The Kalmar War was one of a series of wars between Sweden and Denmark. It was caused by rivalry between the two powers in the Baltic and by Swedish efforts to gain control of Finnmark, the area to the north of Lapland (it should be remembered that Norway and Denmark were united at this period, giving Denmark a presence in the arctic north.
Christian IV of Denmark had been in favour of a war with Sweden for some time. He had been unable to convince the Danish Council of his case in the previous decade, but in 1611 convinced them to support him by threatening to declare war in his role as duke of Holstein.
Christian commanded a small professional army and a powerful navy. The Swedes were distracted by events to their east, where Muscovy was suffering her “time of troubles” (1604-1613), a period of political chaos, and had been involved in a war with Poland since 1600. In the 1611 campaign Christian was able to capture the city of Kalmar, on the south east coast of Sweden, with an army that contained on 4,580 infantry and 645-700 cavalry. The same campaign saw the death of Charles IX of Sweden. He was replaced by the young Gustavus Adolphus.
The campaign of 1612 was dominated by inconclusive border warfare. The Danish navy had control of the sea, allowing Christian to capture Öland, and in June 1612 the Danes captured Älvsborg, on the west coast of Sweden.
The war came to an end in January 1613 at the Peace of Knarod, which saw Finnmark go to Denmark and Älvsborg ransomed back to Sweden (Finnmark is now the northernmost part of Norway). Despite his victory in the war, Christian recognised the problems with his army, and in the following years made an attempt to strengthen it.