Editing Thirty Years' War (section)

=== Danish intervention (1625–1629) ===
{{Location map many |Lower Saxony |caption=Key locations, 1625–1629 on the map of the modern federal state [[Lower Saxony]] |border=black |width=280 |float=right |relief=yes
|label=Bremen |pos=top |lat_deg=53.083333 |lon_deg=8.8
|label2=Osnabrück |pos2=left |lat2_deg=52.2833 |lon2_deg=8.1464
|label3=Halberstadt |pos3=bottom |lat3_deg=51 |lat3_min=54 |lon3_deg=11 |lon3_min=03
|label4=Lübeck (Duchy of Holstein) |pos4=bottom |lat4_deg=53 |lat4_min=52 |lon4_deg=10 |lon4_min=41
|label5=Magdeburg |pos5=left |lat5_deg=52 |lat5_min=08 |lon5_deg=11 |lon5_min=37
|label6=Hamburg |pos6=left |lat6_deg=53 |lat6_min=35 |lon6_deg=10 |lon6_min=00
|label7=Lutter |pos7=left |lat7_deg=52 |lat7_min=00 |lon7_deg=10 |lon7_min=16
|label8=Verden |pos8=bottom |lat8_deg=52 |lat8_min=55 |lon8_deg=9 |lon8_min=14
|label9=Kassel |pos9=left |lat9_deg=51 |lat9_min=19 |lon9_deg=9 |lon9_min=30
|label10=Wolfenbüttel |pos10=left |lat10_deg=52.262222|lon10_deg=10.636944
}}

With Saxony dominating the [[Upper Saxon Circle]] and [[Brandenburg]] the [[Lower Saxon Circle|Lower]], both ''kreise'' had remained neutral during the campaigns in Bohemia and the Palatinate. However, Frederick's deposition in 1623 meant 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. These fears seemed confirmed when Tilly restored the [[Roman Catholic Diocese of Halberstadt]] in early 1625.{{Sfn|Wedgwood|1938|pp=179–181}}

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]].{{Sfn|Lockhart|2007|pp=107–109}} In 1621, Hamburg accepted Danish "supervision", while his son [[Frederick III of Denmark|Frederick]] became joint-administrator of [[Prince-bishopric of Lübeck|Lübeck]], [[Prince-Archbishopric of Bremen|Bremen]], and [[Prince-Bishopric of Verden|Verden]]; possession ensured Danish control of the [[Elbe]] and [[Weser]] rivers.{{Sfn|Murdoch|2000|p=53}}

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.{{Sfn|Wilson|2009|p=387}}

In the June 1624 [[Treaty of Compiègne (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 (1625)|Treaty of The Hague]], the Dutch and English agreed to finance Danish intervention in the Empire.{{Efn|As well as being brother-in-law to Frederick of the Palatinate, James I was also linked to Christian IV of Denmark, having married his elder sister [[Anne of Denmark]] (1574–1619).{{Sfn|Wilson|2009|p=382}}}} Hoping to create a wider coalition against Ferdinand, the Dutch invited France, Sweden, Savoy, and the [[Republic of Venice]] to join, but it was overtaken by events.{{Sfn|Davenport|1917|p=295}} In early 1626, [[Cardinal Richelieu]], main architect of the alliance, faced a new Huguenot rebellion at home and in the March [[Treaty of Monzón]], France withdrew from northern Italy, re-opening the Spanish Road.{{Sfn|Wedgwood|1938|p=208}}

[[File:Danish phase Bul Map.png|thumb|left|upright=1.0|Danish intervention]]

Dutch and English subsidies enabled Christian to devise an ambitious three part campaign plan; while he led the main force down the Weser, Mansfeld would attack Wallenstein in [[Archbishopric of Magdeburg|Magdeburg]], supported by forces led by Christian of Brunswick and [[Maurice, Landgrave of Hesse-Kassel|Maurice of Hesse-Kassel]]. The advance quickly fell apart; Mansfeld was defeated at [[Battle of Dessau Bridge|Dessau Bridge]] in April, and when Maurice refused to support him, Christian of Brunswick fell back on [[Wolfenbüttel]], where he died of disease shortly after. The Danes were comprehensively beaten at [[Battle of Lutter|Lutter]] in August, and Mansfeld's army dissolved following his death in November.{{Sfn|Wedgwood|1938|p=212}}

Many of Christian's German allies, such as [[Landgraviate of Hesse-Kassel|Hesse-Kassel]] and Saxony, had little interest in replacing imperial domination with Danish, while few of the subsidies agreed to by the Treaty of The Hague were ever paid. [[Charles I of England]] allowed Christian to recruit up to 9,000 Scottish mercenaries, but they took time to arrive, and while able to slow Wallenstein's advance were insufficient to stop him.{{Sfn|Murdoch|Grosjean|2014|pp=43–44}} By the end of 1627, Wallenstein occupied [[Mecklenburg]], [[Capitulation of Franzburg|Pomerania]], and [[Jutland]], and began making plans to construct a fleet capable of challenging Danish control of the Baltic. He was supported by Spain, for whom it provided an opportunity to open another front against the Dutch.{{Sfn|Wilson|2009|p=426}}

On 13 May 1628, his deputy [[Hans Georg von Arnim-Boitzenburg|von Arnim]] besieged [[Siege of Stralsund (1628)|Stralsund]], the only port with facilities large enough to build this fleet. However, this threat led [[Gustavus Adolphus]] to send several thousand Scots and Swedish troops to Stralsund, commanded by [[Alexander Leslie]] who was also appointed governor.{{Sfn|Murdoch| Grosjean|2014|pp=48–49}} Von Arnim was forced to lift the siege on 4 August, but three weeks later, Christian suffered another defeat at [[Battle of Wolgast|Wolgast]]. He began negotiations with Wallenstein, who despite his recent victories was concerned by the prospect of Swedish intervention, and thus anxious to make peace.{{Sfn|Lockhart|2007|p=170}}

[[File:Albrecht von Wallenstein.jpeg|thumb|right|upright=0.7|[[Albrecht von Wallenstein]] achieved great military success for the Empire but his power threatened both Ferdinand and the German princes.]]

With Austrian resources stretched by the outbreak of the [[War of the Mantuan Succession]], Wallenstein persuaded Ferdinand to agree with relatively lenient terms in the June 1629 [[Treaty of Lübeck]]. Christian retained his German possessions of [[Duchy of Schleswig|Schleswig]] and Holstein, in return for relinquishing Bremen and Verden, and abandoning support for the German Protestants. While Denmark kept Schleswig and Holstein until 1864, this effectively ended its reign as the predominant Nordic state.{{Sfn|Lockhart|2007|p=172}}

Once again, the methods used to obtain victory explain why the war failed to end. Ferdinand paid Wallenstein by letting him confiscate estates, extort ransoms from towns, and allowing his men to plunder the lands they passed through, regardless of whether they belonged to allies or opponents. In early 1628, Ferdinand deposed the hereditary [[Adolf Frederick I, Duke of Mecklenburg|Duke of Mecklenburg]], and appointed Wallenstein in his place, an act which united all German princes in opposition, regardless of religion. This unity was undermined by Maximilian of Bavaria's desire to retain the Palatinate; as a result, the Catholic League argued only for a return to the position prevailing pre-1627, while Protestants wanted that of 1618.{{Sfn|Wedgwood|1938|pp=232–233}}

Made overconfident by success, in March 1629 Ferdinand passed an [[Edict of Restitution]], which required all lands taken from the Catholic church after 1555 to be returned. While technically legal, politically it was extremely unwise, since doing so would alter nearly every single state boundary in north and central Germany, deny the existence of Calvinism and restore Catholicism in areas where it had not been a significant presence for nearly a century. Well aware none of the princes involved would agree, Ferdinand used the device of an imperial [[edict]], once again asserting his right to alter laws without consultation. This new assault on "German liberties" ensured continuing opposition and undermined his previous success.{{Sfn|Wedgwood|1938|pp=242–244}}

At the same time, his Spanish allies were reluctant to antagonise German Protestants as their [[Eighty Years' War, 1621–1648|war in the Spanish Netherlands]] had now shifted in favour of the Dutch Republic. The financial predicament of the Spanish Crown steadily deteriorated in the 1620s, particularly after the [[Dutch West India Company]] captured their [[Spanish treasure fleet|treasure fleet]] at [[Battle in the Bay of Matanzas|Matanzas]] in 1628. The War of the Mantuan Succession further diverted Spanish resources from the Netherlands,{{sfn|Israel|1995b|p=497}} while the loss of [[Siege of 's-Hertogenbosch|'s-Hertogenbosch]] to the [[Dutch States Army|Dutch Army]] under [[Frederick Henry, Prince of Orange|Frederick Henry]] in 1629 caused dismay in Madrid.{{sfn|Israel|1995b|p=511}}