A kingdom in northwestern Europe, located strategically between the North Sea and the Baltic Sea. Denmark was one of the chief Viking centers and for centuries was one of the leading powers in northern Europe. At one time or another during the Middle Ages, Denmark ruled Norway, Sweden, Finland, Iceland and England. During the 17th-19th centuries, Danish power declined, and defeats by Sweden, Britain and Prussia forced it back to, roughly, its present boundaries. After 1815, Denmark adopted a policy of neutrality, which it maintained for 130 years. This policy was abandoned after World War II, during which the country was occupied by Germany. Denmark was a charter member of NATO and joined the Common Market in 1960. A rich country agriculturally, Denmark has undergone an industrial boom since 1945. A long tradition of democracy and social cooperation mark the country's political life.