All ships sailing to the Baltic had to pass through the Sont. This narrow strait - between present day Denmark and Sweden - was the cause of several conflicts between the two countries. Because of the importance of the Baltic trade for the Dutch Republic, the Dutch got involved in these conflicts.
The Dutch traders were willing enough to pay toll to the Danish. But when the Danish king increased the charges considerably in 1644, the Dutch added war ships to their trade convoys to make the Danish change their mind... and with success.
In 1658 Sweden threatened to annex Denmark in order to get complete control over the Sont. Such a monopoly posed a threat to the Dutch Republic's trade. A Dutch war fleet, commanded by Michiel de Ruyter, came to the aid of the Danish. The sudden death of the Swedish king in 1660 brought an end to the threatening situation. Dutch ships could again sail to the Baltic undisturbed.