The Viking Age was characterised by a significant expansion of activity, in Sweden’s case largely toward the east. Many Viking expeditions set off from Sweden to both plunder and trade along the Baltic coast and the rivers that stretched deep into present-day Russia. The Vikings traveled as far as the Black and Caspian Seas, where they developed trading links with the Byzantine Empire and the Arab kingdoms. Christianity first reached Sweden with a mission led by Ansgar, who visited in the 9th century, but the country was not converted to Christianity until the 11th century.

The various provinces of Sweden were absorbed around 1000 AD into a single unit, but the crown began to gain significant influence only during the late 13th century. In 1280 King Magnus Ladulås (1275–90) issued a statute authorising the establishment of a nobility and the organisation of society on the feudal model.

Trade grew during the 14th century, especially with the German towns grouped under the leadership of Lübeck. By the mid-16th century, this group, known as the Hanseatic League, dominated Swedish trade, and many towns were founded as a result of lively commercial activity. However, the Black Death, which reached Sweden in 1350, led to a long period of economic and population decline.

In 1389, the crowns of Denmark, Norway and Sweden were united under the rule of the Danish Queen Margareta. In 1397, the Kalmar Union was formed, with the three Scandinavian countries under a single monarch. However, the union (1397–1523) was scarred by internal conflicts that culminated in the ‘Stockholm Bloodbath’ in 1520, when 80 Swedish nobles were executed at the instigation of the Danish union king, Kristian II. The act provoked a rebellion, which in 1521 led to the deposition of Kristian II and the seizure of power by a Swedish nobleman, Gustav Vasa, who was elected king of Sweden in 1523.

The foundations of the Swedish state were laid during the reign of Gustav Vasa. The church was nationalised, its estates confiscated by the crown, and the Protestant Reformation was introduced. Power was concentrated in the hands of the king and hereditary monarchy came into force in 1544.

Since the dissolution of the Kalmar Union, Swedish foreign policy had been aimed at gaining dominion over the Baltic Sea, leading to repeated wars with Denmark from the 1560s onward. After Sweden intervened in 1630 with great success in the Thirty Years’ War on the side of the German Protestants, and Gustav II Adolf became one of Europe’s most powerful monarchs, Sweden defeated Denmark in the two wars of 1643–45 and 1657–58. Finland, provinces in northern Germany and the present-day Baltic republics also belonged to Sweden, and after the Peace of Westphalia in 1648 and the Peace of Roskilde with Denmark in 1658, Sweden was a great power in northern Europe. The country even founded a short-lived colony in what is now Delaware in North America. However, Sweden had a largely agrarian economy and lacked the resources to maintain its position as a great power in the long run.

After its defeat in the Great Northern War (1700–21) against the combined forces of Denmark, Poland and Russia, Sweden lost most of its provinces on the other side of the Baltic Sea and was reduced essentially to the same frontiers as present-day Sweden and Finland. During the Napoleonic Wars, Sweden surrendered Finland to Russia. As compensation, the French marshal Jean Baptiste Bernadotte, who had been elected heir to the Swedish throne in 1810, succeeded in obtaining Norway, which was forced into a union with Sweden in 1814. This union was peacefully dissolved in 1905 after many internal disputes.