The European Union (EU) is a political and economic partnership that represents a unique form of cooperation among sovereign states. It is the latest stage in a process of European integration begun after World War II, initially by six Western European countries, to promote peace and economic recovery. Its founders hoped that by pooling sovereignty in certain sectors (primarily economic ones at first), integration would foster interdependence and make another war in Europe unthinkable. The EU currently consists of 28 member states, including most of the formerly communist countries of Central and Eastern Europe (see map in the Appendix ). The EU has been built through a series of binding treaties, and has characteristics of both a supranational entity (in specified areas, sovereignty is shared and EU institutions hold executive authority) and an intergovernmental organization (in other areas, cooperation is based on consensus among the member state governments). Over the years, member states have sought to harmonize laws and adopt common policies on an increasing number of issues. EU members share a customs union; a single market in which goods, services, people, and capital move freely (known as the "four freedoms"); a common trade policy; a common agricultural policy; and a common currency (the euro), which is used by 19 member states (collectively referred to as the "eurozone"). Twenty-two EU members (and four non-EU countries) participate in the Schengen area of free movement, which allows individuals to travel without passport checks. (See Table 1 for eurozone and Schengen membership.) In addition, the EU has taken steps to develop common foreign and security policies and has sought to build common internal security measures. Various European policymakers and analysts have likened the European integration project to a bicycle, which must keep going forward to avoid falling over. Currently, however, the EU faces a range of political and economic pressures—including successful populist, antiestablishment political parties in many EU countries—and multiple internal and external challenges, which have raised questions about the EU's future shape and character. Although many experts maintain that the EU will continue to pedal along, others worry that the EU bicycle appears wobbly. Perhaps the most prominent challenge for the EU is the United Kingdom's (UK's) expected exit from the EU (known as "Brexit"). In a public referendum in June 2016, British voters favored leaving the EU by 52% to 48%. The UK has been engaged in withdrawal negotiations with the EU but remains a member of the EU until it formally exits the bloc (which is widely expected to occur in March 2019). In addition, the EU faces a number of other salient issues; these issues include addressing concerns about democratic backsliding in some member states (especially Poland and Hungary), managing migratory pressures and integration of newcomers, dealing with a resurgent Russia, and combating a heightened terrorism threat. Despite Brexit, the other 27 EU member states appear committed to sustaining the EU and are considering further EU reforms. In the longer term, some analysts suggest the EU likely faces a fundamental choice between those supporting further integration as the solution to the bloc's woes and those contending that integration has gone too far and should be put on hold (or possibly even reversed in certain areas). Whereas some experts argue that "more EU" is necessary to deliver robust economic growth and ensure security, others are skeptical that national governments will be inclined to cede more authority to a Brussels bureaucracy viewed as opaque and out of touch with the problems of average citizens. Successive U.S. Administrations and many Members of Congress have strongly supported the European integration project as a key pillar of the transatlantic relationship. In the aftermath of World War II, the United States viewed European integration as a way to entrench democratic systems and free markets, and the creation of NATO was meant to provide collective defense and security. With the end of the Cold War, the United States was a key advocate for EU enlargement to the countries of Central and Eastern Europe, believing it would help to promote stability and prosperity in the region. Today, the United States and the EU share a dynamic political partnership and a huge trade and investment relationship. Although U.S.-EU relations have experienced numerous ups and downs in the past and political and economic tensions have arisen periodically, U.S. and EU policymakers have tended to view the overall partnership as mutually beneficial. During the Obama Administration, the numerous challenges facing the EU worried many U.S. officials who feared that a politically divided, economically weak, and internally preoccupied EU would threaten European stability, jeopardize U.S.-EU economic relations, and make the EU a less robust and effective global partner. In the 114 th Congress, several hearings addressed some of the specific issues facing the EU, including the Greek debt crisis, the conflict in Ukraine, Europe's migration and refugee crisis, and potential threats to Europe posed by the Islamic State terrorist organization. Some hearings also considered the EU's future development more broadly and its possible strategic and economic implications for the United States. Many in the EU are concerned about the future trajectory of U.S.-EU relations under the Trump Administration and about whether the United States will continue to be a reliable partner in the years ahead. President Trump's reported questioning of the EU's value and utility is largely unprecedented and in contrast to long-standing U.S. support for the European integration project. EU leaders have been taken aback by what they regard as President Trump's hostility toward the bloc. U.S.-EU divisions have emerged on a growing number of issues, from trade to climate change to the 2015 nuclear deal with Iran. Some analysts suggest that managing relations with the United States under the Trump Administration has emerged as another, somewhat unexpected, challenge for the EU. At the same time, the EU hopes to preserve close U.S.-EU ties, and EU policymakers continue to seek to cooperate with the Trump Administration where possible on issues of common interest and concern. Congressional interest in the EU, its ongoing challenges, and the state of U.S.-EU relations has persisted in the 115 th Congress. Various congressional hearings have focused on Brexit, EU policy toward Russia, and European efforts to address societal and security concerns posed by continued migration. In the 115 th Congress, some House and Senate Members also have sought to reassure EU officials and member state governments of continued U.S. support for the EU, in part through visits to Brussels and key European capitals, the reestablishment of the EU Caucus in the House, and the introduction of resolutions reaffirming the importance of a strong U.S.-EU partnership. In the aftermath of World War II, leaders in Western Europe were anxious to secure long-term peace and stability in Europe and to create a favorable environment for economic growth and recovery. In 1951, six countries—Belgium, the Federal Republic of Germany, France, Italy, Luxembourg, and the Netherlands—decided to establish the European Coal and Steel Community (ECSC), which is regarded as the first step in the European integration project. The ECSC was envisioned as a single market in which sovereignty over coal and steel would be pooled and production controlled by an independent supranational authority. In embarking on this plan, the six founders hoped to greatly diminish the chances of another catastrophic conflict in Europe by binding their economies together, controlling the raw materials of war, and promoting political reconciliation (especially between France and Germany). The ECSC began operations in 1952; over the next five years, coal and steel trade among the six members increased 129%. In light of the ECSC's success, in 1957, the six ECSC countries signed two new treaties in Rome: one treaty established the European Economic Community (EEC) to develop common economic policies and merge the separate national markets into a single market for goods, people, capital, and services; the other created a European Atomic Energy Community (EURATOM) to ensure the use of nuclear energy for peaceful purposes. These two treaties, commonly referred to as the "Treaties of Rome," came into force in 1958. In 1967, the ECSC, the EEC, and EURATOM collectively became known as the European Community (EC). The EC first added new members in 1973, with the entry of the United Kingdom, Ireland, and Denmark. Greece joined in 1981, followed by Spain and Portugal in 1986. The Single European Act modified the EC treaties in 1987 to facilitate the creation of the single market and ultimately resulted in the mostly free movement of goods, people, capital, and services (known as the "four freedoms") within the EC. On November 1, 1993, the Treaty on European Union (also known as the Maastricht Treaty) went into effect, encompassing the EC and establishing the modern-day European Union. The EU was intended as a significant step on the path toward not only greater economic integration but also closer political cooperation. The Maastricht Treaty contained provisions that resulted in the creation of the eurozone, in which participants share a common currency, a common central bank (the European Central Bank, or ECB), and a common monetary policy (there is no common fiscal policy, however, and member states retain control over national spending and taxation, subject to certain conditions designed to maintain budgetary discipline). The Maastricht Treaty also set out a blueprint for greater coordination on foreign policy and internal security issues. Since the mid-1990s, EU member states have worked to forge a Common Foreign and Security Policy (CFSP), including a Common Security and Defense Policy (CSDP), and sought to establish common policies in the area of Justice and Home Affairs (JHA). In the late 1990s, the Schengen Agreement of 1985—which established the framework for eliminating border controls among participating states—became EU law. With the end of the Cold War, the EU pursued further enlargement. Austria, Finland, and Sweden joined in 1995.