What campaign by Sweden led to Denmark losing a third of its territory with the treaty of  Treaty of Roskilde?
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. Treaty 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 (Båhuslen), Scania (Skåne) and Trøndelag, as well as her claims to Halland. 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.
March Across the Belts