Eric XIV (Swedish: Erik XIV; 13 December 1533 – 26 February 1577) was King of Sweden from 1560 until he was deposed in 1568. Eric XIV was the eldest son of Gustav I (1496–1560) and Catherine of Saxe-Lauenburg (1513–1535). He was also ruler of Estonia, after its conquest by Sweden in 1561. While he has been regarded as intelligent and artistically skilled, as well as politically ambitious, early in his reign he showed signs of mental instability, a condition that eventually led to insanity. Some scholars claim that his illness began early during his reign, while others believe that it first manifested with the Sture Murders. Eric, having been deposed and imprisoned, was most likely murdered. Eric XIV was born at Tre Kronor castle, at 9 o'clock on the morning of 13 December 1533. Before the age of two, he lost his mother. In 1536, his father, Gustav Vasa, married Margaret Leijonhufvud (1516–1551), a Swedish noblewoman. He was crowned as Eric XIV, but was not necessarily the 14th king of Sweden named Eric. He and his brother Charles IX (1604–11) adopted regnal numbers according to Johannes Magnus's partly fictitious history of Sweden. There had, however, been at least six earlier Swedish kings with the name of Eric. In domestic politics, Eric's ambitions were strongly opposed by the Swedish nobility, including his half-brother, later John III of Sweden (1537–92). John was the Duke of Finland and was married to a Polish princess, which made him friendly with Poland. John pursued an expansionist policy in Livonia (now Estonia, Latvia, and Lithuania) which led to contention between the brothers. In 1563, John was seized and tried for high treason by Eric's order. Unlike his father, who had been satisfied with ruling an independent state, Eric tried to expand his influence in the Baltic region and in Estonia, beginning the process that resulted in Sweden becoming a great power in the 17th century. This expansionism resulted in a clash with his cousin, Frederick II of Denmark (1534–1588). Most of Eric XIV's reign was then dominated by the Livonian War and the Scandinavian Seven Years' War against Denmark (1563–1570), during which he successfully repelled most Danish attempts at conquest, but was unable to keep his own acquisitions.

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