Margaret I (Danish: Margrete Valdemarsdatter; March 1353 – 28 October 1412) was ruler of Denmark, Norway, and Sweden (which included Finland) from the late 1380s until her death, and the founder of the Kalmar Union that joined the Scandinavian kingdoms together for over a century. She had been Norway's queen consort 1363–1380 and Sweden's 1363–1364, since then titled Queen. Margaret was known as a wise, energetic and capable leader, who governed with "farsighted tact and caution," earning the nickname "Semiramis of the North". She was derisively called "King Breechless", one of several derogatory nicknames invented by her rival Albert of Mecklenburg, but was also known by her subjects as "the Lady King", which became widely used in recognition of her capabilities. Knut Gjerset calls her "the first great ruling queen in European history."
The youngest daughter of King Valdemar IV of Denmark, Margaret was born at Søborg Castle. She was a practical, patient administrator and diplomat, albeit one of high aspirations and a strong will, who intended to unite Scandinavia forever into one single entity with the strength to resist and compete against the might of the Hanseatic League. With her husband, King Haakon VI of Norway, Margaret had a son, King Olaf II of Denmark, who died before her. She was ultimately succeeded by a grandnephew, Eric of Pomerania. Although Eric came of age in 1401, Margaret continued for the remaining 11 years of her life to be sole ruler in all but name. Her regency marked the beginning of a Dano-Norwegian union which was to last for more than four centuries.
Some Norwegian and Swedish historians have criticized Margaret for favouring Denmark and being too autocratic, though she is generally thought to have been highly regarded in Norway and respected in Denmark and Sweden. She was painted in a negative light in contemporary religious chronicles, as she had no qualms suppressing the Church to promote royal power. Margaret is known in Denmark as "Margrethe I" to distinguish her from the current queen, who chose to be known as Margrethe II in recognition of her predecessor.