The Kalmar War lasted from 1611 to 1613.
Denmark had dominion over the strait between the Baltic Sea and the North Sea. Sweden sought an alternative trade route to avoid paying Denmark's Sound Toll through Northern Norway. Sweden sought control the land route through sparsely populated Lapland. In 1607, Charles IX of Sweden declared himself "King of the Lapps in Nordland" and began "collecting" taxes in Norwegian territory, even south of Tromsø.
Since Sound Dues to pass through the strait between the Baltic and the North Sea were Denmark's main source of income, Denmark did not want alternative trade routes established, particularly when established through Norwegian territory. Denmark protested.
King Charles IX of Sweden ignored King Christian IV of Denmark and Norway's protests. Finally, in 1611 in response to Sweden's claim of a traditionally Dano-Norwegian area in Northern Norway, Denmark invaded Sweden. A force of 6000 men laid siege on the city of Kalmar, ultimately taking it. Norwegian forces, although stationed on the border, were instructed not to enter Sweden.
On October 20th, King Charles IX of Sweden died and was succeeded by his son, Gustavus Adolphus. On ascending the throne, Gustavus Adolphus sued for peace, but Christian IV saw an opportunity for larger victories, and strengthened his armies in southern Sweden.
England and the Netherlands were also invested in the Baltic Sea trade, and pressured to curtail Denmark's power by ending the Kalmar War before a decisive victory could be attained. The Danes, while well-equipped and strong, had relied heavily on mercenary forces and Christian IV, low on funds, was finally amenable to persuasion in 1613. With England's intercession, the Treaty of Knäred was signed on January 20, 1613.
Denmark reached its victory, restoring Norwegian control of Sweden's land route through Lapland by incorporating Lapland into Norway (and thus under Danish rule). Further, Sweden had to pay a high ransom for two fortresses captured by Denmark. Sweden, however, achieved a major concession — the right of free trade through the Sound Strait, becoming exempt of the Sound toll (a right shared by Britain and Holland).
Although a side-note to the war, the Battle of Kringen, in which Scottish mercenary forces were defeated by Gudbrandsdal militiamen from Lesja, Dovre, Vaage (Vågå), Fron, Lom and Ringebu is a noted military event in Norway, celebrated to this day.