How Climate Change Helped the Swedes March Across A Frozen Sea And Defeat The Danes

 
On the morning of January 30, 1658, some 9,000 cavalrymen and their horses, as well as 3,000-foot soldiers and the rest of the army’s equipment, stepped on the ice. iStock
 
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It all started in 1655 during the Second Northern War. Sweden was fighting against the Polish-Lithuanian Commonwealth, Russia, Brandenburg-Prussia, the Habsburg Monarchy, Denmark-Norway, and sometimes, the Dutch Republic. On July 21 of that year, Charles X Gustav of Sweden invaded Poland-Lithuania and advanced across the country so fast; they called it the Swedish Deluge.

By September 8, he took Warsaw, and by October’s end, most of the regions armed forces had surrendered to him. The Polish king, however, did not. Rather than surrender, John II Casimir Vasa fled to the Habsburgs. Given Europes continually changing set of alliances, Charles knew that it was only a matter of time before the other nations began attacking him.

There was also the matter of securing his hold over Poland-Lithuania. Not only were the Swedes foreign invaders, but they were predominantly Protestant while the Poles and Lithuanians were mainly Catholics, creating tensions between the occupiers and the occupied. Then there was John. The Polish king wanted his country back, so other nations began supporting him against Sweden.

John II Casimir, King of Poland and Grand Duke of Lithuania. Oil on Copper plate by Marcello Bacciarelli seometime between 1768 and 1771, now at the Royal Castle in Warsaw, Poland.
John II Casimir, King of Poland and Grand Duke of Lithuania. Oil on copperplate by Marcello Bacciarelli sometime between 1768 and 1771, now at the Royal Castle in Warsaw, Poland.

By January 1656, Charles found himself bogged down in a series of skirmishes that seemed to get him nowhere. Worse, the Poles and Lithuanians were taking back their country. Taking advantage of the situation, Russia got involved so it could get a share of the spoils. The countdown had begun, and it was only a matter of time before the other nations joined the fray.

So now Charles was trapped. He had taken Poland-Lithuania, but could not hold it effectively. His brother-in-law, Frederick of Hesse, had been killed when guerrilla forces attacked the Swedish garrison at Socian in October 1655. Local resistance continued to mount.

Johan Philip Lenke's oil on canvas, "The Swedes crossing the ice over to Zealand in 1658"
Johan Philip Lenke’s oil on canvas, “The Swedes crossing the ice over to Zealand in 1658.”

Abandoning his new acquisition would have been the best thing to do, but the Swedish king could not just leave. If he did, he would lose face. The other nations would take it as a sign of weakness, putting Sweden in serious trouble.

Fortunately, there was Sweden’s old enemy: Denmark. Nervous about Swedish expansionism, King Frederick III of Denmark and Norway signed a manifesto justifying war against Sweden on June 1, 1657. It was never formally declared, however, as Frederick needed time to prepare his forces and to work out more alliances in preparation for a fight.

Frederick III of Denmark and Norway. Painted by an unknown artist sometime in the 1600s, it rests in the Frederiksborg Museum in Hillerød, Denmark.
Frederick III of Denmark and Norway. Painted by an unknown artist sometime in the 1600s, it rests in the Frederiksborg Museum in Hillerød, Denmark.

It was precisely the face-saving excuse Charles needed and the Little Ice Age would work in his favor.

Charles marched his 6,000-strong army out of Poland and into the western and central parts of Denmark, sweeping aside the Danish forces as they made their way toward Jutland. The Swedish army was not a large army, but they were among the best trained and most well-equipped militia at the time. They reached the newly-built Danish fortress of Friedriksodde along the east coast of Jutland and took it on October 24.

Equipped with the captured supplies from the fortress and all of Jutland under his control, Charles decided to do something mad – attack the Danish islands. That was not the insane bit; an assault by sea was the usual way to go about it, but it was December, and the coldest anyone had ever known.

The seas around the islands had frozen solid. That meant no boats, but the king had an idea, so he turned to his engineer.

Count Erik Jönsson Dahlbergh was not born a noble. He was an orphan of peasant stock but proved to be such a capable military man and engineer, as well as a brilliant designer of fortifications, that he was made a noble. Within his lifetime, they called him the “Vauban of Sweden,” in honor of Sébastien Le Prestre de Vauban, the most famous military engineer of his time.

Strömer's 1849 lithograph of Count Erik Dahlbergh
Strömer’s 1849 lithograph of Count Erik Dahlbergh

Dahlbergh headed out on the ice and returned days later with his assessment. On the morning of January 30, 1658, some 9,000 cavalrymen and their horses, as well as 3,000-foot soldiers and the rest of the army’s equipment, stepped on the ice.

The Little Belt strait is ordinarily free-flowing. It stretches for about 31 miles in length and is some 17 miles wide, and contains many small islands. The Swedes headed for Funen, the third biggest Danish island.

Taken on 11 April 2004, the New Little Belt Bridge spans the Little Belt between Jutland and Funen Island
Taken on 11 April 2004, the New Little Belt Bridge spans the Little Belt between Jutland and Funen Island

Despite Dahlbergh’s assessment, the ice warped beneath the weight of the men. In some places, they sank up to their knees, but the “Vauban of Sweden” lived up to his reputation – the ice did not break. They made it to Funen, brushing aside some 3,000 defenders and establishing themselves on the island.

Their next destination was Zealand Island. To reach it, they had to cross over the Great Belt, some 37 miles long and 10 to 20 miles wide. Dahlbergh again went out to assess the ice, but when he came back, the news was not good. A direct route was ruled out as the ice would not hold them.

Instead, they had to take a longer route toward Langeland Island and then on to Lolland, Denmark’s fourth largest. Charles went ahead with the cavalry on the evening of February 5 and reached Langeland later that night. The infantry and artillery had to wait until the next day, as Dahlbergh felt it would give the ice time to repair itself.

The Great Belt Fixed Link (East Bridge) which spans the Great Belt
The Great Belt Fixed Link (East Bridge) which spans the Great Belt. By TobiasKierk – CC BY-SA 4.0

By February 8, they were all on Zealand. Subjecting the entire army to a forced march, they reached Copenhagen on February 15, taking the Danes completely by surprise, as what sensible person travels in such weather?

Frederick (who was sensible) was waiting for the spring thaw to begin his war on Sweden. As Charles had decided to bring the war to him, and because Frederick did not like the idea of fighting on his front lawn, the latter capitulated.

In signing the Treaty of Roskilde on February 26, 1658, Frederick handed over Denmark’s entire eastern half to Sweden.