Treaty of Roskilde: After a devastating defeat in the Northern Wars (1655–1661), Frederick III, the King of Denmark–Norway is forced to give up nearly half his territory to Sweden.

The Treaty of Roskilde was negotiated at Høje Taastrup Church and was concluded on 26 February (OS) or 8 March 1658 (NS) during the Second Northern War between Frederick III of Denmark–Norway and Karl X Gustav of Sweden in the Danish city of Roskilde. After a devastating defeat, Denmark–Norway was forced to give up a third of its territory to save the rest, the ceded lands comprising Blekinge, Bornholm, Bohuslän, Scania and Trøndelag, as well as Halland.

"Northern Wars" is a term used for a series of wars fought in northern and northeastern Europe from the 16th to the 18th century, primarily between the territorial rivals of the Swedish Empire, Tsardom of Russia, Poland–Lithuania, and Denmark–Norway. The Great Northern War is generally considered to have concluded the Northern Wars with the decline of Sweden and establishment of the Russian Empire as the principal power of the region, however there are different scholarly opinions on which war constitutes the First Northern War and an internationally agreed-on nomenclature for these wars has not yet been devised.

Frederick III was King of Denmark and Norway from 1648 until his death in 1670. He also governed under the name Frederik II as diocesan administrator of the Prince-Bishopric of Verden, and the Prince-Archbishopric of Bremen (1635–45).

Denmark–Norway was a 16th-to-19th-century multi-national and multi-lingual real union consisting of the Kingdom of Denmark, the Kingdom of Norway, the Duchy of Schleswig, and the Duchy of Holstein. The state also claimed sovereignty over three historical peoples: Frisians, Gutes and Wends. Denmark–Norway had several colonies, namely the Danish Gold Coast, Danish India, and the Danish West Indies.