War with Denmark–Norway
Labiau involved an essential modification of Charles's Baltic policy; but the alliance with the elector of Brandenburg had now become indispensable for him on almost any terms. The difficulties of Charles X in Poland are believed to have caused him to receive the tidings of the Danish-Norwegian declaration of war on 1 June 1657 with extreme satisfaction. He had learnt from Torstensson that Denmark was most vulnerable if attacked from the south, and he attacked Denmark with a velocity which paralysed resistance. At the end of June 1657, at the head of 8,000 seasoned veterans, he broke up from Bromberg (Bydgoszcz) south of Pomerania and reached the borders of Holstein on 18 July. The Danish army dispersed and the Swedes recovered the duchy of Bremen. In the early autumn Charles's troops swarmed over Jutland and firmly established themselves in the duchies. But the fortress of Fredriksodde (Fredericia) held Charles's smaller army at bay from mid-August to mid-October, while the fleet of Denmark–Norway, after two days' battle, compelled the Swedish fleet to abandon its projected attack on the Danish islands. The position of the Swedish king had now become critical. In July Denmark–Norway and Poland-Lithuania concluded an offensive and defensive alliance. Still more ominously for the Swedes, the elector of Brandenburg, perceiving Sweden's difficulties, joined the league against Sweden and compelled Charles to accept the proffered mediation of Oliver Cromwell, Coenraad van Beuningen and Cardinal Mazarin. The negotiations foundered, however, upon the refusal of Sweden to refer the points in dispute to a general peace-congress, and Charles received encouragement from the capture of Fredriksodde, 23–24 October, whereupon he began to make preparations for conveying his troops over to Funen in transport vessels. But soon another and cheaper expedient presented itself. In the middle of December 1657 began the great frost, which would prove so fatal to Denmark–Norway. In a few weeks the cold had grown so intense that the freezing of an arm of the sea with so rapid a current as the Small Belt became a conceivable possibility; and henceforth meteorological observations formed an essential part of the strategy of the Swedes.
On 28 January 1658, Charles X arrived at Haderslev in South Jutland. His meteorologists estimated that in a couple of days the ice of the Little Belt would become firm enough to bear even the passage of a mail-clad host. The cold during the night of 29 January became most severe; and early in the morning of the 30th the Swedish king gave the order to start, the horsemen dismounting on the weaker spots of ice and cautiously leading their horses as far apart as possible, until they swung into their saddles again, closed their ranks and made a dash for the shore. Swedish arms quickly overpowered the Danish troops lining the opposite coast and won the whole of Funen with the loss of only two companies of cavalry, which disappeared under the ice while fighting with the Danish left wing. Pursuing his march, Charles X, with his eyes fixed on Copenhagen, resolved to cross the frozen Great Belt also. However, he accepted the advice of his chief engineer officer Erik Dahlberg, who acted as pioneer throughout and chose the more circuitous route from Svendborg, by the islands of Langeland, Lolland and Falster, in preference to the direct route from Nyborg to Korsør, which would have had to cross a broad, almost uninterrupted expanse of ice. A council of war, which met at two o'clock in the morning to consider the practicability of Dahlberg's proposal, dismissed it as hazardous. Even the king wavered; but when Dahlberg persisted in his opinion, Charles overruled the objections of the commanders. On the night of 5 February the transit began, the cavalry leading the way through the snow-covered ice, which quickly thawed beneath the horses' hoofs so that the infantry which followed after had to wade through half an ell (nearly 2 feet) of sludge, facing the risk that the ice would break beneath their feet. At three o'clock in the afternoon, with Dahlberg leading the way, the army reached Grimsted in Lolland without losing a man; on 8 February, Charles reached Falster. On 11 February he stood safely on the soil of Zealand. A Swedish medal struck to commemorate the transit of the Baltic Sea bear the inscription: Natura hoc debuit uni. Sweden had achieved a rare war exploit, in Sweden considered to be matched only by the crusade of the Livonian Order led by William of Modena to conquer Saaremaa (Osel) in January 1227 and afterwards when two Russian armies crossed the frozen Gulf of Bothnia from Finland to mainland Sweden in March 1809 during the Finnish War. It is believed that the effect of this achievement on the Danish government found expression in the Treaty of Taastrup on 18 February, and in the Treaty of Roskilde (26 February 1658), whereby Denmark–Norway sacrificed a great part of her territory to save the rest. However, Charles X continued the war efforts against Denmark–Norway after a council held at Gottorp on 7 July, even though he was in defiance of international equity. Without warning, Denmark–Norway was attacked a second time.
On 17 July he again landed on Zealand and besieged Copenhagen with its king Frederick III of Denmark and Norway, but the city managed to hold out long enough for the Dutch fleet under Lieutenant-Admiral Jacob van Wassenaer Obdam to relieve the city, defeating the Swedish fleet in the Battle of the Sound on 29 October 1658, and Copenhagen repelled a major assault afterwards.
