From the Document: "The European Union (EU) is a political and economic partnership that represents a unique form of cooperation among sovereign countries. The EU is the latest stage in a process of integration begun after World War II, initially by six Western European countries, to foster interdependence and make another war in Europe unthinkable. The EU currently consists of 27 member states, including most of the countries of Central and Eastern Europe, and has helped to promote peace, stability, and economic prosperity throughout the European continent."
CRS Report for Congress, RS21372
Congressional Research Service: https://crsreports.congress.gov/