The union was formed by Queen Margaret I (1353 - 1413) in the Swedish town of Kalmar, close to the Danish border, after Danish and Swedish troops in 1389 had defeated the Swedish king, Albert of Mecklenburg, and he subsequently failed to pay the tribute of 60,000 silver marks within three years after his release. King Albert, born in Germany, was ill-liked by the Swedish nobility and their rebellion had received help from the Danes, who intended the union to serve as a check on the growing power of the German Hanseatic League. Queen Margaret who was a daughter of the late Danish king Valdemar Atterdag and wife of the late Norwegian king Haakon VI, maneuvered to have her grand-nephew, Eric of Pomerania elected king over the three countries. Queen Margaret promised to protect the political influence and privileges of the nobility under the union, but king Eric wanted to strengthen the monarchy.
The Swedes where not happy with the Danes' frequent wars on Schleswig, Holstein, Mecklenburg and Pomerania which were a disturbance for Swedish exports to the European continent (notably iron). Furthermore, the centralizing of government to Denmark caused suspicions. The Swedish Privy Council wanted to retain a fair degree of self-government. The unity of the union eroded in the 1430s, even to the point of armed rebellion, leading to the expulsion of Danish forces from Sweden. Eric was deposed in 1438-1439 as the union king and succeeded by the childless Christopher of Bavaria. In the power vacuum that arose following Christopher’s death in 1448, Sweden elected Charles VIII king with the intent to reestablish the union under the Swedish Crown. Charles was elected king of Norway in the following year, but the counts of Holstein were more influential than the Swedes and the Norwegians together and made the Danish Privy Council appoint Christian I of Oldenburg as king, and were not about to surrender their claim to the union throne that easily. During the next seven decades struggle for power and the wars between Sweden and Denmark would dominate the union.
After the successful re-taking of Sweden by Christian II and the subsequent Stockholm bloodbath in 1520, the Swedes started yet another rebellion which ousted the Danish forces once again in 1521. While independence had been reclaimed the election of king Gustav Vasa on June 6, 1523 restored sovereignty for Sweden and finally dissolved the union.
The last structures remained until 1536 when Norway, in the aftermath of a Danish civil war, formally lost its independent status as a separate kingdom and was made a province of Denmark. This also led the former Norwegian possessions of Iceland, Greenland and the Faroe Islands, to come directly under the Danish crown. In 1814 Denmark was forced to cede Norway to Sweden and this would in the middle of the 19th century give rise to the Scandinavian movement which sought to reunite the countries of the Kalmar Union, except Finland, under one monarch.