European regional integration has been one of the most significant developments in international politics since the mid-twentieth century. The European Union (EU), as the most advanced form of regional integration in the world, represents both a political experiment and a theoretical challenge. Explaining how and why European states chose to pool sovereignty and construct supranational institutions has been the concern of a wide array of scholars in political science and international relations. Theories of European integration not only illuminate the EU’s evolution but also help assess the dynamics of cooperation, conflict, and sovereignty in regional governance. Several major theoretical approaches—such as federalism, functionalism, neofunctionalism, intergovernmentalism, liberal intergovernmentalism, and constructivism—have attempted to explain the nature and trajectory of European integration. The earliest attempts to explain integration emerged out of federalist thought, which saw the creation of a United States of Europe as the ultimate solution to Europe’s recurring conflicts. Federalist thinkers argued that integration should proceed by establishing a constitutional order in which sovereignty would be divided between a central authority and member states. Federalism thus emphasized deliberate political design, institution-building, and constitutional settlements to prevent wars and ensure unity. Although federalism inspired figures such as Altiero Spinelli and influenced early post-war initiatives, it lacked the explanatory power to account for the incremental and often pragmatic nature of actual integration processes. Rather than a sudden leap toward federation, integration unfolded through sectoral cooperation and functional arrangements. Functionalism, associated with David Mitrany, offered a different framework by emphasizing the role of technical cooperation in solving practical problems. Mitrany argued that states would delegate authority to specialized agencies in specific issue areas—such as transport, communication, or energy—because these functions required collective management. Functionalism predicted that cooperation in technical fields would depoliticize integration, leading to a gradual expansion of joint governance without necessarily creating a federal state. The functionalist logic underpinned the establishment of European institutions such as the European Coal and Steel Community (ECSC) in 1951, where cooperation in one key sector was seen as a way to build trust and interdependence. However, functionalism underestimated the political dimension of integration and failed to explain why states would accept supranational authority in sensitive areas. Neofunctionalism, developed by Ernst B. Haas, sought to address these shortcomings by combining functional logic with political dynamics. Neofunctionalism argued that integration generates a process of “spillover” in which cooperation in one area creates pressures for integration in related areas. For example, cooperation in coal and steel required coordination in energy, transport, and eventually broader economic policies. Spillover could be functional, political, or cultivated. Functional spillover referred to the technical necessity of extending integration into adjacent fields, while political spillover captured the tendency of interest groups, bureaucrats, and elites to redirect demands toward the supranational level. Cultivated spillover highlighted the role of supranational institutions such as the European Commission in pushing integration forward. Neofunctionalism explained the