Treaty of Copenhagen (1660)

The Treaty of Copenhagen was signed on 27 May 1660, and marked the conclusion of the Second Northern War between Sweden and the alliance of Denmark-Norway and the Polish-Lithuanian Commonwealth. This treaty was a smaller follow-up treaty to that of the Treaty of Roskilde, which this then declared the official boundaries of Denmark, Sweden, and Norway; boundaries which are almost exactly the same to this day.

Charles X of Sweden would not accept any other outcome than Sweden's receipt of Akershus county, in exchange for the return of Trøndelag and Bornholm to Denmark-Norway; Frederick III of Denmark on the other hand refused to abide by the terms of the Treaty of Roskilde, instead wanting to revert to the conditions of the Second Treaty of Brömsebro (1645). Both kings were stubborn, and had to depend on the mediating powers, France and England on the Swedish side, and the Dutch Republic on the Danish side, to bring about a treaty.

Frederick argued to his Dutch allies that Swedish control of Scania would present an eternal lingering threat to Copenhagen, and proposed a Danish-Dutch alliance to retake Scania. Frederick had the support of Brandenburg and Austria, but the Dutch had already agreed with England that preventing the western and eastern shore of Øresund from being controlled by the same country would in turn prevent future raises of the Øresund toll, which would cause disruption in the Dutch and English Baltic trade. In addition, Cardinal Mazarin threatened a French invasion of the Netherlands if it were to assist Denmark against Sweden.

...
Wikipedia