Swedish intervention in the Thirty Years' War began in July 1630 when troops under Gustavus Adolphus landed in Pomerania. Under his leadership, the Protestant cause, previously on the verge of defeat, won several major victories and changed the direction of the war.
Following the Edict of Restitution by Emperor Ferdinand II on the height of his and the Catholic League's military success in 1629, Protestantism in the Holy Roman Empire was seriously threatened. In July 1630, King Gustav II Adolf of Sweden landed in the Duchy of Pomerania to intervene in favor of the German Protestants. Although he was killed in battle at Lützen, southwest of Leipzig, the Swedish armies achieved several victories against their Catholic enemies. However, the decisive defeat at Nördlingen in 1634 threatened continuing Swedish participation in the war. In consequence, the Emperor made peace with most of his German opponents in the Peace of Prague – essentially revoking the Edict of Restitution – while France directly intervened against him to prevent the Habsburg dynasty from gaining too much power at its eastern border.