International law, also known as public international law and law of nations, is the set of rules, norms, and standards generally accepted in relations between nations. It establishes normative guidelines and a common conceptual framework to guide states across a broad range of domains, including war, diplomacy, trade, and human rights. International law thus provides a means for states to practice more stable, consistent, and organized international relations.
The sources of international law include international custom (general state practice accepted as law), treaties, and general principles of law recognized by most national legal systems. International law may also be reflected in international comity, the practices and customs adopted by states to maintain good relations and mutual recognition, such as saluting the flag of a foreign ship or enforcing a foreign judgment.
International law differs from state-based legal systems in that it is primarily—though not exclusively—applicable to countries, rather than to individuals, and operates largely through consent, since there is no universally accepted authority to enforce it upon sovereign states. Consequently, states may choose to not abide by international law, and even to break a treaty. However, such violations, particularly of customary international law and peremptory norms (jus cogens), can be met with coercive action, ranging from military intervention to diplomatic and economic pressure.
The relationship and interaction between a national legal system (municipal law) and international law is complex and variable. National law may become international law when treaties permit national jurisdiction to supranational tribunals such as the European Court of Human Rights or the International Criminal Court. Treaties such as the Geneva Conventions may require national law to conform to treaty provisions. National laws or constitutions may also provide for the implementation or integration of international legal obligations into domestic law.
The term "international law" is sometimes divided into "public" and "private" international law, particularly by civil law scholars, who seek to follow a Roman tradition. Roman lawyers would have further distinguished jus gentium, the law of nations, and jus inter gentes, agreements between nations. On this view, "public" international law is said to cover relations between nation-states and includes fields such as law, law of sea, international criminal law, the laws of war or international humanitarian law, international human rights law, and refugee law. By contrast "private" international law, which is more commonly termed "conflict of laws", concerns whether courts within countries claim jurisdiction over cases with a foreign element, and which country's law applies.
A more recent concept is "supranational law", which concerns regional agreements where the laws of nation states may be held inapplicable when conflicting with a supranational legal system to which the nation has a treaty obligation. Systems of supranational law arise when nations explicitly cede their right to make certain judicial decisions to a common tribunal. The decisions of the common tribunal are directly effective in each party nation, and have priority over decisions taken by national courts. The European Union is most prominent example of an international treaty organization that implements a supranational legal framework, with the European Court of Justice having supremacy over all member-nation courts in matter of European Union law.
The term "transnational law" is sometimes used to a body of rules that transcend the nation state.