Denmark–Norway has a reputation as a relatively powerful kingdom at this time.

European politics of the sixteenth century had revolved largely around the struggle between Catholic and Protestant forces, so it had seemed almost inevitable that Denmark, a strong, unified Lutheran kingdom, would get drawn into the larger war when it came.

The Thirty Years' War goes badly for the Protestant states in the early 1620s, and a call goes out to Denmark–Norway to "save the Protestant cause".

King Christian IV, who is also a duke of the Holy Roman Empire on the basis of his possessions in Holstein, decides to intervene in the conflict raging in northern Germany.

The campaign ends in defeat, and Jutland is occupied by the imperial army of Albrecht von Wallenstein.

In the Treaty of Lübeck, Christian makes peace and agrees to not intervene in Germany again.

The war in Germany has been very expensive and Christian IV sees no other recourse than to raise the Sound tolls.

Unfortunately, this act pushes the Netherlands away from Denmark and into the arms of Sweden.