Gustavus's excellent education, personal endowments, and early experience in affairs of state prepared him for his crucial role in Sweden and Europe. With the help of his great chancellor, Axel Oxenstierna, he insured internal stability by granting concessions to the turbulent nobility, and he terminated (1613) the Kalmar War with Denmark by buying off the Danes. This enabled him to undertake a successful campaign against Russia, which was forced to cede (1617) Ingermanland.
Gustavus at first stayed out of the Thirty Years War, which had begun in 1618. However, his resumption (1621) of the intermittent warfare between the Swedish and Polish branches of the house of Vasa led to his entry into that vast conflict. His primary objects in invading Poland were to consolidate Swedish hegemony over the Baltic by acquiring Polish Livonia and to reduce the threat posed by the Catholic Sigismund III of Poland to Swedish Protestantism.
The victories of the armies of Holy Roman Emperor Ferdinand II in the Thirty Years War soon caused the king to draw closer to the German Protestant princes. In 1628 he promised his aid to Christian IV of Denmark in the defense of Stralsund. In 1629, through the mediation of Cardinal Richelieu of France, he obtained the truce of Altmark with Poland, gaining a large part of Livonia and several good Baltic ports; a secret treaty with France promised a French subsidy if Gustavus entered Germany.
For the Protestant cause and also to gain control of the S Baltic coast, the king landed in Pomerania with 13,000 troops in 1630; these were soon augmented until 40,000 were at his disposal. Gustavus's invasion of Mecklenburg failed when the Mecklenburgers refused to heed his appeal to rise against the chief imperial general, Wallenstein, who was their new ruler. Early in 1631 the Franco-Swedish treaty was openly ratified at Bärwalde, and after the fall of Magdeburg, Saxony and Brandenburg accepted the king's conditions for an alliance with Sweden.
The spectacular sweep of the Swedish army through Germany then began. In Sept., 1631, Gustavus defeated the new imperial commander, Tilly, at Breitenfeld near Leipzig in the first Protestant victory of the war. He then marched west, reaching Mainz by Christmas, while the Saxon army moved into Bohemia. Resuming his campaign early in 1632, Gustavus returned east, defeated (April) the imperial troops at the crossing of the Lech (where Tilly was mortally wounded), and entered Bavaria. Wallenstein, reinstated as commander by the emperor, speedily put a large army into the field and forced the king to fall back to Nuremberg.
Wallenstein set up his camp at nearby, and the two armies remained facing each other for more than two months (July–Sept.) without doing battle. Finally Gustavus attacked Wallenstein's camp, but he failed and retired toward Würzburg, leaving a strong garrison at Nuremberg. Wallenstein then invaded unprotected Saxony, causing Gustavus to hasten north. At Lützen the two armies met on Nov. 16. The Swedes won the battle, but Gustavus was killed. Oxenstierna continued to direct Swedish policy under Gustavus's daughter, Queen Christina, while eventually Baner, and later Torstensson, took the king's place in the field.