Conflict of laws

Conflict of laws, often called private international law, refers to the domestic rules a jurisdiction uses when a case, transaction, or event involves more than one legal system. It covers three main areas: jurisdiction (whether a court can hear the case), foreign judgments (how one jurisdiction enforces another’s court rulings), and choice of law (which substantive laws apply). These questions can appear in any private law matter, but they are most common in contract and tort cases. In practice, such issues also arise in international arbitration, especially where countries have separate procedural rules for domestic and international proceedings. In dualist systems like Colombia, for instance, classifying an arbitration as domestic or international can determine the applicable procedures and grounds for annulment; international awards typically face a narrower review focused on due process and international public policy, not a full rehearing of the merits.

The term “conflict of laws” is mainly used in the United States, Canada, and increasingly the United Kingdom. Elsewhere, “private international law” is more common—for example, Switzerland’s Federal Act on Private International Law governs which law applies in cross-border disputes. Some scholars from countries that prefer “conflict of laws” find “private international law” misleading, since this body of law is not international but domestic; it only involves international law when a nation has treaty obligations, and even then only as domestic law makes those treaties enforceable. The term “private international law” stems from the private/public law divide in civil law systems, where it refers to domestic private law that addresses international issues, not an agreed international code. Importantly, while conflict of laws typically deals with international disputes, the applicable law is always domestic. Unlike public international law, it does not regulate relations between countries but how each country internally handles the affairs of individuals connected to multiple jurisdictions. Domestic law can, of course, be shaped by international treaties a country has joined. In federal states where subnational entities make significant laws—notably the United States—conflict-of-laws issues often arise in purely domestic contexts, involving laws of different states or provinces rather than foreign nations.

Western legal systems first recognized a core principle of conflict of laws—that foreign law should sometimes apply to foreign cases—in the twelfth century. Before that, personal law prevailed, where each person’s applicable laws came from the group they belonged to. Initially, the approach was simply to choose the most fair jurisdiction’s law, but over time it shifted toward more defined rules. Law professor Bartolus de Saxoferrato systematically summarized these rules in the mid-fourteenth century, and his work was cited for centuries. In the seventeenth century, Dutch scholars including Christian Rodenburg, Paulus Voet, Johannes Voet, and Ulrik Huber expanded the field with two key ideas: first, nations are fully sovereign within their borders and cannot be forced to apply foreign law in their own courts; second, for international conflict of laws to work rationally, nations must practice comity—enforcing each other’s laws out of mutual interest. Comity has evolved since then but still holds that every state is sovereign, and the most just exercise of authority often involves recognizing another state’s laws and judgments. Many countries, like Canada, still see comity as the foundation of private international law. In some, such as the United States and Australia, comity is written into the constitution. In the U.S., conflict-of-laws issues date back at least to the Constitution’s framing, with concerns about what law federal courts would apply in cases between parties from different states. Over a hundred such cases arose within two decades of ratification, though the term “conflict of laws” was not yet used. The Constitution created a “plurilegal federal union” in which these issues continue to arise.