Question: What were the dates of the campaign that eventually led to the Treaty of Roskilde?
Context: 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 decisively delineated the mutually recognized boundaries of Denmark, Sweden, and Norway; boundaries which are almost exactly the same to this day. 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.  The risky but vastly successful crossing was a crushing blow to Denmark, and led to the Treaty of Roskilde later that year, which handed Scania to Sweden. 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.  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. The Treaty of Taastrup (or the Høje Taastrup Peace) was a preliminary accord signed on 11 February 1658 between Charles X Gustav of Sweden and King Frederick III of Denmark.  The treaty was signed at the Høje Taastrup Church.  Individuals such as Count Corfitz Ulfeldt participated in the peace negotiations after Denmark lost in the Second Northern War.  The agreement was finalized through the Treaty of Roskilde.  The original copy of the treaty is at the National Museum of Denmark ("Nationalmuseet") in Copenhagen. Christopher James Adler (born November 23, 1972) is an American drummer, best known as a member of the groove metal band Lamb of God.  Additionally he became a member of thrash metal band Megadeth from 2015-2016, playing on their fifteenth album "Dystopia" and most dates for the album's tour with his eventual replacement Dirk Verbeuren covering dates he was unable to attend due to Lamb of God commitments.  These commitments eventually led to his departure from the band by a mutual decision as he did not wish to leave Lamb of God. " Dystopia" won the Best Metal Performance at the 59th Grammy Awards. Leader Dominic Stirling (19 January 1906 – 7 February 2003) was a missionary surgeon and former Health Minister in Tanzania.  Born in Finchley, England and raised in Sussex Weald, Stirling attended Bishop's Stortford College and the University of London.  After a brief period of general practice, Stirling joined the Universities' Mission to Central Africa and was deployed to Tanzania.  He spent 14 years of service to the UMCA in Lulindi.  He then converted to Catholicism and joined the Benedictine Mission, working with them in Mnero, where he built another hospital.  After 15 years he left to Kibosho, on the slopes of the Kilimanjaro, where he worked for 5 more years.  During his medical missionary career, he emphasised the training of local nurses, establishing a precedent for official nurse recognition in Tanzania.  His experience in Africa eventually led him to the political career, and in 1958 Leader Stirling was elected (unopposed) to the first Parliament of Tanzania.  He held this position for the next 22 years, being the last 5 as Health Minister by appointment of Julius Nyerere.  Besides his medical and political work, Stirling was also interested in Scouting.  His successful efforts to establish a Scout movement in Tanzania eventually led him to the post of Chief Scout of Tanzania in 1962, following the formation of the Republic. The Piet Retief Delegation massacre occurred when Voortrekkers under Piet Retief migrated into Natal in 1837 and negotiated a land treaty in February 1838 with the Zulu King Dingane.  Upon reconsideration, Dingane doublecrossed the Voortrekkers, killing the delegation of 100 including their leader Piet Retief on 6 February 1838.  The land treaty was later found in Piet Retief's possession.  It gave the Voortrekkers the land between the Tugela River and Port St. Johns.  This event eventually led to the Battle of Blood River and the eventual defeat of Dingane. The Irish Republican Army was a guerrilla army that fought the Irish War of Independence against Britain from 1919–1921.  It saw itself as the legitimate army of the Irish Republic declared in 1919.  The Anglo-Irish Treaty, which ended this conflict, was a compromise which abolished the Irish Republic, but created the self-governing Irish Free State, within the British Empire.  The IRA was deeply split over whether to accept the Treaty.  Some accepted, whereas some rejected not only the Treaty but also the civilian authorities who had accepted it.  This attitude eventually led to the outbreak of the Irish Civil War in late June 1922 between pro- and anti-Treaty factions. The Treaties of Roskilde of 18 and 22 November 1568 were peace treaties between the kingdoms of Denmark-Norway and the allied Free and Hanseatic City of Lübeck on one side, and the Swedish kingdom on the other side, supposed to end the Northern Seven Years' War after the "de facto" succession of the later king John III in Sweden.  Negotiated on John's initiative, he refused ratification, viewing the concessions his envoys made in Roskilde as too far-reaching.  Most notably, these concessions included Swedish obligations to pay Denmark her war costs, and to cede Swedish Estonia.  Thus, the war dragged on until it was concluded by the Treaty of Stettin (1570). Joachim Gersdorff (12 November 1611 - 19 April 1661) was a Danish politician, from 1650 to 1660 Steward of the Danish Realm.  It was Gersdorff who negotiated the Treaty of Roskilde on Denmark's part during the Second Northern War, a war he had himself been in favour of entering.  The treaty, which was concluded in Roskilde on 8 March 1658 (NS), ceded Scania, Halland, Blekinge and Bornholm to Sweden.
Answer: between 30 January and 8 February 1658