Question: What campaign by Sweden led to Denmark losing a third of its territory with the treaty of  Treaty of Roskilde?
Sentence (from "Treaty of Roskilde"): The Treaty of Roskilde was concluded on 26 February (OS) or 8 March 1658 (NS) during the Second Northern War between Frederick III of Denmark–Norway and Charles X Gustav of Sweden in the Danish city of Roskilde.
Relevant evidence: Yes