A failed siege of Copenhagen reshaped the borders of Northern Europe forever
When Sweden attempted to erase Denmark from the map in 1658, they triggered a massive international coalition. What began as a bold bid for Baltic dominance devolved into a desperate struggle for survival, ultimately resulting in a 1660 treaty that defined the modern borders of Denmark, Sweden, and Norway.
The Dano-Swedish War of 1658–1660 was a direct consequence of the Treaty of Roskilde, which had forced Denmark to cede significant territory to Sweden. King Charles X Gustav of Sweden, seeking to avoid the disbandment of his army and desiring to wage war across Europe without Danish interference, used minor Danish delays in fulfilling the peace terms as a pretext for a total invasion. His goal was to vanquish Denmark as a sovereign state and divide the country into four administrative regions.
In August 1658, Swedish forces surrounded Copenhagen, beginning a siege that would last 22 months. The Swedish king initially opted to starve the city into submission rather than launch an immediate assault, a decision that allowed the Danish population to rally behind King Frederick III and fortify their defenses. The conflict escalated when the Dutch Republic, honoring a 1649 treaty, intervened by sending a relief fleet that broke the Swedish naval blockade in the Øresund straits. This intervention proved decisive, as it allowed vital supplies and reinforcements to reach the beleaguered capital.
As the war dragged on, a broad coalition including Brandenburg, Poland, and Austria joined the fray, attacking Swedish interests across the Baltic. The Swedish military suffered a catastrophic defeat at the Battle of Nyborg in late 1659, where their forces were decimated by the combined Danish and allied armies. Following the death of Charles X in February 1660, the political landscape shifted rapidly. The resulting Treaty of Copenhagen forced Sweden to return the island of Bornholm to Denmark and the province of Trøndelag to Norway. These territorial adjustments established the political borders that persist between these nations today, marking the end of Sweden's attempt to dominate the Baltic through the total destruction of its neighbor.
Source: Dano-Swedish War (1658–1660)
