An international regime is the set of implicit or explicit principles, norms, rules, and decision-making procedures that international actors’ expectations converge around in a given area of international relations. These regimes guide and structure interactions between international actors and, in some cases, may evolve into an intergovernmental organization.
International law, also known as public international law and the law of nations, is the set of rules, norms, legal customs and standards that states and other actors feel an obligation to, and generally do, obey in their mutual relations. In international relations, actors are simply the individuals and collective entities, such as states, international organizations, and non-state groups, which can make behavioral choices, whether lawful or unlawful. Rules are formal, typically written expectations that outline required behavior, while norms are informal, often unwritten guidelines about appropriate behavior that are shaped by custom and social practice. It establishes norms for states across a broad range of domains, including war and diplomacy, economic relations, and human rights.
The term public international law embraces a wide variety legal regimes governing the conduct and relationships between states, between states and international organizations, and between entities and persons both natural and legal. Public international law defines the criteria for statehood, and legal theorists argue that it establishes states as the principal actors in the international legal system. (While the traditional view was that only states were subjects of international law, with the founding of the United Nations, that view expanded to include intergovernmental organizations. Contemporary conceptions of international law are much broader, and include the interactions such as the ones listed at the beginning of this paragraph.) Public International law also governs the outer bounds of permissible treatment of individuals by states with comprehensive international law regimes dealing with non-combatants, including prisoners of war, civilians, and refugees, as well as human rights.
Regime theory is a theory within international relations derived from the liberal tradition which argues that international institutions or regimes affect the behavior of states or other international actors. It assumes that cooperation is possible in the anarchic system of states, as regimes are, by definition, instances of international cooperation.
Stephen D. Krasner was a key figure in establishing the theory as a prominent topic of study in IR, in part through the 1983 edited collection International Regimes. Robert Keohane's 1984 book After Hegemony has been described as regime theory's "fullest expression."