Frederick IV (October 11, 1671 - October 12, 1730) king of Denmark and Norway from 1699. Son of Christian V.

For much of Frederik IV's reign Denmark was engaged in the Great Northern War (1700-1721) against Sweden. A first short-lived encounter 1700 ended with a Swedish invasion and threats from Europe's western naval powers. In 1709 Denmark again entered the war encouraged by the Swedish defeat at Poltava. Frederick IV commanded the Danish troops at the battle of Gadebusch in 1712. Although Denmark emerged on the victorious side, she failed to reconquer lost possessions in southern Sweden. The most important result was the destruction of the pro-Swedish duchy of Holstein-Gottorp re-establishing Denmark's domination in Schleswig-Holstein.

His most important domestic reform was the abolition in 1702 of the so-called vornedskab, a kind of serfdom which had fallen on the peasants of Zealand in the later Middle Ages. His efforts were largely in vain because of the introduction of adscription in 1733.

After the war, trade and culture flowered. The Danish theatre was created and the great dramatist Ludvig Holberg began his career. Also the colonisation of Greenland was started by the missionary Hans Egede. Politically this period was marked by the king's connection to the Reventlows, the Holsteiner relatives of his last queen, and by his growing suspicion toward the old nobility.

During Frederick's rule Copenhagen was struck by two disasters: the plague of 1711, and the great fire of October 1728 which destroyed most of the medieval capital. Although the king had been persuaded by Ole Rømer to introduce the Gregorian calendar in Denmark-Norway in 1700, the astonomer's observations and calculations were among the treasures lost to the fire.

Frederik IV, having taken twice visited Italy, had two pleasure palaces built in the Italian baroque style: Frederiksberg Palace and Fredensborg Palace, both considered monuments to the conclusion of the Great Northern War.

Frederick was deemed a man of responsibility and industry — often regarded as the most intelligent of Denmark's absolute monarchs. He seems to have mastered the art of remaining independent of his ministers. Lacking all interest in academic knowledge, he was nevertheless a patron of culture, especially in art and architecture. His main weaknesses were probably pleasure-seeking and womanising (he is the only Danish king known to have committed bigamy), which sometimes distracted him.

His mother was Charlotte, daughter of William VI, Landgrave of Hesse-Kassel. Without divorcing his first queen, Louise of Mecklenburg-Güstrow whom he had wed 5 December 1695, Frederick carried off the 19 year-old Countess Anne Sophie Reventlow from her home in Clausholm near Randers on 26 June 1712 and secretly wed her at Skanderborg. At that time he accorded her the title "Duchess of Schleswig" (derived from one of his own subsidiary titles). Three weeks after Queen Louise's death in Copenhagen on 4 April 1721, he married her again, this time declaring her queen (the only wife of an hereditary Danish king to bear that title who was not a princess by birth). Of the eight children born to him of these two wives, only two survived to adulthood, Christian VI and the spinster princess, Charlotte-Amalia, both from the first marriage.

Nonetheless, much of the king's life was spent in strife with kinsmen. Charles XII of Sweden and Frederick IV, Duke of Holstein-Gottorp were his first cousins and had waged war upon his father jointly. Initially defeated by the Swedes and forced to recognize the independence of Holstein-Gottorp, Frederick finally drove Duke Charles Frederick out of Schleswig in 1713, and avoided the revenge contemplated by the duke's mother-in-law, Catherine I of Russia.

The Reventlows took advantage of their kinship to the king to aggrandize. Within a year of conferring the crown matrimonial on Countess Reventlow, Frederick also recognized as dynastic the issue of the morganatic marriages of two of his kinsmen, Duke Philip Ernest of Schleswig-Holstein-Glucksburg (1673–1729) and Duke Christian Charles of Schleswig-Holstein-Plön-Norburg (1674-1706), to non-royal noblewomen. The other Schleswig-Holstein dukes of the House of Oldenburg perceived their interests to be injured, and Frederick found himself embroiled in their complicated lawsuits and petitions to the Holy Roman Emperor. Also offended by the countess's elevation were King Frederick's younger, unmarried siblings, Princess Sophia Hedwig (1677–1735) and Prince Charles (1680–1729, who withdrew from Copenhagen to their own rival court at the handsomely re-modelled Vemmetofte Cloister (later a haven for dowerless damsels of the nobility.

During King Frederick's last years he was afflicted with weak health and private sorrows that inclined him toward Pietism. That form of faith would rise to prevalence during the reign of his son. On his death in 1730, Frederick IV was interred in Roskilde Cathedral.

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