Question: What campaign by Sweden led to Denmark losing a third of its territory with the treaty of  Treaty of Roskilde?
Context: 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). The Concert of The Hague, signed on 21 May 1659, was an outline of the common stance of England, France and the Dutch Republic regarding the Second Northern War.  The powers agreed that the Swedish Empire and Denmark–Norway should settle for a peace treaty based on the Treaty of Roskilde, including free navigation through The Sound and the Baltic Sea based on the Treaty of Elbing.  The subsequent Dano-Swedish Peace of Copenhagen largely followed the terms dictated by the Concert of the Hague. 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. The Treaty of Paris, signed on 6 January 1810, ended the war between France and Sweden after Sweden's defeat by Russia, an ally of France, in the Finnish War of 1808-1809.  Russia had previously been an ally of Sweden in the Third and Fourth Coalitions against France, but after Russia's defeat at Friedland, she joined France and attacked Sweden so as to compel her to join Napoleon's Continental System.  Indeed, the primary result of the treaty was Sweden's agreement to join the Continental System, so that Sweden would not trade with the UK.  Shortly after the treaty was signed, on 21 August 1810, one of Napoleon's marshals, Jean-Baptiste Bernadotte, was elected crown prince of Sweden, and he went on to found the House of Bernadotte, which remains the Royal House of Sweden.  The peace resulting from the treaty lasted until Napoleon's refusal to permit Sweden to annex Norway, which was then under the sovereignty of Denmark, an ally of France.  This was followed in January 1812 by French occupation of Swedish Pomerania for violation of the Continental System, since Sweden was still trading with the UK, and, in April, Sweden's conclusion of the Treaty of Petersburg with Russia against France. The Treaty of Tsarskoye Selo was a territorial and dynastic treaty between the Russian Empire and the Kingdom of Denmark.  Signed on 1 June 1773, it transferred control of ducal Schleswig-Holstein to the Danish crown in return for Russian control of the County of Oldenburg and adjacent lands within the Holy Roman Empire.  The treaty reduced the fragmentation of Danish territory and led to an alliance—The Eternal Alliance (Danish: "Den Evige Alliance" )—between Denmark and Russia that lasted into the Napoleonic Wars, effectively ending in 1809, when Russian diplomats suggested to Sweden that Norway be a possible compensation for Finland. 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 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. 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. 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 Third Treaty of San Ildefonso was a treaty between France and Spain in which Spain returned the colonial territory of Louisiana to France.  The treaty was concluded on 1 October 1800 between Louis Alexandre Berthier representing France and Don Mariano Luis de Urquijo for Spain.  The treaty was negotiated under some duress, as Spain was under pressure from Napoleon, although Spain did gain the Tuscany area.  The terms of the treaty did not specify the boundaries of the territory being returned, which later became a point of contention between Spain and the United States after the Louisiana Purchase in 1803, in which France sold its territory to the United States.  This treaty also affirmed the earlier Treaty of Alliance signed at San Ildefonso on 19 August 1796.
Answer: March Across the Belts