During the seventh and eighth centuries, the Swedes were merchant seamen well known for their far-reaching trade. In the ninth century, Nordic Vikings raided and ravaged the European Continent as far as the black and Caspian Seas. During the 11th and 12th centuries, Sweden gradually became a unified Christian kingdom that later included Finland. Queen Margaret of Denmark united all the Nordic lands in the "Kalmar Union" in 1397. Continual tension within the countries and within the union gradually led to open conflict between the Swedes and the Danes in the 15th century. The union's final disintegration in the early 16th century brought on a long-lived rivalry between Norway and Denmark on one side and Sweden and Finland on the other

During and after World War I, in which Sweden remained neutral, the nation benefited from the worldwide demand for Swedish steel, ball bearings, wood pulp, and matches. Postwar prosperity provided the foundations for the social welfare policies characteristic of modern Sweden. Foreign policy concerns in the 1930s centered on Soviet and German developmentism, which stimulated abortive efforts at Nordic defense cooperation. Sweden followed a policy of armed neutrality during World War II and currently remains nonaligned. Sweden became a member of the European Union in 1995.