States are the most influential actors in the international system and as such relations between states are important in terms of cooperation. State relations have been addressed through both international law and supranational entities, including both intergovernmental organizations (IGOs) and nongovernmental organizations (NGOs). International law formalizes operating rules between states and international organizations are nonstate actors formed to serve the greater good.

International law is the regulation of relations among sovereign states emerging from customary practices. Its development was prompted by changes in the European political system, particularly the end of the Thirty Years War and the emergence of the modern sovereign state system. The sixteenth century Dutch lawyer Hugo Grotius is often accredited as the father of international law. His work is regarded as the first comprehensive treatise on international law and later served as the basis of positivism, the codification of customary practices between nation states. Cooperative structures, such as the Concert of Europe formed in 1815, were formed to maintain the balance of power and thus peace.

The evolution of international organizations was also prompted by the development of the sovereign state system in the seventeenth century. IGOs in particular addressed practical areas of exchange, including communications and transportation. The origins of the modern political IGO can be pinpointed to 1919, when the League of Nations was created after World War I. The League of Nations was limited by the lack of American participation and, despite US attempts to support the League through mechanisms such as the Kellogg-Briand Pact, it ultimately failed. However, the Atlantic Charter laid a foundation for another intergovernmental organization: the United Nations (UN), which was established in 1945. 

As states became increasingly reliant on each other in relationships of complex interdependence, more and more regional IGOs were established to strengthen such relationships. Many broad associations have copied the UN example, including the Organization of American States and the African Union. Regional organizations are built on the geographical proximity of states and common interests that dictate that they work together, or are formed to serve specific purposes driven by functional needs, with the European Union providing the best example.

Another prominent type of international organizations is NGOs. NGOs are best suited to fulfill some functions of IGOs when states are stymied by their own interests, lack expertise, or cooperative agreements cannot be met. They are generally task-oriented, driven by people with common interests, and frequently organized around specific issue areas. For the most part, NGOs can be categorized in terms of political advocacy, economic development, and environmental and humanitarian orientations. This type of organization has flourished. Today, we may think of these actors as civil society.

The founding premise of the UN is to promote peace, an end that has traditionally been met through peacekeeping. In the post–Cold War world, that mission turned towards peacemaking, and in modern times, peacebuilding. In such operations, NGOs can sometimes bridge the gap between the UN and locals, amplifying peacebuilding efforts.

Additionally, after the Cold War, there has been an expansion of international law. In 1998, the International Criminal Court (ICC) was established. Since then, the ICC has considered 16 cases across seven countries.

Overall, the greatest factor limiting IGOs are its member states. Many treaties and regimes have been limited or failed due to lack of involvement or ratification by the United States. But, global issues require cooperation on a global issue, so international law and international organizations will remain critical to addressing and overcoming such challenges.