No Swedish king has done so much for education as Gustavus Adolphus. To the University of Upsala he donated 300 of his hereditary estates, founding its library, improving its courses, banishing misrule, and appointing his old teacher, John Skytte, its chancellor. He created the German University of Dorpat in Esthonia, in 1632; later for some time moved to Pernau. Colleges were established in the larger towns. The king was, through his thorough studies of Swedish laws and conditions, in a position to take an active part in the reforms which he promulgated, never resting long in one place, but travelling from one point to another, where his presence was most necessary; shaping plans and reforms by his own judgment, to have them indorsed by the next Riksdag, and then enforcing them himself. Especially the army passed through an evolution, thanks to new methods, devised by the king, who was to win his victories through the introduction of improved tactics and divisions, by means of which the troops were easier to move and the co-operation between the various weapons increased.
In 1618 the “Thirty Years’ War” began. The dethroned Frederic of the Palatinate turned, among others, to Gustavus Adolphus for support, which the latter was not able to give in a direct way. But he promised to attack Poland as soon as the armistice was at an end, thereby making it impossible for Sigismund to support Emperor Ferdinand with troops. In 1621, Gustavus Adolphus commenced operations against Poland, taking the command himself. Riga and Mitau were captured, the former important commercial centre regaining its privileges, but sending representatives to the Swedish Riksdag and accepting a Swedish governor. After having conquered Livonia, Gustavus Adolphus entered Courland the following year, when an armistice was agreed to. Gustavus followed the events in Germany with increasing interest, forming the plan of an alliance between the Protestant powers. Learning that the emperor was willing to support Sigismund, Gustavus Adolphus offered to invade Silesia. But as Christian IV. of Denmark was anxious to lead the Protestant forces, Gustavus Adolphus quietly withdrew, resuming action against Poland. After a victory at Wallhof, he entered Polish Prussia, where he was dangerously wounded at Dirschau. The Poles were reinforced by imperial troops, but suffered a defeat at Gurzo; the Swedish general, Herman Wrangel, winning the day. When the considerable reinforcements of 10,000 men joined the Poles, the Swedes receded in good order. A smaller conflict occurred at Stuhm, famous because Gustavus Adolphus was twice in danger of his life during the struggle, which otherwise was of no importance. An imperial trooper caught him by the belt and tried to drag the king with him. According to the report of Axel Oxenstierna, the king loosened the belt and let it go. In so doing, he also lost his hat, which was carried to Vienna and preserved as a token of the “great victory.” Another trooper, shortly afterward, caught the king by the arm, aiming at the head with his sword. In the critical moment, Eric Soop, the colonel of a Swedish cavalry regiment, appeared, killing the trooper with a pistol-shot. Gustavus Adolphus referred to this struggle as the “hottest bath” that he was ever in.