It has been said that during the time of the Kalmar Union the true power around the Baltic Sea was not held by the King of the Union or local nobles, but by the Hanseatic League, or Hansa.
The Hansa was a confederation of powerful merchants founded in Lubeck, Germany that controlled all trade to and from the Baltic. Previous to the league’s rise to power Scandinavia controlled trade around the Baltic, and this lead to much competition between the league and the Kalmar Union.
The league held such power that it functioned almost as an independent country. Between 1361-1370 league cities waged war against Denmark, sacking Copenhagen and Helsingborg and ultimately forcing the kings of Denmark and Norway to grant the league 15% of their trade profits.
As the league began to lose power in the 16th century it opened up the Baltic and gave Denmark and Sweden greater significance in Northern European politics as control of Baltic trade fell back into the hands of Denmark-Norway and the quickly growing Swedish Empire.