Since the end of World War II, in part due to industrial size and the onset of the Cold War, the United States has often been a proponent of reduced tariff-barriers and free trade. The U.S. helped establish the General Agreement on Tariffs and Trade  and later the World Trade Organization ; although it had rejected an earlier version in the 1950s . Since the 1970s, U.S. governments have negotiated managed-trade agreements, such as the North American Free Trade Agreement  in the 1990s, the Dominican Republic-Central America Free Trade Agreement  in 2006, and a number of bilateral agreements . In Europe, six countries formed the European Coal and Steel Community  in 1951 which became the European Economic Community  in 1958. Two core objectives of the EEC were the development of a common market, subsequently renamed the single market, and establishing a customs union between its member states. After expanding its membership, the EEC became the European Union  in 1993. The European Union, now the world's largest single market, has concluded free trade agreements with many countries around the world.
Question: When did the countries that later formed the European Union first join?
Answer: 1951