Frederick I (Swedish: Fredrik I; German: Friedrich von Hessen-Kassel; 28 April 1676 – 5 April 1751) was prince consort of Sweden from 1718 to 1720, and King of Sweden from 1720 until his death and (as Frederick I) also Landgrave of Hesse-Kassel from 1730. He was the son of Charles I, Landgrave of Hesse-Kassel, and Princess Maria Amalia of Courland. In 1692 the young prince made his Grand Tour to the Dutch Republic, in 1695 to the Italian Peninsula and later he studied in Geneva. After this he had a military career, leading the Hessian troops as Lieutenant General in the War of Spanish Succession on the side of the Dutch. He was defeated in 1703 in the Battle of Speyerbach, but participated the next year in the great victory in the Battle of Blenheim. In 1706 he was again defeated by the French in the Battle of Castiglione. Both in 1716 and 1718 he joined the campaign of Charles XII of Sweden against Norway, and was appointed Swedish Generalissimus. He married his second wife, Princess Ulrika Eleonora of Sweden, in 1715. He was then granted the title Prince of Sweden, with the style Royal Highness by the estates, and was prince consort there during Ulrika Eleonora's rule as queen regnant from 1718 until her abdication in 1720. He was the only Swedish prince consort there had been to date. Frederick I had much influence during the reign of his spouse. Frederick succeeded Ulrika Eleonora on the throne upon her abdication in his favor in 1720, elected by the Swedish Estates. The defeats suffered by Charles XII in the Great Northern War ended Sweden's position as a first-rank European power. Under Frederick, this had to be accepted. Sweden also had to cede Estonia, Ingria and Livonia to Russia in the Treaty of Nystad, in 1721.Frederick I was a very active and dynamic king at the beginning of his 31-year reign. But after the aristocracy had regained power during the wars with Russia, he became not so much powerless as uninterested in affairs of state. In 1723, he tried to strengthen royal authority, but after he failed, he never had much to do with politics. He did not even sign official documents; instead a stamp of his signature was used. He devoted most of his time to hunting and love affairs. His marriage to Queen Ulrika Eleonora was childless, but he had several children by his mistress, Hedvig Taube. Frederick became Landgrave of Hesse only in 1730, ten years after becoming King of Sweden. He immediately appointed his younger brother William governor of Hesse. As Landgrave, Frederick is generally not seen as a success. Indeed, he did concentrate more on Sweden, and due to his negotiated, compromise-like ascension to the throne there, he and his court had a very low income. The money for that very expensive court, then, since the 1730s came from wealthy Hesse, and this means that Frederick essentially behaved like an absentee landlord and drained Hessian resources to finance life in Sweden.

Read more: Wikipedia