States group together into supranational organizations - cooperating groups of states that operate on either a regional or international level
United Nations (UN) - 193 states as members, serves as a platform for dialogue between countries. All UN members sit in the General Assembly, but 15 sit on the Security Council, which is the only body that can authorize military force. 5 countries (USA, UK, France, Russia, China) have permanent seats on the SC and also hold veto power.
NATO - Military alliance initially designed by the USA to prevent the spread of communism
Warsaw Pact - Mutual-defense alliance between Eastern European satellite states and Soviet Union. Now defunct.
COMECON - Promote trade amongst communist states. Now defunct.
European Union (EU) - Initially formed to help Western Europe recover from WW2. Expansion has included majority of Eastern Europe. Key goals are economic and political cooperation. The EU has moved towards integration, with members pooling their sovereignty to gain collective political, economic, and social power (ex: unified monetary policy - introduction of the Euro)
Other organizations include the Organization of American States, Organization on Security and Cooperation in Europe, the African Union, Arab League, and the British Commonwealth.