(1660), treaty between Sweden and Denmark-Norway that concluded a generation of warfare between the two powers. Together with the Treaty of Roskilde, the Copenhagen treaty largely fixed the modern boundaries of Denmark, Norway, and Sweden.

In the Roskilde treaty (signed Feb. 26, 1658) Denmark ceded its most fertile corn-growing provinces, Skåne, Blekinge, and Halland, as well as the Baltic Sea island of Bornholm and the Trøndelag region of central Norway to Sweden. Less than six months later, without warning, Sweden's King Charles X Gustav again invaded Denmark, seized Fünen, and attacked Zealand, but a Dutch fleet broke through the Swedish blockade of Copenhagen in October. The war's turning point was the Danish defense of Copenhagen, led by the heroic King Frederick III, in February, 1659. A year later Charles X was planning a further attack on Denmark when he died suddenly of an illness, leaving a four-year-old son heir to the throne. Shortly thereafter Sweden and Denmark negotiated peace.

Signed on May 27, 1660, the Treaty of Copenhagen recovered Fünen and Bornholm for Denmark and Trøndelag for Norway. Denmark's former mainland provinces east of The Sound (Øresund), however, remained part of Sweden. As a consequence of the peace, the Danish nobility, who had not supported the Danish war effort, became the scapegoats for the country's losses; and in a coup d'état, Frederick was named a hereditary and absolute king.