During the Second Northern War, King Charles X Gustav of Sweden was bogged down in his war with Poland, unable to reach a decisive conclusion to the hostilities despite taking Warsaw. King Frederick III of Denmark provided a way out of Poland when he declared war on Sweden. The Swedish army could now march from Poland without it looking like a rout. The Swedish army marched to the western and central parts of Denmark from Poland, without returning to Sweden, to avoid fighting its way through then-Danish Scania. Forced marches of several tens of kilometers a day, brought 6,000 Swedish soldiers to Jutland. The Swedish army was not large, but the soldiers were some of the best equipped and best trained troops in Europe. The army swept away the Danish resistance; the remaining Danish did not risk a pitched battle with the Swedes. On 25 August 1657, the Swedish army stood before the newly completed Danish fortress Fredriksodde on Jutland's east coast. The fortress had a complement of about 8,000 men. This fortress did not exist in 1645. Fredriksodde was besieged by general Karl Gustav Wrangel for two months, and then stormed on the night of 24 October. Few Swedes perished and 6,000 Danes were captured. In addition, the Swedish army now had all of the Jutland army's supplies. All of Jutland was soon under Swedish control and Charles X started preparations for an attack against the Danish islands. In the middle of December the weather shifted, turning into the coldest winter in memory. The seawater between the islands froze, making a ship-borne assault impossible. Engineer Erik Dahlberg was dispatched by the king to ascertain whether the ice would support the weight of the Swedish cavalry and artillery. Dahlberg reported that a crossing over the ice was feasible.
Crossing
Early in the morning of 30 January 1658, the army was lined up to cross the Little Belt to reach Funen. The army consisted of about 9,000 cavalrymen and 3,000 foot soldiers. The ice warped under the weight of the soldiers; on occasions water reached up to the men's knees. Close to the shore of Funen a skirmish broke out with about 3,000 Danish defenders, but these were brushed aside quickly and the army was safe on Funen. Now investigations were made to find the best way over the ice that covered the Great Belt in order to reach Zealand. Again Erik Dahlberg led the investigation, and he advised taking the longer route via Langeland and Lolland rather than the more direct route across the Belt. The night of 5 February the king set off with the cavalry across the ice and safely reached Lolland later in the day. The infantry and the artillery followed the next day. Thus, on 8 February, the Swedish host was safely on Zealand, and on 15 February, after forced marches, it reached the outskirts of Copenhagen. The Danes, who had thought the Swedes would start their offensive in the spring at the earliest, panicked and yielded. Negotiations were started and on 26 February the Treaty of Roskilde was signed by the two parties.
Aftermath
The March Across the Belts was a risky but successful gamble which turned out to be entirely to King Charles X Gustav's benefit. The news of the victory spread quickly; it was an achievement that drew admiration all over Europe. For Denmark, the outcome of the treaty of Roskilde was disastrous: the entire eastern part of the kingdom was ceded to Sweden, and Denmark's existence as an independent state lay in doubt.
