Draft:Danish intervention in the Thirty Years' War

The Danish intervention in the Thirty Years' War, (Danish: Kejserkrigen, German: Niedersächsisch-Dänischer Krieg) translated to the Emperor's War and Lower Saxon-Danish War respectively, was a intervention by the Danish king, Christian IV of Denmark, in the Lower Saxon War which name the intervention sometimes also has been referred to.

Notably the intervention was not Danish, since Denmark was never officially at war because of the rigsråd's lack of support to Christian IV. Instead Christian was elected military commander (German: Kreisoberst) of the Lower Saxon Circle to command a mixed force of 20,000 mercenaries and local conscripts in May 1625. After a brief campaign in 1625, England and the Dutch Republic agreed to subsidice money to Denmark in the Treaty of the Hague. These subsidies led enabled Christian to devise a three part campaign plan. Christian would attack down the Weser, while general Ernst von Mansfeld would attack Bohemian general, Wallenstein, in Magdeburg, supported by forces of Christian of Brunswick and Maurice of Hesse-Kassel.

The three part campaign plan was a complete failure for Christian, who suffered greatly at Dessau Bridge and Lutter am Barenberge. Christian retreated to Holstein, later Jutland, both which got occupied by Imperial forces in September 1627. Having retreated to the islands, emboldened Christian wanted to launch a counter-offensive on occupied Jutland, which he thought would allow Denmark to secure an honourable peace settlement. Meanwhile the construction of an Imperial fleet began at Wismar, Greifswald, Rostock, and the Jutish port of Aalborg at the end of 1627, which threatned Danish Dominium maris baltici.

Christian IV's strategy in 1628 was to make a series of amphibious assults on Jutland and Holstein in the hopes to regain a foothold on the Danish mainland, while diversionary actions in Mecklenburg and Pomerania were to draw Wallenstein’s attentions away from Jutland and Holstein. Christian made succesful footholds on Fehmarn and Eckernförde in Holstein, yet suffered a defeat at Wolgast. Though the Danish campaign of 1628 combined with a Dano-Swedish mutual defensive pact in April 1628, which posed a threat to the empire, led to negotiations between Christian IV, Wallenstein and Johann Tserclaes. After prolonged negotiations the two parties came to a conclusion on 22 May 1629 in the Treaty of Lübeck. The treaty restored to Christian his pre-war possessions of Jutland and Holstein, yet obliged him to cede his claims to his Lower Saxon bishoprics, and to discontinue his alliances with the North German states. Additionally he was not to further interfere with imperial affairs in the future.

Danish policies in Germany
Christian IV had great political interests in northern Germany. He aimed to expand Danish commercial influence in northern Germany in the expense of the declining Hanseatic League, and was quick to claim the Elbe and Weser estuaries.

He excluded Hamburg merchants from Icelandic trade, and reversed Hanseatic trade privileges in Norway. By such means he forced Hamburg in 1603 to acknowledge Danish suzerainty, yet in 1618 the Reichskammergericht ruled that Hamburg was a 'free city' and could not be claimed by Denmark. In response Christian founded Glückstadt, which gradually reduced maritime trafic between the North Sea and Hamburg, and Hamburg again surrendered its sovereignty to Christian in June 1621.

Also in 1621, the Prince-Archbishopric of Bremen joined Denmark's political orbit. Christian's son, Frederick, had become administrator of the archbishopric through bribery. Additionally Frederick became administrator of Verden in 1623, and Halberstadt in 1624. For his youngest son, Ulrik, he accuired Schwerin and a canonry in Bremen. Though not all of Christian's attempts to expand commercial and dynastic influence in northern Germany succeded. He failed to secure Cammin for Ulrik, and in 1625 failed to buy Rügen for 150.000 Danish rigsdaler.