The Second Northern War between Sweden and Denmark-Norway took a dramatic turn in 1657 when Denmark-Norway, seizing an opportunity while Sweden was preoccupied in Poland, launched an invasion as an act of revenge. However, the campaign quickly became disastrous for Denmark-Norway. Firstly, the Netherlands, Denmark's powerful ally, chose to remain neutral, since Denmark was the aggressor and Sweden the defender. Secondly, the unusually severe winter of 1657–1658 caused the Belts to freeze, allowing Swedish King Charles X Gustav to lead his army across the ice, an unprecedented feat, to launch a surprise invasion of Zealand.
This invasion forced Denmark-Norway to capitulate, and in the Treaty of Roskilde (1658), Denmark-Norway suffered a crushing defeat, ceding significant territories to Sweden. These included Skåne, Halland, Blekinge, and Bornholm from Eastern Denmark, as well as the counties of Båhuslen in Norway and Trøndelag. Additionally, Holstein-Gottorp came under Swedish influence, providing a strategic foothold for future invasions.
However, the war did not end there. Merely three months after the Treaty of Roskilde, Charles X Gustav, emboldened by his success, decided to eliminate Denmark entirely and unite Scandinavia under his rule. The Swedish army once again advanced towards Copenhagen. This time, King Frederick III of Denmark resolved to fight back. He inspired the citizens by declaring he would "die in his nest," rallying them to defend the city against the Swedish siege.
Denmark before 1658. @ Vesconte
Sweden’s unprovoked aggression triggered Denmark-Norway's alliance with the Netherlands. A formidable Dutch fleet arrived in Copenhagen with critical supplies and reinforcements, enabling the city to withstand the Swedish assault. At the same time, Brandenburg-Prussia, the Polish–Lithuanian Commonwealth, and the Habsburg monarchy joined the conflict against Sweden, and the fighting continued through 1659.
In early 1660, as Charles X Gustav was preparing an invasion of Norway, he unexpectedly died from illness. Following his death, the warring parties sought peace, resulting in the Treaty of Copenhagen in 1660. This treaty restored Trøndelag to Norway and Bornholm to Denmark, but Sweden retained Bahusia and Terra Scania. The settlement, accepted by other European powers like the Netherlands, established the borders between Norway, Denmark, and Sweden, which largely remain unchanged today. The outcome of these conflicts marked Sweden’s emergence as the dominant power in Scandinavia, surpassing Denmark.