Sweden had been one of the most successful participants in the Thirty Years' War, securing significant victories under King Gustavus Adolphus and, after his death, under Count Axel Oxenstierna, the Lord High Chancellor of Sweden. However, the country's expansionist ambitions were constantly threatened by Denmark–Norway, which almost completely surrounded Sweden from the south (Blekinge, Scania, and Halland), the west (Bohuslän), and the north-west (Jämtland and Härjedalen). The Danish-imposed Sound Dues, a toll on ships passing through Danish waters into the Baltic Sea, also irritated the Swedes and contributed to tensions. By the spring of 1643, the Swedish Privy Council, confident in Sweden’s military strength, decided that territorial gains could be made at Denmark–Norway's expense. Count Oxenstierna crafted a plan for war, initiating a surprise multi-front attack on Denmark in May 1643.
Swedish Field Marshal Lennart Torstensson was tasked with the primary offensive. Starting from Moravia, his forces marched into Danish-controlled Holstein on December 12, 1643. By the end of January 1644, Torstensson had secured the Jutland Peninsula. In February, Swedish General Gustav Horn led an army of 11,000 men into the Danish provinces of Halland and Scania, occupying much of the territory except for the fortress town of Malmö.
Despite being caught off guard, King Christian IV of Denmark–Norway responded swiftly, relying on his fleet to protect the Danish home islands. On July 1, 1644, his forces narrowly won the Battle of Colberger Heide. However, a major defeat followed on October 13, 1644, at the Battle of Fehmarn, where the combined Swedish and Dutch fleets destroyed much of the Danish navy. Christian IV also hoped that Norway, led by his son-in-law Governor-General Hannibal Sehested, would relieve pressure on Danish territories by attacking Sweden from the north.
However, Norway was reluctant to participate in the war, as the Norwegian populace opposed attacking Sweden, fearing retaliation. The resulting campaign was dubbed the "Hannibal War" by critics. Despite resistance, Norwegian forces did engage Swedish troops along their shared border. An attack led by Jacob Ulfeld into Swedish Jämtland failed, with Swedish forces occupying Jämtland and advancing into the Norwegian Østerdal before being repelled.
Governor-General Sehested planned to attack Swedish Värmland with his army and another under Henrik Bjelke but was instead ordered to assist King Christian IV in defending Gothenburg. Upon arriving, Sehested saw the king join his fleet, where he heroically fought to prevent Torstensson's army from advancing onto the Danish islands, even after being wounded. Sehested also launched successful attacks on the Swedish city of Vänersborg, while Norwegian troops under George von Reichwein and Henrik Bjelke crossed the border into Swedish Värmland and Dalsland.
Denmark before 1645. @ Vesconte
By 1645, Christian IV's forces were exhausted, and Denmark–Norway had to accept the mediation of France and the United Provinces to negotiate peace. The Treaty of Brömsebro, signed on August 13, 1645, was a humiliating defeat for Denmark–Norway. The terms of the treaty required Denmark–Norway to cede the Norwegian provinces of Jämtland, Härjedalen, and Idre & Särna to Sweden, as well as the Danish islands of Gotland and Ösel in the Baltic Sea. The Swedish also took control of the Danish province of Halland for 30 years as a guarantee of the treaty. Additionally, Sweden was granted exemption from the Sound Dues, significantly diminishing Denmark's control over the Baltic trade. The Danish crown prince, Frederick II, was forced to resign from his posts in the Prince-Bishoprics of Verden and Bremen, which were subsequently occupied by Sweden and later became Swedish fiefs under the Holy Roman Empire in 1648.
This outcome dramatically shifted the balance of power in the Baltic region, as Sweden gained dominance over the Baltic Sea and unrestricted access to the North Sea, breaking Denmark–Norway’s encirclement. Following Denmark–Norway’s defeat, Swedish forces under Torstensson resumed operations in the Thirty Years' War, pursuing the Imperial army south into Bohemia. At the Battle of Jankau near Prague in 1645, Torstensson’s army won another decisive victory, allowing Swedish forces to occupy parts of Bohemia and threaten both Prague and Vienna.
The Treaty of Brömsebro fundamentally altered the power dynamics of Northern Europe, with Sweden emerging as a dominant force in the Baltic region, while Denmark–Norway began seeking opportunities to recover from their losses, foreshadowing future conflicts in the region.