Lennart Torstensson, his master of strategy, had shown Charles X. how Denmark was to be attacked. With an army of only 8,000, but consisting of the choicest and most victorious troops in all Europe, Charles X. hastened in rapid marches through Pomerania and Mecklenburg, recaptured Bremen, and invaded through friendly Holstein all of Schleswig and Jutland, defeating the larger but inexperienced Danish army and capturing the strong fortress of Fredericia.
Yet the new position was as precarious as the one in Poland, and Charles had to use all the skill of his diplomacy to save his little army from an assault by inimical Europe. France and England seemed unwilling to render him effective help. But when the elector of Brandenburg, who had taken upon himself the leadership of Sweden’s enemies, turned to the emperor, emphasizing the necessity of crushing the Swedish power in one blow, he received the following surprising answer: “The king of Hungary has no reason to be the enemy of the king of Sweden.” Charles had reached a secret understanding with Austria. By this move he gained time. Through what seemed almost a miracle, he was not only to save his army but lead it on to victory after a strategic deed, in originality and daring unique in the history of the world.
The year of 1658 commenced with severe frosts. Charles X. conceived the daring plan of attacking the Danish isles by leading his army over the frozen sounds. He concluded to cross the sound of Lille Belt, opposite the islet of Brandsœ. His quartermaster-general, Eric Dahlberg, an engineer of great genius, ascertained that the ice was safe. One frosty winter morning, the 30th of January, the Swedish army, reinforced to 9,000 men, marched down on the ice, safely reaching Brandsœ at sunrise. A Danish army, arranged in order of battle in the island of Funen, was defeated. While crossing over to Funen, the ice cracked under two squadrons of cavalry, those who followed not daring to proceed. The king himself hurried past the dangerous place, pointing out a safe course, and the troops followed him.
The most dangerous part remained to cross, the much wider sound of Store Belt, in order to reach Seeland. Charles first thought of taking the direct route of two miles, but commissioned Dahlberg to explore the condition of the ice across to the smaller islands to the south. Dahlberg did so, and said he would wager his head for its perfectly safe condition. In enthusiasm, Charles clapped his hands exclaiming: “Now, brother Frederic, we will converse in good Swedish!” In the night between the 5th and 6th of February, the Swedish army marched from Svendborg in Funen over the ice to Langeland. “It was terrible,” wrote an eye-witness, “to march through the night over this frozen sea, where the horses’ hoofs had thawed down the snow on the ice, which was below two feet of water, and where we, in every moment, were in fear of striking the open sea.” At dawn the army landed in Langeland. During the rest for breakfast, frozen beer was chopped and distributed in pieces to the soldiers. The march continued over the still wider sound to Laaland, Eric Dahlberg in front, directing the march. Reaching Grimsted in Laaland at three o’clock in the afternoon, and proceeding to Nakskov, Charles Gustavus was met, at midnight, by the burgomaster and council of said town, who surrendered its keys. The 9th of February, the army stood in Falster, and a few days later was collected at the captured castle of Vordingborg in Seeland. Peace was hastily offered and agreed to on the 17th, and the treaty of peace signed the 28th of February, 1658, at Rœskilde. The conditions were severe, Denmark ceding the provinces of Scania, Halland, Bleking and Bohuslæn, the whole district or diocese of Drontheim in Norway, and the island of Bornholm, and agreeing to hold the Baltic closed to hostile fleets with the help of Sweden. The last clause was a piece of a Scandinavian policy devised by the Swedish king.