The Swedish-Danish War of 1658-60 broke out only six months after the end of the war of 1657-58. That war had been started by the Danes, who saw Swedish involvement in Poland-Lithuania as a chance to regain areas lost to Sweden in earlier wars, especially Torstensson’s War (1643-45). Frederik III of Denmark had been mistaken in his hope that Charles X of Sweden would be too distracted to react forcefully. After Swedish troops reached the edge of Copenhagen in February 1658, Frederik agreed to the Peace of Roskilde (8 March 1658) in which he surrendered Denmark’s provinces on the southern end of the Scandinavian peninsula, promised to provide Sweden with 2,000 soldiers and to pay the costs of the Swedish army at Copenhagen until it withdrew.
Having made peace with the Danes, Charles now found himself in a difficult position. He was still at war with Poland-Lithuanian and with Muscovy. His former ally Frederick William of Brandenburg had changed sides in return for recognition of his sovereignty over Ducal Prussia. He needed to keep his army intact but could not afford to do so in Sweden. After three years of campaigns in Poland-Lithuania most of the areas he had captured were back in Polish hands. An attack on Prussia would be dangerous, a campaign in Livonia likely to be expensive and unproductive.
Fortunately for Charles, a number of disputes over the terms of the Treaty of Roskilde now developed. The Danes attempted to provide the 2000 men they had promised, but 1,064 deserted. They attempted to reduce the amount of money they were paying to maintain the Swedish army. Finally, one clause of the treaty, excluding foreign fleets from the Baltic, was entirely ignored. In July 1658 Charles declared war on Denmark and launched a second attack on Copenhagen.
This time they found the Danish capital a much tougher nut to crack. Frederik III was determined to defend the city, which was protected by a combination of modern defences and water and was garrisoned by up to 10,000 men. A long siege would be needed to take the city. Worse was to come. The Danes had a defensive treaty with the Dutch, which was triggered by the Swedish attack. In November 1658 a Dutch fleet broke the naval blockade of Copenhagen, while Dutch troops joined the garrison.
Swedish control of Jutland was broken by an allied army 25,000 strong containing Austrian, Brandenburg and Polish contingents. The Swedes maintained themselves on the islands until the end of 1659, but were defeated at Nyborg (24 November 1659) and forced to evacuate the island of Fyn. The few Swedish conquests still remaining in Poland-Lithuania came under attack, although neither Elbing nor Marienburg fell.
Despite the military setbacks and the diplomatic pressure on Sweden, it was the death of Charles X, on 23 February 1660, that effectively ended the war. Peace came with the Treaty of Oliva (3 May 1660) that ended the Northern War of 1655-60 and the Treaty of Copenhagen (6 June 2007) that ended the Danish War. Denmark recovered Trondheim and the island of Bornholm, but lost her former provinces in the southern tip of the Scandinavian peninsula.