May 27
Treaty of Copenhagen Concludes Second Northern War
The 1660 agreement ended the Second Northern War by confirming Swedish gains east of the Sound while returning key Norwegian and Danish territories, setting borders that have largely endured.
Summary
In the mid-17th century, the Second Northern War pitted Sweden against Denmark-Norway and allies amid shifting Baltic power dynamics. Swedish King Charles X Gustav had earlier forced harsh terms via the Treaty of Roskilde in 1658, seizing Danish territories. After Charles X's death in early 1660, Danish King Frederick III sought revisions with support from the Dutch Republic and others. Mediators from France, England, and the Netherlands facilitated talks near Copenhagen. On May 27, 1660, the treaty was signed in tents between the city and the Swedish camp, restoring Trøndelag and Bornholm to Denmark-Norway while confirming Swedish control over Scania and other eastern Sound provinces. The agreement largely fixed modern borders between Denmark, Norway, and Sweden.
Context
The Second Northern War arose from long-standing rivalries over control of the Baltic Sea and its lucrative trade routes, pitting the expanding Swedish Empire against Denmark-Norway and its allies. Sweden had emerged from the Thirty Years' War as a dominant northern power, seeking to secure its position through territorial acquisitions that would neutralize Danish influence over the Øresund strait.
What Happened
In 1658, Sweden's aggressive campaign forced Denmark-Norway to accept the harsh Treaty of Roskilde, ceding fertile provinces and strategic islands. Swedish King Charles X Gustav soon renewed hostilities with a surprise invasion, besieging Copenhagen, but a Dutch naval intervention and determined Danish resistance stalled the advance. Charles X died suddenly in February 1660, opening the way for renewed diplomacy under the mediation of France, England, and the Dutch Republic.
Aftermath
Negotiations intensified after the Swedish king's death, with Danish King Frederick III leveraging support from Brandenburg, Austria, and Poland while the Dutch prioritized preventing any single power from dominating the Sound. Direct talks between Danish and Swedish representatives, facilitated by figures such as Hannibal Sehested, produced the final terms signed on 27 May 1660 in tents positioned between Copenhagen and the Swedish encampment at Carlstad.
Legacy
The treaty restored Trøndelag to Norway and Bornholm to Denmark while confirming Swedish retention of Scania, Halland, Blekinge, and Bohuslän, thereby establishing the modern borders of Denmark, Norway, and Sweden with only minor subsequent adjustments. It reinforced Swedish naval and commercial preeminence in the Baltic for decades and prompted internal political shifts in Denmark, including the consolidation of absolute monarchy under Frederick III.
Why It Matters
The treaty immediately stabilized the Baltic region after years of conflict and established enduring national boundaries that persist today. It also reinforced Swedish naval dominance in the Baltic while curbing further Danish territorial losses, influencing trade routes and power balances in Northern Europe for generations.
Related Questions
What territories changed hands under the Treaty of Copenhagen?
Sweden retained Scania, Halland, Blekinge, and Bohuslän but returned Trøndelag to Norway and Bornholm to Denmark.
How did the death of Charles X Gustav affect the peace process?
His death in February 1660 removed the chief Swedish advocate for continued war, allowing negotiators to reach a compromise more readily.
Why were the Dutch Republic involved in the negotiations?
The Dutch provided naval aid to Denmark and mediated to protect their Baltic trade interests by preventing any single power from controlling both shores of the Sound.
What immediate political change occurred in Denmark after the treaty?
The peace contributed to a coup that established Frederick III as hereditary absolute monarch, diminishing noble influence.
Did the treaty end all hostilities in the region?
It concluded the Second Northern War between Sweden and Denmark-Norway, though broader European rivalries persisted.
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Sources
- Treaty of Copenhagen (1660), Wikipedia. Accessed 2026-07-10.
- Treaty of Copenhagen, Encyclopædia Britannica. Accessed 2026-07-10.