Emergence as a great power
Sweden emerged as a great European power under Axel Oxenstierna and King Gustavus Adolphus. As a result of acquiring territories seized from Russia and the Polish–Lithuanian Commonwealth, as well as its involvement in the Thirty Years' War, Sweden found itself transformed into the leader of Protestantism.
During the Thirty Years' War, Sweden managed to conquer approximately half of the member states of the Holy Roman Empire. The fortunes of war would shift back and forth several times. After its defeat in the Battle of Nördlingen (1634), confidence in Sweden among the Swedish-controlled German states was damaged, and several of the provinces refused further Swedish military support, leaving Sweden with only a couple of northern German provinces. After France intervened on the same side as Sweden, fortunes shifted again. As the war continued, the civilian and military death toll grew, and when it was over, it had led to severe depopulation in the German states. Although exact population estimates do not exist, historians estimate that the population of the Holy Roman Empire fell by one-third as a result of the war.
Sweden founded overseas colonies, principally in the New World. New Sweden was founded in the valley of the Delaware River in 1638, and Sweden later laid claim to a number of Caribbean islands. A string of Swedish forts and trading posts was constructed along the coast of West Africa as well, but these were not designed for Swedish settlers.
Peace of Westphalia
At the conclusion of the Thirty Years' War, the Peace of Westphalia in 1648 granted Sweden territories as war reparations. Sweden demanded Silesia, Pomerania (which had been in its possession since the Treaty of Stettin (1630), and a war indemnity of 20,000,000 Riksdaler.
Through the efforts of Johan Oxenstierna and Johan Adler Salvius it obtained:
These German possessions were to be held as fiefs of the Holy Roman Empire. This allowed Sweden a vote in the Imperial Diet and enabled it to "direct" the Lower Saxon Circle alternately with Brandenburg. France and Sweden, moreover, became joint guarantors of the treaty with the Holy Roman Emperor and were entrusted with carrying out its provisions, as enacted by the executive congress of Nuremberg in 1650.
After the peaces of Brömsebro and Westphalia, Sweden was the third-largest area of control in Europe by land area, only surpassed by Russia and Spain. Sweden reached its largest territorial extent during this time under the rule of Charles X Gustav (1622–1660) after the Treaty of Roskilde in 1658.