Christian IV,1577–1648, king of Denmark and Norway (1588–1648), son and successor of Frederick II. After assuming (1596) personal rule from a regency, he concentrated on building the navy, industry, and commerce. He rebuilt Oslo and renamed it Christiania. Aroused when Charles IX of Sweden asserted authority over Lapland, he made war on Sweden (the so-called Kalmar War, 1611–13) and largely dictated the peace. In the Thirty Years War, urged on by England, France, and the Netherlands, he invaded (1625) Germany to defend Protestantism. Defeated (1626) by Tilly at Lutter, he was driven back in 1627. Schleswig, Holstein, and Jutland were overrun and plundered; Stralsund was besieged by the imperial troops under Wallenstein. Christian, with the help of Gustavus II of Sweden, raised the siege of Stralsund, but in 1629 he signed with Holy Roman Emperor Ferdinand II a separate peace that was lenient to Denmark. His anti-Swedish policy brought on a war with Sweden (1643–45) in which Christian lost the Norwegian provinces of Jamtland and Harjedalen. His son Frederick III succeeded him.
