The Principle of Complementarity and Ongoing ICC Investigations:

Functional Priority, Systemic Coherence, and the Limits of Universal Jurisdiction

I. Introduction

The principle of complementarity constitutes the structural cornerstone of the system established by the Rome Statute of the International Criminal Court (ICC). Rather than conferring primacy upon the Court, the Statute enshrines a model of subsidiarity: national jurisdictions retain primary responsibility, and the ICC intervenes only when States are unwilling or unable genuinely to investigate or prosecute.¹

However, a more complex question arises when the ICC has already formally opened an investigation and is actively exercising jurisdiction: to what extent should third States refrain from initiating or continuing proceedings based on universal jurisdiction concerning the same situation or conduct?

This article argues that, once the ICC is actively exercising jurisdiction over a situation, systemic coherence and the object and purpose of the Rome Statute favour recognising a form of functional priority of the Court vis-à-vis parallel third-State universal jurisdiction proceedings. This priority is not hierarchical in a formal sense, but it derives from the need to preserve the integrity, coherence, and effectiveness of international criminal adjudication.

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II. Complementarity as a Structural Principle

Article 17 of the Rome Statute establishes admissibility criteria grounded in unwillingness or inability.² Complementarity is therefore both a jurisdictional filter and a normative allocation of responsibility between national and international levels.³

The jurisprudence of the ICC Pre-Trial Chambers has clarified that complementarity operates through a comparative analysis between national proceedings and the case before the Court, requiring that domestic proceedings encompass substantially the same person and conduct.⁴

As Stahn observes, complementarity is not merely a procedural mechanism but a structural principle shaping the distribution of authority in international criminal law.⁵ Similarly, Kleffner emphasises that complementarity reflects a systemic preference for national enforcement, yet within a coordinated framework designed to avoid impunity.⁶

Traditionally, the debate has focused on vertical relationships between the ICC and territorial or national States. Less attention has been paid to horizontal tensions between the ICC and third States invoking universal jurisdiction.

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III. Ongoing ICC Investigations and Third-State Universal Jurisdiction

Universal jurisdiction allows States to prosecute certain international crimes irrespective of territorial or nationality links.⁷ While its normative justification lies in combating impunity, its simultaneous exercise alongside ICC proceedings may generate fragmentation.

When the ICC has formally opened an investigation under Article 53 and is actively exercising jurisdiction, the initiation of parallel proceedings by third States concerning the same conduct risks:

1. Evidentiary fragmentation;

2. Conflicting judicial assessments;

3. Strategic forum competition;

4. Undermining cooperation with the Court.

Ambos notes that the Rome Statute system presupposes coordination rather than competition between jurisdictions.⁸ Although the Statute does not explicitly regulate third-State universal jurisdiction in such contexts, its object and purpose — to establish a permanent international criminal court complementary to national jurisdictions — support an interpretation favouring systemic coherence.⁹

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IV. Functional Priority of the ICC

The Rome Statute does not establish formal primacy of the ICC. However, once admissibility has been affirmed and investigations are ongoing, a de facto functional priority emerges.

This functional priority rests on four considerations:

1. Systemic Integrity

Allowing concurrent third-State proceedings may jeopardise the consistency of international criminal adjudication. The ICC was designed as a centralised forum capable of addressing situations comprehensively.¹⁰

2. Cooperation Framework

Part 9 of the Rome Statute establishes a detailed cooperation regime. Parallel proceedings by third States may interfere with evidence collection and witness protection mechanisms coordinated by the Court.¹¹

3. Protection of the Accused

Simultaneous proceedings risk infringing principles related to fairness and legal certainty, including ne bis in idem considerations under Article 20.¹²

4. Object and Purpose of the Statute

Under Article 31(1) of the Vienna Convention on the Law of Treaties, interpretation must consider object and purpose.¹³ The Rome Statute seeks to end impunity through an integrated system; fragmentation undermines this objective.

Therefore, while universal jurisdiction remains legally permissible, systemic logic favours judicial restraint where the ICC is genuinely and actively exercising jurisdiction.

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V. Limits and Exceptions

Functional priority is not absolute. Third-State jurisdiction may remain appropriate where:

(a) The ICC investigation becomes dormant or ineffective;
(b) The Court declines to prosecute particular conduct not encompassed in its case selection;
(c) Cooperation with the ICC is structurally blocked.

In such circumstances, universal jurisdiction may serve as a safety net rather than a competing forum.

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VI. Conclusion

Complementarity was conceived as a principle of subsidiarity, not primacy. Yet when the ICC has opened and is actively pursuing an investigation, systemic coherence favours recognising a functional priority of the Court over parallel universal jurisdiction proceedings by third States.

This priority is neither hierarchical nor absolute. It arises from the structural logic of the Rome Statute system, the need to preserve judicial coherence, and the objective of avoiding fragmentation in international criminal adjudication.

Universal jurisdiction remains an essential tool against impunity. However, its exercise should not undermine the coordinated architecture established by the Rome Statute.

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Footnotes (OSCOLA)

1. Rome Statute of the International Criminal Court (adopted 17 July 1998, entered into force 1 July 2002) 2187 UNTS 90 (Rome Statute) Preamble para 10.

2. Rome Statute (n 1) art 17.

3. Carsten Stahn, A Critical Introduction to International Criminal Law (CUP 2019) 217–224.

4. Prosecutor v Thomas Lubanga Dyilo (Decision on the Prosecutor’s Application for a Warrant of Arrest) ICC-01/04-01/06 (10 February 2006) paras 29–37.

5. Stahn (n 3) 223–230.

6. Jann K Kleffner, Complementarity in the Rome Statute and National Criminal Jurisdictions (OUP 2008) 102–115.

7. Cedric Ryngaert, Jurisdiction in International Law (2nd edn, OUP 2015) 187–205.

8. Kai Ambos, Treatise on International Criminal Law: Volume I (OUP 2013) 88–94.

9. Rome Statute (n 1) Preamble para 4.

10. Stahn (n 3) 229–232.

11. Rome Statute (n 1) arts 86–93.

12. Rome Statute (n 1) art 20.

13. Vienna Convention on the Law of Treaties (adopted 23 May 1969, entered into force 27 January 1980) 1155 UNTS 331, art 31(1)