The Swedish-Danish War of 1657-58 was part of the wider Northern War of 1655-60. That war had begun with a period of dramatic successes for Charles X of Sweden in Poland-Lithuania, but by 1657 the Polish recovery was well underway. The treaty of Vienna of 27 May 1657 had seen that recovery given a boost by an alliance with the Emperor Leopold.
When the news of this treaty reached Frederik III of Denmark he immediately declared war on Sweden (June 1657). In 1657 Denmark still held a number of provinces around the southern tip of the Scandenavian peninsular, including Scan, Bohuslän and Blekinge. Until 1645 she had also held Halland, leaving the Swedes with a narrow foothold on the North Sea at Gothenberg, and the island of Gotland in the Baltic, both of which had been lost during Torstensson’s War. Denmark was also united with Norway.
Frederik opened hostilities with an invasion of Sweden’s German possessions in Bremen Verden, attacking from Holstein. In July Danish forces invaded Jämtland (east of Trondheim), also lost to Sweden in 1645 and Västergötland, the area north east of Gothenburg, threatening land communications between Gothenburg and Stockholm.
Since the start of 1657 Charles had been campaigning in Poland-Lithuania with his ally George Rákóczi, prince of Transylvania. News of the Danish invasions forced him to abandon that campaign, and along with it his best chances of a victory in Poland. Rákóczi soon suffered defeat at the hands of the Poles and was forced to surrender.
Charles had more success in Denmark. At the head of an army of just under 13,000 men, he invaded Holstein from the east. A detachment under Karl Gustav Wrangel quickly restored the situation in Bremen, while the main army advanced towards the fortress of Fredriksodde. That fortress held out until October, when it was stormed by the Swedes. The previous month had seen the Swedish navy fight an inconclusive battle with the Danes at Møn, failing to win control of the belts (the area of sea that separated the island of Zealand from Jutland.
Charles now occupied Jutland, but it appeared that he had lost his chance to cross to Zealand. He solved this problem by crossing to the islands over the frozen sea in the Little and Great Belts. On 9 February 1658 the Swedish army crossed over the Little Belt to the island of Fyn, defeating a small Danish force on the way. He then turned to the south east, using the islands of Langeland, Lolland and Falster to bridge the gap to Zealand. This was a very daring move – the gap between Langeland and Lolland is over six miles wide – and it brought Charles a dramatic victory. On 25 February the Danes found a 5,000 strong Swedish army at the gates of Copenhagen.
Frederick III was forced to agree to all of Charles’s demands (Peace of Roskilde, 8 March 1658). This saw Denmark surrender all of its provinces on the southern tip of Scandinavia (Scania, Bohuslän and Blekinge), as well as the island of Bornholm at the western end of the Baltic, and Trondheim on the Norwegian coast. The Danes were also to provide Charles with 2,000 soldiers and pay the costs of his army until May.
The peace did not last for long. The sudden end of the war against Denmark left Charles with a problem. He needed to keep his army intact to guard against a Polish or Muscovite attack, but another campaign in Poland would be unpopular and probably expensive, while a campaign in Livonia would be difficult. In July 1658 Charles found enough excuses to declare war on Denmark (Swedish-Danish War, 1658-60).