Both sides of the Oresund or Sound strait that separated the Baltic from the north sea were owned by the Danish crown. The Sound dues made the Danish king relatively independent from the nobility and tax revenues. To firm his grip on this strategic position, the king built fortesses on both sides on the river, including the impressive Kronburg castle.
For the Dutch republic, the Baltic trade route became the most important of them all. Poland supplied the country of grain, Sweden supplied iron and wood for ships.
When in 1642 the Danish king decided to drastically raise the due tarrifs, the Dutch decided to display their power. They send a fleet of 42 warships to escort a convoy of 900 merchantmen and send it though the strait. The next year, an even bigger fleet crossed the straight without paying at all.
This display of force, together with some skillful diplomacy of fleet commander Witte de With made the the Danish king to grant the Dutch a very favourable treaty. The two nations would remain allies for the rest of the century.
This picture shows a fleet of fluyts passing the Sound in 1644. Right is the Danish castle Kronburg.
Far right is the flagship Brederode commanded by Witte de With.