The Kalmar War (1611–1613) was a conflict between Denmark-Norway and Sweden, marking a critical moment in their struggle for dominance in the Baltic Sea. At the time, Denmark-Norway controlled key trade routes through the Sound Strait, which Sweden sought to bypass by expanding through Lapland. Tensions over control of these routes and territory in northern Norway led to war when Denmark-Norway, under King Christian IV, declared war on Sweden after Swedish King Charles IX claimed land in the Norwegian north.
The war initially saw Denmark-Norway gaining the upper hand, capturing the strategic city of Kalmar and several other fortresses, including Älvsborg and Gullberg, cutting off Sweden's access to the North Sea. However, the war became a drawn-out conflict, with Swedish King Gustavus Adolphus (who succeeded Charles IX in 1611) leading raids and resisting Danish advances despite several setbacks, including almost drowning during the Battle of Vittsjö.
The war concluded with the Treaty of Knäred in 1613. Although Denmark-Norway managed to reaffirm its control over Lapland and exacted a hefty ransom for captured fortresses, Sweden secured the right to free trade through the Sound, a significant economic concession that allowed it to bypass Denmark's tolls. This treaty also demonstrated the exhaustion of both sides, with Denmark-Norway relying on mercenaries and limited by financial strain, while Sweden's military potential, under Gustavus Adolphus, was rising.
While Denmark-Norway defended its dominium maris baltici (Baltic dominance) in this war, Sweden would soon recover and emerge as a significant Baltic power under Gustavus Adolphus, setting the stage for future conflicts, such as the Torstenson War (1643–1645), where Sweden would ultimately gain dominance in the Baltic region. The rivalry between the two nations continued to influence their relations well into the 19th and early 20th centuries.