aty ▁of ▁Br öm se bro ▁in ▁ 1 6 4 5 , ▁Den mark – N or way ▁had ▁to ▁c ede ▁some ▁of ▁their ▁territ ories , ▁including ▁Norwegian ▁territ ories ▁J em t land , ▁Her jed alen ▁and ▁Id re ▁& ▁S erna , ▁and ▁the ▁Dan ish ▁Balt ic ▁Sea ▁islands ▁of ▁Got land ▁and ▁Ö sel . ▁The ▁Th irty ▁Years ' ▁War ▁thus ▁began ▁the ▁rise ▁of ▁Sweden ▁as ▁a ▁great ▁power , ▁while ▁it ▁marked ▁the ▁start ▁of ▁decl ine ▁for ▁the ▁Dan ish . ▁ ▁Second ▁Northern ▁Wars ▁ ▁The ▁D ano - S wed ish ▁War ▁( 1 6 5 7 – 1 6 5 8 ), ▁a ▁part ▁of ▁the ▁Second ▁Northern ▁War , ▁was ▁one ▁of ▁the ▁most ▁dev ast ating ▁wars ▁for ▁the ▁D ano - N or weg ian ▁kingdom . ▁After ▁a ▁huge ▁loss ▁in ▁the ▁war , ▁Den mark – N or way ▁was ▁forced ▁in ▁the ▁Tre aty ▁of ▁Ros k ilde ▁to ▁give ▁Sweden ▁nearly ▁half ▁its ▁territory . ▁This ▁included ▁Norwegian ▁province ▁of ▁Tr ø nd el ag ▁and ▁B å hus len , ▁all ▁remaining ▁Dan ish ▁provinces ▁on ▁the ▁Swedish ▁main land , ▁and ▁the ▁island ▁of ▁Born holm . ▁ ▁However , ▁two ▁years ▁later , ▁in ▁ 1 6 6 0 , ▁there ▁was ▁a ▁follow - up ▁treat y , ▁the ▁Tre aty ▁of ▁C open hagen , ▁which ▁gave ▁Tr ø nd el ag ▁and ▁Born holm ▁back