How Do International Law and Domestic Law Interact, and Which Prevails in Case of Conflict?

Japanese Attorney at Law - Bengoshi L.L.

• 10 min read

In an increasingly interconnected world, the lines between international law and domestic (national) law are more frequently tested and scrutinized. For businesses operating across borders, for individuals whose rights may be defined by international standards, and for states themselves, understanding this relationship is paramount. Key questions arise: How does international law become part_of a country's internal legal system? What happens when a domestic law clashes with an international obligation? Which legal order takes precedence? This article explores the complex interplay between these two spheres of law, examining theoretical frameworks, national practices, and the mechanisms by which domestic courts apply international law.

Theoretical Underpinnings: Monism and Dualism

Traditionally, two main theories have sought to explain the relationship between international law and domestic law:

  1. Dualism: Championed by jurists like Heinrich Triepel, dualism posits that international law and domestic law are two distinct, separate, and independent legal orders. They differ in their sources (international law arises from the common will of states, domestic law from the will of a single state), their subjects (international law primarily governs states, domestic law governs individuals and entities within a state), and their substance. Under a strict dualist view, international law can only apply within the domestic sphere if it is specifically "transformed" or "incorporated" into domestic law through an act of the national legislature (e.g., by passing a statute that enacts the treaty's provisions). Without such transformation, international law remains outside the domestic legal system, binding the state only on the international plane.
  2. Monism: In contrast, monism views international law and domestic law as parts of a single, unified legal system. Within this broad theory, there are variations:
    • Monism with Primacy of International Law: Figures like Hans Kelsen argued that all law ultimately derives its validity from a single fundamental norm (Grundnorm), and in a unified system encompassing both international and domestic law, international law is superior. Domestic law that conflicts with international law could be considered invalid, at least from a theoretical internationalist perspective.
    • Monism with Primacy of Domestic Law: This less common view effectively subordinates international law to domestic constitutional principles, essentially treating international law as an extension of domestic law ("external public law") rather than a superior order.

While this theoretical debate was once central to international legal scholarship, its practical significance in determining a state's international obligations has diminished. It is a firmly established principle of international law, codified in Article 27 of the Vienna Convention on the Law of Treaties (1969), that a state "may not invoke the provisions of its internal law as justification for its failure to perform a treaty." Similarly, Article 3 of the International Law Commission's Articles on Responsibility of States for Internationally Wrongful Acts (2001) states that the characterization of an act of a state as internationally wrongful is governed by international law and is not affected by the characterization of the same act as lawful by internal law.

However, the monist-dualist debate continues to inform how different national legal systems approach the domestic reception and application of international law.

The method by which international law becomes part of a state's internal legal order is determined by that state's own constitutional framework and legal traditions.

1. Treaties

States adopt various approaches to giving domestic legal effect to treaties:

  • Transformation: In strictly dualist systems like the United Kingdom and many Commonwealth countries (e.g., Australia, Canada), treaties do not automatically become part of domestic law upon ratification. Specific enabling legislation is required to "transform" the treaty's provisions into national law. Without such legislation, domestic courts cannot directly apply the treaty, although it remains binding on the state internationally. Some Nordic countries also follow this model.
  • Automatic Incorporation (General Reception): Many other states, often with monist leanings or specific constitutional provisions, provide that duly ratified treaties automatically become part of the domestic legal order, sometimes upon publication. Examples include France, Belgium, the Netherlands, Spain, Portugal, Switzerland, and Russia. The United States Constitution (Article VI, Clause 2) declares that "all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land," suggesting automatic incorporation. However, the U.S. doctrine of "self-executing" vs. "non-self-executing" treaties, as discussed below and significantly shaped by cases like Medellín v. Texas (552 U.S. 491 (2008)), complicates this by requiring implementing legislation for non-self-executing treaties before they can be enforced by U.S. courts.
  • Japan: Article 98, paragraph 2 of the Constitution of Japan states: "The treaties concluded by Japan and established laws of nations shall be faithfully observed." This is generally interpreted by prevailing scholarly opinion and court practice as providing for the automatic incorporation of treaties into Japanese domestic law once they are duly concluded and promulgated, without the need for specific implementing legislation for them to be considered "law of the land."
  • Legislative Approval Granting Domestic Status: Some countries, like Germany, require a specific federal statute (a Zustimmungsgesetz or act of assent) for the ratification of most important treaties. Historically, under the "transformation theory" (Transformationsthese), this act was seen as transforming the treaty into German law. More recently, the prevailing "enforcement theory" (Vollzugslehre) suggests the act of assent merely permits the treaty to be applied domestically as international law, retaining its international character. Greece and Nigeria have similar systems.

2. Customary International Law (CIL)

The domestic reception of CIL often follows a different, generally more direct, path:

  • General Incorporation: Most states, regardless of their approach to treaties, accept that CIL is automatically part of their domestic law without the need for specific legislative incorporation.
    • In common law countries like the United Kingdom, the traditional doctrine is that "international law is part of the law of the land" (as articulated by Blackstone). This principle was also inherited by the United States, as affirmed in cases like The Paquete Habana (175 U.S. 677 (1900)).
    • Many civil law countries have constitutional provisions that explicitly incorporate "general rules of international law" or "generally recognized rules of international law" into their domestic legal order. For example, Article 25 of the German Basic Law states that the general rules of public international law are an integral part of federal law and take precedence over statutes. Article 10 of the Italian Constitution provides that the Italian legal system conforms to the generally recognized rules of international law. Japan's Article 98(2) is also understood to encompass CIL.

Hierarchy: When International and Domestic Law Clash

The crucial question for domestic application is the hierarchical relationship between international law and various forms of domestic law (constitutions, statutes). This is determined by each state's constitutional order.

International Law Perspective

From the standpoint of international law, a state's obligations under international law prevail over its domestic law. As noted, a state cannot invoke its constitution or statutes to excuse a failure to perform an international obligation.

Domestic Law Perspective

The domestic hierarchy varies significantly:

  • International Law vs. National Constitution:
    • Constitution Supreme: In the vast majority of states, the national constitution is considered the supreme law of the land, and international law (both treaties and CIL) is subordinate to it. If a treaty conflicts with the constitution, the treaty may be considered inapplicable domestically, or the constitution might need to be amended before the treaty can be ratified. The Japanese Supreme Court's decision in the Sunagawa Case (Judgment of December 16, 1959), while dealing with the constitutionality of the U.S.-Japan Security Treaty, proceeded on the assumption of the Constitution's supremacy. The German Basic Law (Article 25) places CIL below the constitution, though above federal statutes.
    • Nuances and Exceptions: Some countries, like the Netherlands, have provisions allowing self-executing treaty provisions to prevail even over the constitution (Article 94 of the Dutch Constitution). Certain states, particularly in Latin America and increasingly in Europe, accord a special, sometimes quasi-constitutional or supra-constitutional, status to international human rights treaties. For instance, Article 75(22) of the Argentine Constitution grants constitutional hierarchy to certain human rights treaties. Switzerland (Article 193(4), 194(2) of the Federal Constitution) prohibits constitutional amendments that conflict with peremptory norms of international law (jus cogens).
  • International Law (Treaties/CIL) vs. National Statutes:
    • International Law Superior: In many countries, duly incorporated treaties are considered superior to domestic statutes, whether enacted before or after the treaty. This is the case in France (Article 55 of the Constitution), the Netherlands, and is the prevailing understanding for treaties in Japan. In such systems, courts may set aside a statute that conflicts with a treaty.
    • Equal Rank (Later-in-Time Rule): In other systems, international treaties and federal statutes are considered to be of equal rank. In the United States, for instance, both treaties and federal statutes are "supreme Law of the Land," and in case of conflict, the one later in time prevails (lex posterior derogat priori). A similar approach is often taken in Germany for treaties that have been incorporated via a federal statute of assent.
    • International Law Inferior or Ambiguous: For CIL, its rank relative to statutes can be less clear. In the UK and many Commonwealth nations, statutes can override CIL. In the US, the precise hierarchical status of CIL vis-à-vis federal statutes is complex and debated, though generally, a clear federal statute would prevail. In France, the Cour de Cassation and Conseil d'État have held CIL to be inferior to statutes (Aquarone decision by the Conseil d'État, 1997).

Application of International Law by Domestic Courts

Even when international law is considered part of the domestic legal order and its hierarchical position is established, questions arise about how, and under what conditions, domestic courts can actually apply it.

1. Direct Application (Self-Executing Norms)

Direct application refers to the capacity of an international legal norm to be applied by domestic courts as a rule of decision in a specific case without the need for further implementing legislation. Whether a norm is directly applicable (or "self-executing," a term particularly common in U.S. jurisprudence) is a matter of domestic law, though it often involves interpreting the international norm itself. Several factors are typically considered:

  • Intent of the Drafters/Parties (for Treaties): Courts may look to whether the treaty drafters or the ratifying state (e.g., through its legislature or executive during the ratification process) intended the provision to be immediately operative or to require further legislative action. The PCIJ's Advisory Opinion on the Jurisdiction of the Courts of Danzig (1928) is an early example suggesting that the parties' intent for a treaty to create directly enforceable individual rights is key. In the U.S., if the political branches declare a treaty non-self-executing, courts generally defer to that determination (Medellín v. Texas, 2008).
  • Clarity, Precision, and Completeness of the Norm: To be directly applicable, an international norm must generally be sufficiently clear, precise, and complete to be applied by a court without further legislative elaboration. Vague or programmatic provisions that merely set out goals for states are often deemed non-self-executing. Japanese courts have considered the clarity of treaty provisions in determining direct applicability, for instance, in relation to the International Covenant on Civil and Political Rights (ICCPR).
  • Subject Matter and Addressee of the Norm: If an international norm is addressed exclusively to the state's political branches (legislative or executive) and requires them to take certain actions, courts may find it non-self-executing because its implementation falls outside judicial competence. The Japanese Supreme Court, for example, has held that Article 9 of the International Covenant on Economic, Social and Cultural Rights (ICESCR), concerning the right to social security, declares a political responsibility of the state rather than conferring immediately enforceable individual rights (Judgment of March 2, 1989). Similarly, WTO agreements are generally considered not directly applicable in the domestic law of many members, including the EU and the US, partly because they are seen as establishing obligations between states to be managed through specific international dispute settlement mechanisms.

2. Indirect Application (Principle of Consistent Interpretation)

Even if an international norm is not directly applicable, domestic courts frequently engage in "indirect application" through the principle of consistent interpretation (also known as the presumption of conformity or harmonious interpretation). This principle entails that domestic courts will, wherever possible, interpret and apply national law in a manner that is consistent with the state's international obligations, whether arising from treaties or CIL.

  • Rationale: This interpretive approach is often based on a presumption that the legislature does not intend to enact laws that would place the state in breach of its international commitments. It promotes coherence between a state's domestic and international legal postures.
  • Widespread Practice: This is a widely accepted canon of statutory interpretation in many legal systems. Some constitutions explicitly mandate it (e.g., Article 10(2) of the Spanish Constitution for fundamental rights).
  • Limits: The principle of consistent interpretation has its limits. It cannot be used to contradict the clear and unambiguous meaning of a domestic statute (contra legem interpretation is generally not permissible). If a domestic law is irreconcilably in conflict with an international obligation, and if international law does not have superior hierarchical status in that domestic system, the domestic court may be bound to apply the domestic law, even if it leads to an international breach by the state.
  • Japanese Practice: Japanese courts frequently employ consistent interpretation. For example, the Supreme Court in its 2008 judgment on the Nationality Act (concerning distinctions based on marital status of parents for acquisition of nationality by children born out of wedlock), referred to the "changing domestic and international social environment, including the ICCPR and the Convention on the Rights of the Child" in interpreting Article 14(1) of the Japanese Constitution (equality under the law) (Judgment of June 4, 2008).

The Special Case of Regional Integration Law

Legal systems arising from deep regional integration, most notably European Union (EU) law, present a special case that often transcends the traditional international/domestic law dichotomy.

EU law is characterized by principles such as direct effect (EU law provisions can create rights for individuals which national courts must protect) and primacy/supremacy (EU law takes precedence over all conflicting provisions of national law, including national constitutions, as asserted by the Court of Justice of the European Union in landmark cases like Costa v. ENEL, 1964, and Internationale Handelsgesellschaft, 1970). The ECJ considers EU law to constitute a "new legal order," distinct from traditional international law, for the benefit of which the Member States have limited their sovereign rights.

While this represents the EU's own constitutional perspective, it has sometimes led to tensions with the constitutional courts of Member States, which may assert ultimate national constitutional sovereignty (e.g., the German Federal Constitutional Court's pronouncements in its Maastricht (1993) and Lisbon (2009) judgments regarding the limits of EU competence and the protection of national constitutional identity). Other regional systems, like the Organisation for the Harmonisation of Business Law in Africa (OHADA), also exhibit strong features of direct applicability and primacy for their harmonized laws within member states.

Conclusion

The relationship between international law and domestic law is multifaceted and dynamic, reflecting a continuous dialogue between national sovereignty and the demands of an increasingly interdependent global order. While international law unequivocally binds states on the international plane, its status and effect within domestic legal systems are diverse, primarily governed by each state's constitutional framework. Nevertheless, the overarching trend is towards a greater penetration and influence of international norms in domestic law, whether through automatic incorporation, legislative transformation, direct judicial application of self-executing provisions, or the pervasive principle of consistent interpretation. For legal and business professionals, understanding these mechanisms of interaction is no longer a niche concern but a practical necessity for navigating a world where local and global legal orders are ever more intertwined.