After Frederick's deposition in 1623, John George of Saxony and the Calvinist George William, Elector of Brandenburg became concerned Ferdinand intended to reclaim formerly Catholic bishoprics currently held by Protestants. As Duke of Holstein, Christian IV was also a member of the Lower Saxon circle, while the Danish economy relied on the Baltic trade and tolls from traffic through the Øresund.

Ferdinand had paid Albrecht von Wallenstein for his support against Frederick with estates confiscated from the Bohemian rebels, and now contracted with him to conquer the north on a similar basis. In May 1625, the Lower Saxony kreis elected Christian their military commander, although not without resistance; Saxony and Brandenburg viewed Denmark and Sweden as competitors, and wanted to avoid either becoming involved in the Empire. Attempts to negotiate a peaceful solution failed as the conflict in Germany became part of the wider struggle between France and their Habsburg rivals in Spain and Austria.

In the June 1624 Treaty of Compiègne, France had agreed to subsidise the Dutch war against Spain for a minimum of three years, while in the December 1625 Treaty of The Hague, the Dutch and English agreed to finance Danish intervention in the Empire.