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. 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.
On 5 June 1657, Denmark declared war on Sweden which was under heavy pressure in the Second Northern War against Poland and Russia. 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. The king's rapid march surprised the Danish troops, whose main body was forced to retreat to the fortress of Frederiksodde. The Swedes attacked and conquered Frederiksodde on 27 October. 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. 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.
After investigating the ice conditions, the king began his march on 30 January 1658 from Jutland across the strait of the Little Belt to Funen. Swedish troops defeated the Danes at Tybrind Vig and Iversnæs and occupied Funen after a few days. To avert the risk that his troops would be isolated on Funen, Charles X Gustav investigated the possibility of crossing the ice of the Great Belt to Zealand. On 5 February the ice at southern Funen was deemed sufficiently thick, and the king decided to cross to Langeland. He continued through Lolland and Falster and reached Zealand on 11 February. 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.
Charles X Gustav's gamble ended with a catastrophic defeat for Denmark. The resulting Treaty of Roskilde, signed on 26 February 1658, was highly favorable for Sweden. Denmark ceded Scania, Blekinge, Halland, Bohuslän, Bornholm and Trøndelag to Sweden. Bornholm and Trøndelag were returned to Denmark in 1660 after Charles X Gustav's failed attempt to defeat Denmark completely in a second war. In its historiography, several historians have highlighted the campaign and the resulting peace treaty as the events that gave Sweden its current "natural" borders. 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.
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