In the Second Treaty of Brömsebro (1645), which concluded the war, Denmark-Norway had to make huge territorial concessions and exempt Sweden from the Sound Dues, de facto acknowledging the end of the Danish-Norwegian dominium maris baltici.

In the spring of 1643 the Swedish Privy Council determined that their military strength made territorial gains at the expense of Denmark-Norway likely.

Proceeding from Moravia, his forces entered Danish territory in Holstein on 12 December and by the end of January 1644 the Jutland peninsula was in his possession.

In February 1644, the Swedish General Gustav Horn with an army of 11,000 men occupied much of the Danish provinces of Halland and Scania, except for the fortress town of Malmø.

This attack caught Denmark-Norway unaware and poorly prepared but King Christian IV retained his presence of mind.

He placed his confidence in the fleet to protect the home islands, just winning the Battle of Colberger Heide on 1 July 1644 but suffering a decisive defeat in the Battle of Fehmarn on 13 October 1644 against the Swedish fleet, including leased Dutch ships under Mårten Anckarhielm (previously Maerten Thijssen).

When Sehested arrived the King joined his fleet and performed heroically, even though wounded, preventing Torstensson's army from moving onto the Danish islands.

In Dalarna the landshövding (chief of the land i.e. governor) raised a host of 200 dalecarlian farmers who seized the region of Särna, making it de facto Swedish territory.