In the Baltics, Sweden was still at war with Poland, and in 1629 the temporary Truce of Altmark was made. In Germany, Gustav II Adolf joined the Thirty Years' War between the Catholic emperor and the Protestant princes in 1630 after making landfall in Northern Germany. Sweden's aim in the war was to support the German Protestants and strengthen its own position. However, Gustav II Adolf died in the Battle of Lützen in 1632 and his daughter Christina succeeded him on the throne while still a minor. In practice, the realm was ruled by a caretaker government led by Axel Oxenstierna. In the Peace of Westphalia that had ended the Thirty Years' War in 1648, Sweden secured its position as a superpower. However, the Thirty Years' War and other conflicts in the superpower age drained Sweden's and Finland's resources badly. A significant part of the peasants had to serve in the army and the navy, and many of them died in service.