Scandinavian Peninsula was inhabited by two tribes of the Germanic peoples: the Suiones, or Swedes, in northern Svealand; and the Gothones, or Goths, in southern Gothia. These tribes, although united in religious beliefs, were generally at war with each other. Before the 10th century, details of Swedish history are obscure. In the first half of the 9th century Frankish missionaries began teaching Christianity, which slowly became established in the country. Olaf Skötkonung was the first Swedish king to become a Christian. From about ad 800, Swedish Vikings established colonies in other countries, especially Russia and Eastern Europe, and established trade routes. During the reign of Eric IX, from 1150 to 1160, Swedish power was strengthened. Eric invaded Finland and forced Christianity on those he conquered; during the subsequent two centuries Finland was completely subjugated by the Swedes.

The thick ice cap that covered Sweden during the last glacial period began to recede in the southern region about 14,000 years ago. About 12,000 years later the earliest hunters in the region began following migratory paths behind the retreating ice field. The stratified clay deposits that were left annually by the melting ice have been studied systematically by Swedish geologists, who have developed a dependable system of geochronology that verifies the dates of the thaw. The first traces of human life in Sweden, dating from about 9000 BC, were found at Segebro outside Malmö in the extreme southern reaches of Sweden. Finds from the peat at Ageröd in Skåne dated to 6500 BC reveal a typical food-gathering culture with tools of flint and primitive hunting and fishing equipment, such as the bow and arrow and the fishing spear.

16th century Sweden entered a period of expansion. The Reval district of Estonia put itself voluntarily under Swedish protection in 1561, and as a result of the Livonia War of 1557 to 1582, Sweden acquired all of Estonia from Poland, including the district of Narva. Gradually the kingdom became a power in the Baltic area, and its expansionist policies were furthered by Gustav II Adolph, considered the greatest Swedish king, who succeeded to the throne in 1611. At the beginning of his reign, Sweden was at war with Russia, and in 1617 Gustav ended the conflict with a treaty by which Sweden obtained eastern Karelia and Ingria. A war with Poland (1621-1629) gave Sweden all Livonia, which was, however, not formally renounced by Poland until 1660. In 1630 Gustav, as the champion of Protestantism, entered the Thirty Years’ War. The king died in 1632, but his policies were continued and brilliantly fulfilled by his chancellor, Count Axel Oxenstierna, who directed the Swedish government during the minority of the monarch’s daughter, Christina.