Question: What campaign by Sweden led to Denmark losing a third of its territory with the treaty of  Treaty of Roskilde?
Sentence (from "March Across the Belts"): The March Across the Belts was a campaign between 30 January and 8 February 1658 during the Second Northern War where King Charles X Gustav of Sweden led the Swedish army from Jutland across the ice of the Little Belt to Funen and the Great Belt to reach Zealand.
Relevant evidence: Yes