Sweden enjoys an enviable reputation as a neutral and peace-loving nation. But in bygone times the Swedes were aggressive, and often at war with other countries of northern Europe, particularly with their Danish neighbours.
Such was the case in the middle of the 17th century during the short reign of Charles X.
Now Charles was not as great a warrior as his grandson, Charles XII, but what he may have lacked in skill he made up for in enthusiasm, and he did his best to uphold the fearsome Swedish reputation.
In 1657 while he was busy making war on Poland, the Danes, sneakily and behind his back, attacked several of Sweden's possessions on the Baltic.
Charles abandoned Poland temporarily, and marching across the northern part of continental Europe, he invaded Denmark from the south.
He quickly gained control of Jutland - what we might think of as the "mainland" part of Denmark - while the Danes hurriedly retreated to their capital at Copenhagen.
Denmark is a fragmented country. Copenhagen lies on the large island of Zealand close to the coast of Sweden, and there are many islands, large and small, between it and Jutland.
This complex geography was difficult for Charles: some months earlier he had lost his navy in a previous skirmish with the Danes, and he now had an army of 10,000 men to transport across these various straits if he wished to deliver a coup de grace in Copenhagen.
The weather proved to be Charles's strongest ally. It turned bitterly cold in January 1658, and since Europe at the time was in the "Little Ice Age", that meant very, very cold indeed; a thick layer of ice began to form on the Kattegat and by February 9th the ice was thick enough for the Swedish army to attempt to walk across.
The cavalry led the way over the snow-covered ice, which quickly thawed beneath the horses' hooves so that the infantry following behind had to wade through several inches of sludge, fearing every moment that the rotting ice would break.
But it held, and the following morning the Swedish troops were across the Little Belt on the island of Funen.
The rest was easy. The ice enabled Charles to use the Danish islands as his stepping stones to Copenhagen. He marched his army across set after set of frozen straits, while the Danes, and indeed the rest of Europe, watched amazed. By late February all the crossings were complete; with the invading army on their doorstep, the Danes surrendered, and Sweden's dominant position in northern Europe was consolidated.