Fearing that he might attempt to close the passage of the Sound, the States-General and the Swedish Regency in 1640 concluded a treaty "for securing the freedom and protection of shipping and commerce in the Baltic and North Seas"; and one of the secret articles gave permission to Sweden to buy or hire ships in the Netherlands and in case of necessity to enlist crews for the same.

Outward peace was precariously maintained between the Scandinavian powers, when the seizure of a number of Swedish ships in the Sound in 1643 made Oxenstierna resolve upon a bold stroke.

Without any declaration of war the Swedish general, Torstensson, was ordered to lead his victorious army from North Germany into Denmark and to force King Christian to cease intriguing with the enemy.

Holstein, Schleswig and Jutland were speedily in Torstensson's hands, but the Danish fleet was superior to the Swedish, and he could make no further progress.

Both sides turned to the United Provinces.