proceedings is of importance to Ukraine, which has a well-established domestic practice of prosecuting individuals for international crimes in their absence and seeks to ensure that the envisaged tribunal will be able to deliver justice despite the anticipated challenges of apprehending those most likely to bear responsibility for the crime of aggression.

Specifically, Article 28 allows for the trial and conviction of an accused who is not present, provided that the accused: (i) was duly notified of the proceedings; (ii) was afforded a genuine opportunity to appear; (iii) is represented by counsel of their choosing or by court-appointed counsel; and (iv) retains an unequivocal right to a retrial upon surrender or apprehension.

This structure draws from the jurisprudence of the European Court of Human Rights, particularly in cases such as Sejdovic v. Italy, which recognize the compatibility of in absentia proceedings with the right to a fair trial under Article 6 of the European Convention on Human Rights, so long as procedural guarantees are rigorously upheld and a right to retrial is preserved.

Nevertheless, a decision to authorize in absentia judgments was one that warranted caution and deliberate restraint. There were lessons to be drawn from prior institutional experience, most notably of the Special Tribunal for Lebanon (STL), which conducted its main trial and rendered its final judgment in the absence of the accused. Although arguably procedurally compliant with applicable international human rights law, the STL’s in absentia model was widely perceived to have lacked sufficient legitimacy and impact, undermining public confidence in the STL’s authority and contributing to perceptions of limited deterrent or expressive value.

The Tribunal’s Statute reflects a framework informed by that precedent, allowing proceedings to continue in the absence of the accused but embeds rigorous procedural protections and avoids any presumption that in absentia trials are a preferred or default mode of adjudication. Article 28 thus reflects an effort to reconcile the necessity of forward procedural momentum with the primacy of due process and the perceived legitimacy of adjudicatory outcomes.

By expressly conditioning the permissibility of in absentia judgments on the preservation of core rights, notice, representation, and retrial, the Statute ensures compliance with prevailing human rights norms while guarding against the reputational and normative risks associated with trials conducted in the physical absence of the accused. Particularly with respect to the right to a retrial, the STL arguably undermined this fundamental guarantee through its own dissolution. Article 22(3) of the STL’s Statute preserved the right to a retrial only “before the Special Tribunal,” implying its availability was contingent on the STL’s continued existence. The Tribunal resolves this conundrum. Article 28(3) of its Statute refers to the right to a retrial “before the Special Tribunal or a successor mechanism,” thereby acknowledging the possibility of dissolution and maintaining the right of retrial thorough another forum.

In doing so, it seeks to uphold the integrity of proceedings and the broader legitimacy of the tribunal’s mandate to address crimes of aggression in a procedurally robust and principled manner.

Conclusion

The Special Tribunal for the Crime of Aggression against Ukraine is a targeted response to an egregious violation of the international legal framework. It was established to address a specific war of aggression with broad implications for international peace and security, in a manner that seeks to balance principle with practical feasibility. It did this with a legal response guided largely, but not entirely, by the region affected, which bolsters rather than undermines its legitimacy.

Rather than circumventing existing legal constraints, the Tribunal’s statute operates within them, respecting immunities in accordance with international law while allowing for the possibility of future accountability; grounding jurisdiction in well recognized principles rather than contested legal theories; and maintaining procedural safeguards.

While not a replication of past tribunals, it offers a forward-looking framework for pursuing accountability for the crime of aggression in a specific context. In an era where such conduct often escapes legal consequence, the tribunal reinforces the principle that accountability for the crime of aggression, while complex, remains a legitimate objective of the international legal order.