Second Northern War and Norway In the Second Northern War from 1655 to 1660 , during the reign of Charles X , Sweden was set on expansion . Through military action , Sweden rapidly became the strongest military power in the north . Frederick III was suffering under the humiliating loss of traditional Danish provinces to Sweden in 1645 . As Charles X appeared to be fully occupied in Poland , Frederick III judged the time appropriate for recapture of the other Danish - Norwegian provinces . The King ’ s Council agreed to war , a decision that led rapidly to ruin . The Norwegian phase of the war went well . A Norwegian force of 2000 men recaptured Jæmtland and Herjedalen . A Norwegian force set out from Båhuslen to join the Danish force invading Sweden from Skåne . Reacting swiftly , by forced marches Charles X brought his hardened armies from Prussia to Holstein . Surprising the Danes , he advanced rapidly against limited opposition , taking Schleswig - Holstein and Jutland . Taking advantage of the unusually cold winter which froze the ice , Charles marched his armies across the ice onto the island of Zealand , leaving the humiliated Danes with no choice but to sue for peace on any terms . As a result , the Treaty of Roskilde was negotiated in 1658 . The terms were brutal : Then Charles X ignored the recently negotiated Treaty of Roskilde , when he invested Copenhagen in August 1658 . The Norwegian army mobilized under the leadership of Jørgen Bjelke . His goal was to recapture Trøndelag and to defend the Norwegian border at Halden , which Charles X had demanded be turned over to Sweden as it provided both an excellent port for timber export from the newly acquired Bohuslän and a point from which further invasions could be launched . In September 1658 the new Swedish governor of Bohuslän invaded Norway with 1 , 500 men and attempted to invest Halden . The inhabitants put up a vigorous defense and the Swedes retreated to Bohuslän . Five months later , in February 1659 , the Swedes again attacked . Since the first attack , Bjelke had directed the garrison to be strengthened . Under the leadership of Tønne Huitfeldt , the Norwegians again repulsed the Swedish forces . Concurrently , Huitfeldt began construction of fortifications . Cretzenstein , later to be renamed Fredriksten , was the citadel of the fortification system . In early January of 1660 , the Swedish forces again attacked Halden ; it was to serve as the base for their advance on Akershus in Oslo . In response to a demand of surrender , Huitfeldt stated that the 2 , 100 man garrison would defend Halden to the last man . After the attempt to storm the fortifications was unsuccessful , the Swedes prepared a regular investment . Under heavy bombardment the inhabitants begged the commandant to surrender , but putting his faith in his garrison , Huitfeldt held on . On February 22 , 1660 the Swedes again were forced to retreat to Bohuslän . There they learned that Charles X had died . Peace negotiations were reopened . Although Sweden demanded that Norway vacate all land to the river Glomma , which was to serve as the new border , with the intercession of Hannibal Sehested , a separate Scandinavian treaty , the Treaty of Copenhagen , was negotiated which reaffirmed much of the Treaty of Roskilde , except that Trøndelag was returned to Norway and the island Bornholm to Denmark , and the clause closing the Sound was deleted .