Margaret I was Queen regnant of Denmark, Norway, and Sweden 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 queen consort of Norway from 1363 to 1380 and of Sweden from 1363 to 1364 by marriage to Haakon VI. Margaret was known as a wise, energetic and capable leader, who governed with "farsighted tact and caution," earning the nickname "Semiramis of the North". Also famous derisively as "King Breechless", one of several derogatory nicknames once thought to have been invented by her rival King Albert of Sweden, she was also known by her subjects as "Lady King", which became widely used in recognition of her capabilities. Knut Gjerset calls her "the first great ruling queen in European history."

The Kalmar Union was a personal union in Scandinavia, agreed at Kalmar in Sweden as designed by widowed Queen Margaret of Norway and Sweden. From 1397 to 1523, it joined under a single monarch the three kingdoms of Denmark, Sweden, and Norway, together with Norway's overseas colonies.