You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
alternatehistory.com
In the peace of Roskilde in 1658, Sweden conquered the Scanian lands, Bohuslän, Bornholm and Trondheim (as well as permanently gaining Halland, which had earlier been "borrowed" for 30 years in the peace of Brömsebro in 1645). In the peace of Copenhagen in 1660, Sweden would give Bornholm and Trondheim back to Denmark, while keeping the Scanian lands and Bohuslän.
The question is, what would happen if Sweden kept the Norwegian province of Trondheim (aka Trøndelag)?
This would give Sweden direct access to the Atlantic sea (granted, Sweden's west coast gives direct access as well, but then you have to go through the Skagerak sound which is surrounded by Denmark and Norway..). It would also be a staging point for further conquest of Norwegian lands, perhaps even the North Sea territories of Iceland and Faroe Islands.