The Role of Domestic Law in the Jurisprudence of the International Court of Justice in Diplomatic Protection Cases

Document Type : Research Paper

Authors

1 Assistant Professor, Department of Public Law, Faculty of Law and Political Science, University of Tehran, Tehran, Iran.

2 PhD Candidate in International Law, Department of Public Law, Faculty of Law and Political Science, University of Tehran, Tehran, Iran.

10.22059/jlq.2026.391985.1007980

Abstract

Introduction
The relationship between domestic and international law has long been a subject of debate. At first sight, invoking domestic law in international judicial proceedings may seem inconsistent with the international judicial function of the International Court of Justice (ICJ). Since then, developments in international law have led to broader interaction between domestic and international law. Accordingly, the manner in which the Court has employed domestic law no longer reflects a rigid distinction between domestic and international legal systems as traditionally conceived.
Where international law refers to rules of domestic law, or where such rules constitute an integral part of an international legal norm, the Court may apply domestic law. In other situations, although international law constitutes the law applicable to the dispute as such, specific aspects of the case may nevertheless necessitate recourse to domestic law. In these circumstances, domestic law is said to form part of the applicable law. In this regard, a considerable number of treaties contain provisions that expressly refer to the domestic law of States, or employ concepts whose interpretation presupposes an understanding within a particular domestic legal framework. Moreover, in certain cases the Court is required to apply domestic law to specific factual elements of the dispute, most notably in cases concerning diplomatic protection.
Accordingly, the present study outlines the traditional theoretical approaches concerning the role of domestic law in the international legal order as reflected in the Court’s jurisprudence. It then departs from prevailing assumptions to analyze how, in certain contexts, the Court has integrated domestic law into its judicial reasoning in ways that go beyond its conventional evidentiary function.
 
Method
This study adopts a descriptive-analytical and qualitative methodology based on documentary and library-based research. Employing an inductive approach, it carefully examines the jurisprudence of the International Court of Justice, including separate and dissenting opinions, through a systematic analysis of relevant judgments and advisory opinions, with particular attention to cases involving diplomatic protection. By drawing legal data from primary judicial decisions and other authoritative scholarly sources, the study derives its conclusions from patterns identified within the Court’s practice concerning the role of domestic law.
Conclusion
Building on the approach of its predecessor, the Permanent Court of International Justice (PCIJ), which treated domestic law as mere facts in the Certain German Interests in Polish Upper Silesia case, the jurisprudence of the International Court of Justice has nevertheless evolved. While the ICJ in its early decisions largely reaffirmed the Permanent Court’s position and asserted itself as an organ of the international legal order, it has gradually developed a more nuanced engagement with domestic law.
Domestic law, as an independent and distinct legal system, has nonetheless influenced the development of international law and assisted the Court in fulfilling its judicial functions. Independent systems can exert mutual influence without compromising their autonomy. Domestic and international legal systems are therefore not merely coexisting regimes; they are complementary and interdependent, cooperating to enhance the public order of  the international community.
This interaction is particularly evident in cases concerning diplomatic protection, where the Court has engaged with domestic legal processes, most notably in determining nationality as a prerequisite for the exercise of diplomatic protection. Similarly, domestic law has been invoked as an interpretative element in relation to unilateral declarations of States and declarations accepting the Court’s compulsory jurisdiction. In such instances, the Court neither applies domestic law as a binding rule nor treats it as a mere factual matter. Rather, domestic law operates as a normatively relevant consideration within the Court’s international legal reasoning. Ultimately, a rigid distinction between domestic and international law proves untenable, as matters governed by one legal system may, in specific contexts, enter the normative domain of the other.
 
 

Keywords

References

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Law Quarterly
Volume 55, Issue 4
January 2026
Pages 581-604

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