January 1, 1958: The European Economic Community, known as the Common Market, becomes official. Belgium, France, West Germany, Italy, Luxembourg, and The Netherlands signed the Treaty of Rome in March 1957, which laid the plan for this trade community, and was later joined by the United Kingdom, Denmark, Ireland, Greece, Portugal,and Spain. East Germany also joined after the reunification of Germany.
Source: The History Place & Ecyclopedia Britannica
Read more from the Goizueta Business Library collection:
Ever Closer Union: An Introduction to European Integration