Valdemar Atterdag (c. 1320 – October 24, 1375) was a King of Denmark (1340 - 1375).

He was the youngest son of Christopher II and spent most of his childhood and youth in German exile at the court of Louis IV, Holy Roman Emperor after the defeats of his father. Here he acted as a pretender waiting for a come-back.

He was elected king in 1340, following the assassination of Count Gerhard III, although his territory at this time was limited to the northernmost quarter of Jutland. King Valdemar is a pivotal figure in Danish history; he gradually reacquired the rest of Jutland and Zealand, and finally reunited Denmark by capturing Scania in 1360. His methods were a mixture of heavy taxations, loans from Northern German princes and a regular neutralisation of the minor mortgagees. In 1346 he sold Estonia, which had in reality already become a German colony, in order to get money for the redemption of the mortgaged Danish areas. He was the first Danish king who became the master of Copenhagen which was normally possessed by the bishop of Roskilde.

His policy, which in the start seems to have been generally accepted by the Danes, was soon met with bitter opposition by the magnates of Jutland and two times in the 1350s they stirred up regular rebellions that were crushed by the king. He maintained a very strong royal power based upon his military forces and the loyal part of the magnates that made the fundament of the Danish rulers until about 1440. Many foreigners were appointed as supreme officials and councillors. The most important of them was the German-Slavic nobleman Henning Podebusk who was drost (almost equal to prime minister) around 1365-1388.

King Valdemar did not only reunite Denmark but also successfully revived her role as a local great power. He intervened in both Northern Germany and Sweden and fought against the Hanseatic League by the conquest of the Hansa-dominated island of Gotland in 1361. However, the resulting conflict with the Hanseatic League ended in a Danish defeat, and in 1370 he was forced to submit to the Treaty of Stralsund which meant some economic and formal cessions to the Hansa. This defeat did not prevent him from trying to regain the lost South Jutland, an attempt which was about to succeed at the time of his sudden death.

Valdemar IV is often regarded as one of the most important of all Danish medieval kings. The sources leave the impression of an intelligent, cynic, reckless and clever Machiavellian ruler with a talent for both politics and economy. He was succeeded by his grandson Oluf II of Denmark, the offspring of his daughter Margaret and Haakon VI of Norway, son to Magnus II of Sweden.

His nickname "Atterdag" is usually interpreted as "day again" (its literal meaning in Danish), indicating that he brought new hope to the realm after a dark period of bad kingship. The epithet has also been suggested as a misinterpretation of the Middle Low German phrase "ter tage" ("these days"), which can best be interpreted as "what times we live in!"

Many myths and sagas have been made about this king; for instance he is a local version of "the wild huntsman". A famous saga about his mistress "Tove", who was killed on the initiative of his queen, has been the inspiration of many romantic poets but in fact this saga originally seems to be connected with his ancestor Valdemar I of Denmark.

He is buried in the church at Sorø Academy in Sorø, Denmark.

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