How a frozen sea changed the map of Scandinavia

HostWe usually think of borders as things drawn by people in meeting rooms or decided by where a river flows. But sometimes, a few weeks of really weird weather can do more to shape the map than a century of talk.

HostWe're looking at a moment when a king had to decide if the ice under his feet was a bridge or a trap. How did a deep freeze in the sixteen hundreds lead to the borders we see in the north today?

GuestIt all starts with a king named Charles who was stuck in a bad spot. He was down in Poland with his army, far from home, when his old rivals in Denmark saw an opening and declared war. Charles was a fast mover, though. He marched his battle-hardened troops all the way back up to the edge of Denmark. But once he got to the coast, he was stuck. Denmark is a land of islands, and their navy was the best in the area. The water was like a huge moat protecting their capital city. As long as the sea stayed wet, the Swedish army was just standing on the shore looking at a prize they couldn't reach.

HostSo they were basically trapped on the mainland while the Danes just watched them from across the waves. Walking across the sea sounds like a tall tale, or at least a very quick way to drown.

GuestIt usually would be. But this was during a time we call the Little Ice Age. The whole world was much colder back then, and that winter was a total freak event. It got so cold that even the salt water between the islands started to turn into solid ice. The king sent his lead engineer, a man named Erik, out into the dark to see if the ice could hold. Imagine him kneeling on the frozen sea in the middle of the night, driving a knife into the ice to see how deep it went. He came back and told the king that if they were very careful, they could walk across.

HostBut an army isn't just a few guys on a stroll. You have thousands of men, heavy horses, and big metal cannons. I can't see how a sheet of ice doesn't just shatter the moment they start moving.

GuestThat was the big fear. To make it work, they had to throw out the way armies usually march. They couldn't stay in tight groups. Instead, they spread out into long, thin lines. The goal was to make sure no single spot on the ice had too much weight on it at once. Most of the men even got off their horses and led them by hand to spread out their footprint. It was tense. At one point, the ice did buckle and some groups of riders were swallowed by the sea. But most of them made it across the first stretch of water to an island called Funen.

HostThat's a big win, but they were still not at the capital. They were just on a different island. Between them and the main prize was another seven miles of deep, open sea. Does the ice even stay solid over water that deep?

GuestThat was the gamble. Most of the king's team thought it was suicide. If the ice broke there, the whole army would be gone. But Erik, the engineer, found a trick. He realized that if they took a long, weird zig-zag path through some smaller islands, they could stay over shallower water. The ice was thicker and more stable there. They also chose to move in the dead of night. They did this because even a little bit of winter sun could warm the surface just enough to weaken the way the ice crystals held together.

HostIt's wild to think about. A whole army sliding across the Baltic Sea in the dark while the Danish navy is just sitting right there. Why didn't the ships just blast them?

GuestThey couldn't move. The Danish ships were literally frozen into their harbors. They were stuck in the ice like toy boats in a freezer. The Swedes just marched right past them. When the army showed up on the main island near the capital, the Danes completely panicked. They had always relied on the sea to keep them safe, so they had no plan for a land fight at their front door. They had to give up and sign a peace deal called the Treaty of Roskilde.

HostIt sounds like Denmark just got bullied because of a cold snap. Was this just a temporary loss for them?

GuestNot at all. This was the birth of the map we know today. Denmark had to hand over its best farm lands, places like Scania and Halland. Those parts of the south stayed Swedish from then on. It ended the time when Denmark was the big power in the north and turned Sweden into a major player. All of that happened because the sea stopped acting like water for a few weeks.

HostIt's amazing how much of our history comes down to that engineer on his knees in the dark.

GuestHe was checking if the ice was thick enough to hold the weight of a whole nation's future.

HostThe next time I look at a map of those northern coasts, I'll be thinking about those long, thin lines of men sliding across a sea that should've been their end.