FREDERICK III. (1609-167o), king of Denmark and Nor way, son of Christian IV. and Anne Catherine of Brandenburg, was born on March 18, 1609 at Hadersleben. While still a lad he became successively bishop of Bremen, bishop of Verden and coadjutor of Halberstadt, while at the age of 18 he was the chief commandant of the fortress of Stade. In 1643 he married Sophia Amelia of Brunswick-Luneburg. During the disastrous Swedish War of 1643-45 Frederick was appointed generalissimo of the duchies by his father ; he quarrelled with the Earl-Marshal Anders Bille, who commanded the Danish forces, and the Danish nobility began to regard him with extreme distrust. The death of his elder brother Christian in June 1647 opened to him the prospect of the succession, but the question was still unsettled when Christian IV. died on Feb. 28, 1648. On July 6 Frederick III. was acknow ledged king, after he had signed a Haand f aestning or charter, by which the already diminished royal prerogative was still further curtailed.
Frederick lacked the brilliant qualities of his impulsive, jovial father, but he possessed compensating virtues of moderation, sobriety and self-control. He rightly regarded the accession of Charles X. of Sweden (June 6, 1654) as a source of danger to Denmark. Charles's invasion of Poland (July 1654) came as a distinct relief to the Danes, who had feared an attack on them selves, but even the Polish War was full of latent peril to Den mark. Frederick resolved on a rupture with Sweden at the first convenient opportunity. The Rigsdag of 1657 granted subsidies for mobilization and other military expenses; on April 23 Fred erick received the assent of the majority of the Rigsraad to an attack on Sweden's German provinces; in the beginning of May the still pending negotiations with Sweden were broken off, and on June I Frederick signed the manifesto justifying a war which was never formally declared. The Swedish king traversed all the plans of his enemies by his passage of the frozen Belts, in Jan uary and February 1658 (see CHARLES X. of Sweden). Frederick III. at once sued for peace; and, persuaded by the English and French ministers, Charles finally agreed to be content with muti lating instead of annihilating the Danish monarchy (treaties of Taastrup, Feb. 18, and of Roskilde, Feb. 26, 1658). The con clusion of peace was followed by a remarkable episode. Charles X. was the Danish king's guest for three days (March 3-5) at the castle' of Fredriksborg, and friendship seemed to be estab lished. But on July 17, without any reasonable cause, without a declaration of war, in defiance of all international equity, Charles again attacked Denmark.
The main Swedish army landed at Korsor in Zealand. None had anticipated the sudden and brutal attack, and the Danish capital was inadequately fortified and garrisoned. The Danes had only three days' warning of the approaching danger; and the vast and dilapidated line of defence had at first but 2,000 regular de' fenders. But Government and people displayed exemplary energy, under the constant supervision of the king, the queen and burgo master Hans Nansen. Charles X. began a regular siege, which he abandoned when, on Oct. 29, an auxiliary Dutch fleet, after rein forcing and reprovisioning the garrison, defeated, in conjunction with the Danish fleet, the Swedish navy of 44 liners in the Sound. The traditional loyalty of the Danish middle classes was trans f ormed into a boundless enthusiasm for the king personally, and for a brief period Frederick found himself the most popular man in his kingdom. He used his popularity to convert an elective into an absolute monarchy by the Revolution of 166o (see DENMARK : History). Frederick III. died on Feb. 6, 167o at the castle of Copenhagen.
See R. N. Bain, "Scandinavia," in Cambridge Modern History (1905) ; Knud Fabricius, Kongeloven (192o).