Denmark tried to gather allies from Poland, Austria, and the Netherlands, and by summer 1657 the situation seemed very advantageous to Denmark. But the Danish command, led by King Frederick III, realized that they were not strong enough to confront Sweden on their own. In 1657, Denmark was ready to attack Sweden, seeing an opportunity to tear up the Treaty of Brömsebro signed in 1645, when they were forced to cede to Sweden the provinces of Gotland, Saaremaa, Jämtland and Härjedalen, including Halland for a 30-year period. In 1656, Russian troops had crossed the border into Swedish Livonia and besieged Riga. Both Austria and the Netherlands sent military aid to Poland, and Sweden's former ally, Brandenburg-Prussia, changed sides during the conflict. Poland's neighbors threatened to get involved in the war. The Poles resisted the Swedish troops, who were being ambushed constantly by Polish guerrilla units. Despite several tactical victories on the battlefield, including the conquest of Warsaw, Charles X Gustav was unable to bring the war to an end. However, the war against Poland was slow. The Swedish empire would be expanded, while the control of the lucrative Baltic Sea trade strengthened. In 1655, King Charles X Gustav began a campaign against the Polish–Lithuanian Commonwealth to force King John II Casimir Vasa to renounce all claims to the Swedish crown, as well as conquer the Polish provinces of Courland and Prussia. Likewise, the events gave rise to a polarised debate over the role of quartermaster general Erik Dahlbergh in the king's decision to march across the Great Belt. In its historiography, several historians have highlighted the campaign and the resulting peace treaty as the events that gave Sweden its current "natural" borders. Bornholm and Trøndelag were returned to Denmark in 1660 after Charles X Gustav's failed attempt to defeat Denmark completely in a second war. Denmark ceded Scania, Blekinge, Halland, Bohuslän, Bornholm and Trøndelag to Sweden. The resulting Treaty of Roskilde, signed on 26 February 1658, was highly favorable for Sweden.
With Swedish troops standing 22 kilometres (14 mi) from Copenhagen on 15 February, King Frederick III of Denmark chose to make an unconditional peace with Sweden, ending the Swedish campaign.Ĭharles X Gustav's gamble ended with a catastrophic defeat for Denmark. He continued through Lolland and Falster and reached Zealand on 11 February. On 5 February the ice at southern Funen was deemed sufficiently thick, and the king decided to cross to Langeland. To avert the risk that his troops would be isolated on Funen, Charles X Gustav investigated the possibility of crossing the ice from the Great Belt to Zealand. Swedish troops defeated the Danes at Tybrind Vig and Iversnæs and occupied Funen after a few days. Since the 17th century was the coldest during the Little Ice Age, with the winters in Scandinavia being exceptionally frigid, the king planned to take advantage of the weather by remaining in Jutland until sufficient ice had built up to support the weight of his troops, and then carry out a risky march across the ice.Īfter investigating the ice conditions, the king began his march on 30 January 1658 from Jutland across the strait of the Little Belt to Funen. With Jutland secured, Charles X Gustav sought to continue his campaign towards Copenhagen on Zealand, but the Danish straits and the Danish navy obstructed him. The Swedes attacked and conquered Frederiksodde on 27 October. The king's rapid march surprised the Danish troops, whose main body was forced to retreat to the fortress of Frederiksodde. Although Charles X Gustav was deeply involved in the conflict with the Polish–Lithuanian Commonwealth, he chose to move the bulk of his army to Jutland and invade Denmark. On 5 June 1657, Denmark declared war on Sweden which was under heavy pressure in the Second Northern War against Poland and Russia. It lasted between 30 January and 15 February 1658, ending with a decisive victory for Swedish King Charles X Gustav during his first Danish war. The March Across the Belts ( Swedish: Tåget över Bält) was a military campaign waged by the Swedish Empire across the ice between the Danish islands.