The unfolding of events made war against Poland inevitable. It was first fought in Livonia, where the Swedes suffered a defeat at Kirkholm in 1605; later, however, in Russia, at that time in full internal disintegration, and in the border districts between Finland and Russia. A considerable worsening of the situation in Sweden occurred in 1611, when war also broke out with Denmark. In the same year Charles IX died; his son Gustav II Adolfo (1611-1632) ascended the throne of Sweden, not yet seventeen years old.

The young king’s position was very precarious, both for domestic and foreign politics. On ascending the throne, he was forced to grant the council of state and parliament extensive political rights, and important privileges, both economic and of other nature, to the nobility, in particular an extension of the exemption from taxes. But Gustavo Adolfo was able to save, without great friction, the interests of the Swedish crown.

During his reign the Swedish aristocracy became a class of state officials, at the same time the life and administration of the state was reorganized. This development was beneficial to both the nobility and the state. The parliament, initially without fixed rules, was organized with a law of 1617. Fixed courts were established for high jurisdiction; created or reorganized several central administrative offices. The heads of the various offices were taken from among the members of the council of state; the council of state was thus transformed by a junta of the landowner aristocracy, which met from time to time into a permanent senate, composed of expert officials, based in Stockholm. The king’s first coadjutor was the head of the kingdom chancellery, the kingdom chancellor, Axel Oxenstjerna. The local administration was also reorganized. As for public finances, the king tried to replace as much as possible the old taxes of the peasants, in kind, with new taxes in money, partly indirect. Gustavo Adolfo’s government was also important for economic life: new cities were created, the mining industry gave more income.

Gustavo Adolfo in foreign policy was able to find the way that from an originally unfavorable situation led to the maximum political power of Sweden. The Danish war ended in 1613 with the Peace of Knäred, which cost Sweden great economic sacrifices, but no territorial cession. The war in Russia continued until 1617 and ended, after several projects had failed: Sweden received East Karelia and Ingria. Of greater importance was the Polish war, in which Gustavo Adolfo personally commanded his troops by developing his characteristic and modernly conceived tactic which was to make him one of the greatest captains of all time. After Riga in 1621 and all of Livonia in 1625 were conquered by the Swedes, the battlefield between 1626-28 moved to East Prussia, where several fortresses fell into the hands of the Swedes and Danzig was surrounded. The large proceeds from the shipping duties on the Vistula were donated to the Swedish treasury. Finally in 1629 an armistice was concluded at Altmark for 6 years: during this period Sweden had to retain Livonia with Riga and, in Prussia, the cities of Elbing, Braunsberg, Memel and Pillau and in addition the collection of duties on the Vistula.

The Swedish-Polish War had been a phase of the great conflict – between Catholicism and Protestantism. When Gustavo Adolfo left the theater of the Polish war, he had already decided to participate in the Thirty Years’ War and to go to Germany. He thought that only there could Swedish Protestantism and its crown be definitively insured. In 1630 he landed in Usedom, occupied Pomerania; and in 1631 he concluded an alliance with France, with which he was promised substantial subsidies. With the great victory of 1631 near Leipzig (Breitenfeld) he became master of the situation; Sweden had suddenly become a great power. Gustavo Adolfo stroked great projects during his triumphal campaign towards southern Germany, most notably the creation of a ‘ North German-Protestant alliance under the leadership of Sweden. His death in Lützen in 1632 suddenly put an end to these projects.

His minor daughter Cristina (1632-54) succeeded him on the Swedish throne; his coadjutor Axel Oxenstjerna first stayed in Germany, where he tried to compensate Sweden for its participation in the war with territorial acquisitions in northern Germany, thus limiting the aims of Swedish foreign policy. Several generals – Gustavo Horn, Giovanni Banér, Lennart Torstensson – commanded the Swedish armies operating in Germany with varying success. The defeat near Nördlingen in 1634, given the political situation of the time, was a real catastrophe for Sweden, which in the renewed armistice of Stuhmsdorf with Poland had to renounce the Prussian cities and thus the large income of the Vistula. The prestige of Swedish arms was restored with the victories of G. Banér at Dittstock in 1636, at Chemnitz in 1639; when Denmark assumed a threatening attitude in 1643, Lennart Torstensson headed from the German theater of war to Jütland; the Danes were defeated and in the Brömsebro peace of 1645 they had to give up Jämtland and Härjedalen (in northern Sweden), Gotland, Ösel and Halland. In the 1648 Peace of Westphalia, Sweden finally gained a large part of Pomerania with Szczecin, Wismar and, on the North Sea, the bishops of Bremen and Verden.

An aristocratic council – a junta of the council of state – under the leadership of Axel Oxenstjerna had ruled the government during the minority of Queen Christina (until 1644), according to a regulation of 1634. This regulation, designed only for the minority of the queen, it acquired great importance for the further constitutional history of Sweden, since by the aristocratic party it was considered a fundamental law, binding even for an adult king. In the meantime, the government college conducted an internal policy on the same basis as Gustavo Adolfo; in order to finance the war, the government sold public assets which were managed by settlers who paid a fixed income. The buyers belonged for the most part to the aristocracy, whose properties were thus enlarged and partly transformed into large estates. This development reached its climax when, after the conclusion of the peace, the officers returning to their homeland received from the crown of the farms as compensation for the service rendered. Since it was not possible to distinguish precisely the tenants of the crown goods from the taxpaying peasants, who owned their farms (allodial goods), it often happened that even the incomes that the crown obtained from the independent peasants passed both by purchase and by donation into the hands of the ‘aristocracy. And so the condition of freedom of the Swedish peasant was threatened by a grave danger. This fact and the difficult financial situation after the war led the lower classes, in the parliament of 1650, to demand from the queen a reduction of the assets of the crown, which had passed into the hands of the nobility. The queen took advantage of such friction between the different classes to obtain the election of her cousin Carl Gustav of the house of the Wittelsbachs, as his successor to the throne. Having succeeded in its intent, it dropped the reduction project. When Cristina, who had already had Catholic sympathies early, had abdicated in 1654 and Charles X Gustavo (1654-60) was ascended to the throne, a moderate reduction of the assets of the crown was decreed in 1655.