The European Union is a unique economic and political union between 28 European countries that was created in the aftermath of the Second World War. The first steps were to foster economic cooperation: the idea being that countries that trade with one another become economically interdependent and so more likely to avoid conflict. The result was the European Economic Community , created in 1958, and initially increasing economic cooperation between six countries: Belgium, Germany, France, Italy, Luxembourg and the Netherlands. Since then, 22 other members joined and a huge single market has been created and continues to develop towards its full potential. After a long period of suspense and many difficulties, the Treaty of Lisbon came into force on 1 December 2009. It has changed legal and institutional framework of the European Union to some degree. Thus, the Treaty is of constitutional importance for the Union. It has changed the previous Treaties on which the Union was based. The new Treaty reflects the efforts of the Union to adapt to its new circumstances that occurred primarily due to a huge enlargement. What began as a purely economic union has evolved into an organization spanning policy areas, from climate, environment and health to external relations and security, justice and migration.