sold to the United Kingdom in 1850. History Origins of the Union The three kingdoms Denmark, Norway and Sweden united in the Kalmar Union in 1397. Sweden broke out of this union and re-entered it several times, until 1521, when Sweden finally left the Union, leaving Denmark–Norway (including overseas possessions in the North Atlantic and the island of Saaremaa in modern Estonia). Norway also wanted to leave the union in the 1530s, but was unable to do so due to Denmark's superior military might. In 1537, Denmark invaded Norway, and annexed Norway under the Oldenburg monarch Christian III. Northern Seven Years' War The outbreak of the Northern Seven Years' War in 1563 is mainly attributed to Denmark's displeasure over the dismantling of the Kalmar Union in the 1520s. When the Danish-Norwegian king Christian III included the traditionally Swedish insignia of three crowns into his own coat of arms, the Swedes interpreted this as a Danish claim over Sweden. In response, Erik XIV of Sweden (reigned 1560–1568) added the insignia of Norway and Denmark to his own coat of arms. After Swedish king Erik introduced obstacles in an attempt to hinder trade with Russia, Lübeck and the Polish–Lithuanian Commonwealth joined Denmark–Norway in a war alliance. Denmark–Norway then carried out some naval attacks on Sweden, which effectively started the war. After seven years of fighting, the conflict concluded in 1570 with a status quo ante bellum. Kalmar War Because of Denmark–Norway's dominion over the Baltic Sea (dominium maris baltici) and the North Sea, Sweden had the intention of avoiding paying Denmark's Sound Toll. Swedish king Charles IX's way of accomplishing this was to try to set up a new trade route through Lapland and northern Norway. In 1607 Charles IX declared himself "King of the Lapps in Nordland", and started collecting taxes in Norwegian territory. Denmark-Norway and King Christian IV protested against the Swedish actions, as they had no intentions of letting another independent trade route open, Christian IV also had an intent of forcing Sweden to rejoin its union with Denmark-Norway. In 1611 Denmark-Norway finally invaded Sweden with 6000 men and took the city of Kalmar. On 20 January 1613, the Treaty of Knäred was signed in which Norway's land route from Sweden was regained by incorporating Lapland into Norway, and Swedish payment of the Älvsborg Ransom for two fortresses which Denmark-Norway had taken in the war. However, Sweden achieved an