THE WARS OF GERMAN UNIFICATION: PRUSSIA’S ROAD TO EMPIRE
Part 1 - The Danish Wars (1864): The Schleswig-Holstein Question and the First Steps Towards German Unification
How a dispute over two duchies brought Prussia and Austria together against Denmark, and ultimately helped set the stage for the creation of the German Empire.
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A Dispute that Helped Change the Map of Europe
2. The Second Schleswig War, 1864
ABOVE: King Christian IX of Denmark
Between 1864 and 1871, three wars transformed Prussia from a powerful but divided German kingdom into the centre of a new European empire. In Denmark, Prussia and Austria defeated a small neighbour and gained control of Schleswig-Holstein. In 1866, Prussia turned on its former ally, crushed Austria in seven weeks, and excluded it from German affairs. In 1870–71, Prussia led the German states against France, defeated Napoleon III, and proclaimed a German emperor at Versailles.
This three-part series follows the political manoeuvres, battlefield decisions, and human costs behind those victories. It examines how Bismarck used diplomacy and war to strengthen the Prussian monarchy, how new military organisation and technology changed the battlefield, and how the creation of the German Empire reshaped the balance of power in Europe.
A Dispute that Helped Change the Map of Europe
In February 1864, Prussian and Austrian soldiers crossed the border into Schleswig, beginning a war against Denmark that would have consequences far beyond the small territories at its centre. The immediate dispute concerned the political future of Schleswig and Holstein, two duchies situated between Denmark and the German states. Yet beneath the arguments over borders, constitutions and royal inheritance lay a much larger question: who had the right to determine the future of the German-speaking lands of Central Europe?
The conflict brought together two powerful German states, Prussia and Austria, which had long competed for influence within the German Confederation. For the moment, they were allies. Their combined armies defeated Denmark, and the Danish monarchy was forced to surrender Schleswig, Holstein and Lauenburg. However, victory did not settle the future of the conquered territories. Instead, disagreements over their administration created fresh tensions between the two victors.
Those tensions would contribute directly to the Austro-Prussian War of 1866, when the former allies turned against one another. Prussia's victory in that conflict removed Austria from German political affairs and cleared the way for the creation of a Prussian-led German state.
The Danish Wars therefore occupy an important place in the story of German unification. To understand the events of 1864, however, it is necessary to go back several decades, when nationalism, constitutional reform and competing claims to royal territory first brought Denmark and the German states into conflict.
1. What Led to the Wars?
1.1 The Schleswig-Holstein Question
The Schleswig-Holstein question was one of the most complicated political disputes in nineteenth-century Europe. Its difficulty arose partly from geography, but more importantly from the unusual constitutional arrangements that connected the duchies to the Danish monarchy and the German political world.
Schleswig and Holstein lay immediately south of Denmark. They were historically linked to the Danish crown, but they were not simply ordinary provinces of the Danish kingdom. Each possessed its own institutions, legal traditions and political privileges, while the Danish monarch also held the title of Duke of Schleswig and Duke of Holstein.
There was, however, an important distinction between the two territories. Schleswig had strong historical, cultural and political connections with Denmark and the German lands, but it was not a member of the German Confederation. Holstein, by contrast, belonged to the German Confederation, the loose association of German states established in 1815 after the defeat of Napoleon. The smaller Duchy of Lauenburg, further south, was also connected to the Danish monarchy and belonged to the Confederation.
The population of the duchies added another complication. Schleswig contained both Danish-speaking and German-speaking communities, with Danish speakers particularly numerous in the north and German speakers more prominent in the south. There were also areas where the two languages were used alongside one another. Holstein was overwhelmingly German-speaking.
These distinctions mattered because nationalism was becoming an increasingly powerful political force throughout Europe. During the first half of the nineteenth century, nationalists increasingly argued that people sharing a language, culture and historical identity should belong to the same political state.
In Denmark, national liberals wanted Schleswig incorporated more closely into the Danish kingdom. Their preferred national boundary was the River Eider, which separated Schleswig from Holstein. Their political slogan, often expressed as "Denmark to the Eider", reflected this ambition.
German nationalists took the opposite position. They wanted Schleswig and Holstein to remain united, arguing that their historical and constitutional relationship entitled them to form part of a German political community. Their cause was associated with the traditional principle Up ewig ungedeelt, meaning "Forever undivided".
The principle had its origins in the historical relationship between the two duchies and became an important rallying cry for German nationalists.
Neither position provided an easy solution. Schleswig was not exclusively Danish or German, and the historical rights claimed by the various parties did not fit neatly with the growing idea of national self-determination.
The Schleswig-Holstein question was therefore more than a territorial disagreement. It brought together competing ideas about national identity, constitutional government, royal inheritance and the political future of Central Europe.
1.2 The Revolutions of 1848 and the First Schleswig War
The immediate crisis developed during the revolutionary upheavals of 1848, when demands for constitutional government and national independence spread across much of Europe.
In Denmark, opposition to absolute monarchy had been growing, and national liberals wanted a constitution that would bring Schleswig more closely into the kingdom. The death of King Christian VIII in January 1848 and the accession of his son, Frederick VII, provided an opportunity for political change.
In March 1848, revolutionary demonstrations and political pressure brought an end to Denmark's absolute monarchy. A new government, strongly influenced by national liberal ideas, came to power.
Developments in the duchies moved in the opposite direction. German-minded representatives rejected the prospect of Schleswig being incorporated into Denmark and established a provisional government at Kiel. They sought to unite Schleswig and Holstein under a constitutional arrangement and gain support from the wider German national movement.
The dispute soon became an armed conflict.
The First Schleswig War, also known as the Three Years' War, lasted from 1848 until 1851. It began as a conflict between Denmark and the insurgent Schleswig-Holstein movement, but the involvement of Prussia and other German states turned it into a much wider confrontation.
The opening military operations initially favoured Denmark. Danish forces defeated the Schleswig-Holstein army at Bov on 9 April 1848. However, German Confederation support and the intervention of Prussian troops changed the military balance.
At the Battle of Schleswig on 23 April 1848, the German-backed forces defeated the Danish army and pushed it northwards. The fighting demonstrated that the dispute could no longer be treated as a purely domestic matter within the Danish monarchy.
Prussian intervention was particularly important. Prussia was the strongest German military power after Austria, and its involvement gave the Schleswig-Holstein movement access to resources that it could not have provided for itself.
Nevertheless, the European powers were concerned about the possibility of a wider conflict. Russia, in particular, opposed the expansion of the German national movement at the expense of the Danish monarchy and exerted pressure on Prussia to withdraw.
An armistice was arranged at Malmö in August 1848. It temporarily suspended hostilities but failed to settle the underlying constitutional and national questions.
Fighting resumed in 1849. Danish forces suffered setbacks, including the loss of the naval engagement at Eckernförde in April, but they subsequently achieved important successes. One of the most significant was the Danish capture of the fortress of Fredericia in July 1849, following a successful attack against the besieging Schleswig-Holstein forces.
Prussia's position was increasingly influenced by diplomatic pressure from Russia and the desire to avoid a broader European confrontation. In July 1850, Prussia concluded peace with Denmark, ending its direct involvement in the conflict.
The Schleswig-Holstein forces continued fighting independently, but they were defeated at the Battle of Idstedt on 25 July 1850. The battle was one of the largest and bloodiest engagements of the First Schleswig War and marked a decisive turning point in Denmark's favour.
The remaining resistance gradually collapsed, and the Schleswig-Holstein forces finally laid down their arms in early 1851.
The Danish monarchy had survived, and the attempt to establish an independent Schleswig-Holstein state had failed. However, the victory did not resolve the fundamental constitutional problem.
1.3 The London Protocols of 1851–1852
The European powers now sought to establish a settlement that would preserve the Danish monarchy while preventing the duchies from becoming the source of another international crisis.
The London agreements of 1851–1852 recognised the integrity of the Danish monarchy and addressed the complicated question of royal succession.
Frederick VII had no children, and the future of the monarchy depended upon establishing who would inherit the throne. The agreements recognised Prince Christian of Glücksburg as the future successor, providing an internationally supported solution to the succession problem.
The settlement also sought to preserve the duchies' constitutional position. Schleswig was not to be incorporated outright into the Danish kingdom, while Holstein retained its separate relationship with the German Confederation.
The arrangement was intended to maintain the Danish monarchy as a composite state, consisting of the kingdom and its associated duchies.
However, the agreement did not remove the competing national ambitions that had caused the First Schleswig War. Danish national liberals continued to favour closer integration of Schleswig, while German nationalists remained committed to the traditional unity of Schleswig and Holstein.
The settlement had therefore postponed the problem rather than solved it.
1.4 A New Constitutional Crisis
During the 1850s, the Danish government attempted to establish a workable constitutional arrangement for the entire monarchy.
These efforts encountered persistent opposition, particularly from German representatives in Holstein and from the German Confederation. Attempts to introduce a common constitutional framework were challenged because they were considered incompatible with the duchies' separate legal status.
By the early 1860s, the Danish national liberal movement had regained considerable influence. Its supporters increasingly believed that the constitutional settlement established after the First Schleswig War had prevented Denmark from securing its legitimate national interests.
They favoured the closer incorporation of Schleswig into Denmark, while allowing Holstein to retain a separate constitutional arrangement.
The political situation became increasingly difficult after Frederick VII died on 15 November 1863. He was succeeded by Christian IX, the first Danish monarch from the Glücksburg branch of the royal family.
Only three days after Frederick's death, Christian IX signed the November Constitution, which provided for a common constitutional arrangement covering Denmark and Schleswig, while Holstein was to be treated separately.
The decision was politically popular among Danish nationalists, but it provoked strong opposition among German nationalists and the governments of the German Confederation.
The constitution was regarded by the German side as a breach of the international arrangements established in 1851–1852. The German Confederation consequently authorised federal military action against Holstein and Lauenburg, and Saxon and Hanoverian forces occupied those territories in December 1863.
Prussia and Austria went further. They regarded the constitutional dispute as an opportunity to intervene directly and issued Denmark with an ultimatum demanding the withdrawal of the November Constitution.
The Danish government was given only a very short period to comply. Its refusal to abandon the constitution brought the two German powers into direct military confrontation with Denmark.
1.5 Bismarck and the Road to War
By 1863, the Prussian government was led by Otto von Bismarck, who had become Minister-President in 1862.
Bismarck was determined to strengthen Prussia's position within Germany and was prepared to use diplomacy, political pressure and military force to achieve that objective.
He recognised that the Schleswig-Holstein crisis offered an opportunity to advance Prussian interests. However, he also understood that acting alone could expose Prussia to diplomatic isolation or intervention by other European powers.
Cooperation with Austria was therefore important.
The two powers had competing ambitions within the German Confederation, but they could agree that Denmark should not be allowed to impose its preferred constitutional settlement on the duchies.
Bismarck was also careful to present the intervention as a response to Denmark's violation of existing agreements rather than simply as an attempt by Prussia to acquire territory.
This was politically useful. It gave Prussia and Austria a stronger diplomatic justification for military action and made it more difficult for other European powers to support Denmark openly.
The ultimatum was issued in January 1864. Denmark did not withdraw the November Constitution within the required period, and Prussian and Austrian troops crossed into Schleswig on 1 February.
The Second Schleswig War had begun.
ABOVE: Otto von Bismarck
2. The Second Schleswig War, 1864
2.1 The Opposing Armies
The Danish army entered the war with about 38,000 men organised in four divisions. Its strength lay in disciplined infantry and strong defensive positions, but it was too small to defend the whole frontier of Jutland and the islands against a coalition of Prussia and Austria.
Prussia fielded roughly 38,400 troops, while Austria contributed about 23,000. The allied force therefore enjoyed a substantial numerical advantage from the outset. More importantly, it possessed better railway links, a more developed staff system, and greater freedom to choose where and when to attack.
The Prussian infantry carried the Dreyse needle gun, a breech-loading rifle. A soldier could load and fire from a prone or kneeling position, whereas soldiers armed with muzzle-loading rifles had to stand or kneel to reload. This gave Prussian infantry a major advantage in fire discipline and volume of fire, especially in open ground.
But the needle gun was not a wonder weapon. Its effective range was shorter than that of many contemporary muzzle-loading rifles, and Danish troops armed with rifled muzzle-loaders could inflict serious losses at longer range. The Prussian army had been undergoing major reform. The decisive Prussian advantage was the combination of the rifle with a professional general staff, staff planning, modern artillery, improved mobilisation, railways, and a clear operational plan.
The principal commanders included:
Friedrich von Wrangel, the Prussian field marshal who held overall command of the allied forces during the opening campaign.
Prince Friedrich Karl of Prussia, who commanded important Prussian formations and would later become one of the leading Prussian commanders.
Ludwig von Gablenz, the Austrian general commanding the Austrian contingent.
Christian de Meza, the Danish commander responsible for the army's initial defensive arrangements.
Georg Gerlach, who subsequently assumed command of the Danish forces.
The campaign would demonstrate the growing military strength of Prussia, although it would be a mistake to attribute the eventual victory solely to superior weapons or training. The Danish army faced difficult strategic circumstances and was often compelled to fight from positions that could be outflanked or overwhelmed.
2.2 The Dannevirke: Denmark Abandons Its Historic Defence
The opening objective of the German forces was to drive the Danish army out of Schleswig.
Prussian and Austrian forces crossed into Schleswig on 1 February 1864. The allied plan was to push the Danish army out of its defensive line and force it northward, away from the fortified Danevirke position.
The Austrians advanced directly toward the Danevirke. Prussian forces attempted a more ambitious operation against Mysunde, on the coast of the Schlei inlet east of Schleswig. By crossing the frozen Schlei, they hoped to turn the Danish flank and bypass part of the Danish position.
The Mysunde attack failed. Danish defenders held their positions for about six hours, and the Prussians withdrew. The episode matters because it shows that the campaign did not begin as an unstoppable Prussian advance. Danish troops could resist effectively when they occupied prepared ground and were not outflanked.
The fighting around Königshügel, near the Danevirke, also demonstrated the difficulty of frontal attacks. Danish infantry sortied from the line and engaged Prussian forces in the open. These actions delayed the allies and imposed costs, but they could not alter the basic strategic problem: the Danish army was defending a long line with insufficient manpower.
The Dannevirke, or Danevirke, was the most important Danish defensive position in the south. It consisted of a long system of earthworks and fortifications extending across the Jutland peninsula between the North Sea and the Baltic approaches.
The defences had great historical and symbolic importance. They had been constructed and extended over many centuries to protect the southern approaches to Denmark.
For generations, the Dannevirke had been regarded as a powerful symbol of Danish national security. Its importance in the public imagination was considerably greater than its actual military capabilities in 1864.
The advancing Prussian and Austrian armies greatly outnumbered the Danish defenders. Furthermore, the winter weather created unexpected opportunities for the attackers to manoeuvre around the defensive system.
Danish commander Christian de Meza recognised that the position could not be held safely. Remaining there risked allowing the German forces to outflank the Danish army and cut off its retreat.
On 5–6 February 1864, the Danish army abandoned the Dannevirke and withdrew northwards towards positions at Dybbøl and Fredericia.
The withdrawal was conducted in difficult winter conditions, and the Danish troops suffered considerable hardship. Although the decision saved the army from possible encirclement, it provoked anger in Denmark.
The abandonment of the Dannevirke was widely regarded as a national humiliation. De Meza was subsequently removed from command, despite the fact that his decision had sound military justification.
This episode illustrates an important problem facing governments during wartime: military decisions are not always politically acceptable, particularly when national pride and public expectations are involved.
ABOVE: The Danish Retreat from the Dannevirke Defences
2.3 The Battle of Sankelmark, 6 February 1864
During the Danish retreat, Austrian troops encountered a Danish rearguard near Sankelmark.
The engagement was a hard-fought action in which the Danish troops attempted to delay the pursuing forces and protect the withdrawal of the main army.
The Austrians eventually forced the Danes back, but the engagement demonstrated that the Danish army remained capable of determined resistance.
Although Sankelmark was not decisive in determining the outcome of the war, it deserves attention because it illustrates the difficult conditions under which the Danish army conducted its retreat.
2.4 Fredericia and the Western Flank
While the main armies confronted one another in Schleswig, fighting also spread along the western side of the Jutland peninsula. The fortress of Fredericia, on the east coast of Jutland, became an important Danish position.
Austrian forces under Lieutenant Field Marshal Ludwig von Gablenz advanced toward Fredericia in March. Danish troops fought delaying actions, including the Battle of Vejle on 8 March, as the Austrians brought up artillery and prepared to besiege the fortress.
The Danish command eventually evacuated Fredericia rather than allow its garrison to be trapped. The evacuation was another painful retreat, but it followed the same logic as the abandonment of the Danevirke: preserve troops rather than sacrifice them in a fortress that could be surrounded.
2.5 The Siege of Dybbøl
Following the withdrawal from the Dannevirke, the principal Danish defensive position became the fortified area around Dybbøl, close to the Alssund strait, opposite the island of Als.
The Dybbøl fortifications consisted of ten redoubts arranged in a defensive line. They were intended to protect the approaches to the island and prevent an enemy from advancing further into Jutland. The ground was open in places, but the defenders had prepared trenches, artillery positions, and infantry shelters.
However, the position was not adequately prepared to withstand a prolonged attack by modern artillery.
The Prussians gradually strengthened their positions and constructed trenches closer to the Danish defences. They also brought forward rifled artillery, which had a greater effective range than much of the Danish artillery.
From March onwards, the Prussian bombardment increasingly damaged the Danish fortifications.
The Danish defenders were forced to endure repeated artillery attacks while attempting to repair their positions and maintain their fighting strength.
The bombardment intensified during April. The situation became increasingly desperate, and the Danish military leadership recognised that the position was unlikely to withstand a determined assault.
Nevertheless, the Danish government was reluctant to order a withdrawal. Prime Minister Ditlev Gothard Monrad believed that abandoning Dybbøl without a major engagement would damage Denmark's position in the international peace negotiations.
The decision was controversial, particularly because the military situation was becoming increasingly unfavourable.
2.6 The Battle of Dybbøl, 18 April 1864
The decisive assault began early on the morning of 18 April. The Prussian forces had assembled at their various positions by 2 am.
After a prolonged artillery bombardment, Prussian forces launched an attack against the Danish redoubts.
The bombardment had severely damaged the defences, and the Danish troops were exhausted by weeks of fighting and exposure to artillery fire.
At approximately 10 am, the Prussian infantry advanced against the positions. The assault was concentrated against the southern and central sections of the Danish defensive line. The assault achieved surprise. The Danish defences in the main attack sector were undermanned: roughly 2,200 Danish soldiers faced about 11,000 Prussian storm troops. Prussian infantry crossed the open ground, entered the redoubts, and overwhelmed the defenders in close fighting.
Danish troops attempted to resist and launched counterattacks, but the German numerical and artillery advantages proved decisive.
One particularly determined Danish counterattack was made by the 8th Brigade, which advanced against the Prussian positions near Dybbøl Mill. The effort failed, and the remaining Danish forces were forced to withdraw towards Als.
By early afternoon, the Prussians had secured the fortifications.
The Danish defeat was severe. Thousands of soldiers were killed, wounded, missing or captured, and the loss of Dybbøl removed one of Denmark's most important defensive positions. Casualty figures vary according to what is counted, but the Danish losses were severe. Danish official figures for 18 April record 379 killed, 646 missing, about 1,250 wounded, and roughly 2,500 taken prisoner. Prussian losses were about 1,200 killed and wounded.
The battle became an enduring symbol of Danish national history. Its significance was not confined to the military defeat. It represented the failure of Denmark's attempt to preserve the duchies and defend its traditional southern frontier.
For Prussia, Dybbøl was a major military success. It also demonstrated the effectiveness of modern artillery, careful preparation and coordinated infantry assaults.
However, the battle should not be reduced to a simple story of superior Prussian weapons defeating an inferior Danish army. The defenders fought with considerable determination, while the outcome was strongly influenced by the attackers' numerical superiority, artillery advantage and the inadequate condition of the Danish fortifications.
Dybbøl fell for several reasons.
Concentration of force: Prussia concentrated about 11,000 infantry against a thinly held section of the Danish line.
Artillery preparation: The bombardment damaged earthworks, destroyed or silenced some Danish guns, and exhausted defenders before the infantry assault.
Local superiority: The Danish army could not match Prussian numbers at the decisive point.
The needle gun: In the final close-range fighting, Prussian infantry could deliver rapid fire while advancing or taking cover.
Danish strategic weakness: Denmark had to defend a long frontier and could not concentrate enough troops at Dybbøl without weakening other sectors.
The victory did not prove that Danish soldiers were inferior. It showed what happened when a well-prepared attacking army concentrated overwhelming force against a vulnerable section of a defensive line.
ABOVE: The Prussian storming of Dybbøl on 18 April 1864
2.7 The Occupation of Jutland
The fall of Dybbøl did not immediately end the war.
Prussian and Austrian forces continued their advance into Danish territory. The Austrians moved into Jutland, while the Prussians concentrated on operations against the island of Als and the remaining Danish positions.
The occupation of Jutland was intended to increase the economic and political pressure on Denmark. By taking control of additional territory, the German powers hoped to force the Danish government to accept a settlement on their terms.
The occupation also demonstrated that Denmark's military position was becoming increasingly difficult. Its army had suffered a major defeat, its principal defensive positions were threatened, and the country had little prospect of receiving effective military assistance from the European powers.
2.8 The London Conference: A Last Attempt at Peace
While fighting continued, diplomatic negotiations were taking place in London.
The British government had been concerned about the possibility of a wider European war and attempted to mediate between the combatants.
The London Conference opened in April 1864, with representatives of the principal European powers and the belligerents participating.
An armistice began on 12 May, giving the negotiators an opportunity to seek a settlement.
The central issue was whether Schleswig could be divided along national or linguistic lines.
This proposal had some support among the neutral powers. It recognised that the population of Schleswig was mixed and that a division might provide a compromise between Danish and German claims.
However, the parties could not agree on where the boundary should be drawn.
The Danish government was reluctant to surrender territory that it considered essential to the kingdom's security and national identity. The German representatives also had their own territorial and strategic demands.
Britain attempted to find a compromise, but the negotiations eventually broke down.
The armistice ended in late June, and fighting resumed.
The failure of the London Conference was an important diplomatic defeat for Denmark. It demonstrated that Britain and the other European powers were unwilling to risk a major confrontation with Prussia and Austria in order to protect the Danish monarchy.
2.9 The Battle of Heligoland, 9 May 1864
While the armies were fighting in Schleswig and Jutland, the Second Schleswig War also produced a significant naval engagement in the North Sea. On 9 May 1864, Danish and Austrian-Prussian warships met off the island of Heligoland, an island in the North Sea, then a British possession.
The Danish squadron, commanded by Commodore Edouard Suenson, consisted of two steam frigates, Niels Juel and Jylland, and the corvette Heimdal. Opposing them was an Austrian squadron under Commodore Wilhelm von Tegetthoff, comprising the frigates Schwarzenberg and Radetzky, accompanied by three smaller Prussian vessels.
The engagement developed into a fierce exchange of gunfire at relatively close range. The Danish ships concentrated much of their fire on Schwarzenberg, eventually setting fire to her rigging. With the Austrian flagship badly damaged and the situation becoming increasingly dangerous, Tegetthoff withdrew towards the neutral British waters around Heligoland. The Danish ships could not pursue him into those waters, and the fighting came to an end.
The encounter was regarded as a Danish tactical victory. Danish losses amounted to fourteen sailors killed and fifty-four wounded, while the Austrian-Prussian squadron suffered heavier casualties and damage to its ships. Suenson returned to Denmark to a hero's welcome.
Despite the success, the battle had little influence on the wider course of the war. Denmark had demonstrated that its navy remained a capable fighting force, but victory at sea could not compensate for the defeats suffered by its army. The decisive events were still unfolding on land, and Denmark's military position continued to deteriorate.
2.10 The Battle of Als, 29 June 1864
The final major operation of the war took place on the island of Als.
Following the loss of Dybbøl, Als had become one of the last important Danish defensive positions.
The island was separated from the mainland by the narrow Alssund, and the Danish army hoped that the waterway and its defensive positions would prevent a successful crossing.
The Prussians prepared a surprise assault. The Prussian command chose a crossing point at Øster Snogbæk, near the northern end of the strait. The nearby woods at Sottrup Storskov provided cover for preparations and helped conceal the operation from Danish observers.
The crossing began during the night of 28–29 June 1864. Around 2 am, about 2,500 Prussian soldiers crossed in small boats and pontoon ferries. There was no preliminary artillery bombardment; secrecy and speed mattered more than a preparatory barrage.
The Danish armoured battery Rolf Krake interfered with the crossing and forced the Prussians to pause. A Prussian gun eventually struck the vessel near its bridge, and the Danish ship withdrew. The interruption delayed but did not stop the operation.
Once Prussian troops established a foothold on Als, they built up their force and advanced against the Danish defenders. Roughly 10,000 Danish troops faced about 23,000 Prussians. Danish units fought delaying actions around places such as Kær, but they were progressively pushed back. Some Danish soldiers escaped by sea from the southern peninsula of Kegnæs; many others were captured.
The loss of the island effectively ended Denmark's ability to continue the war. Its remaining forces were no longer capable of resisting the combined Prussian and Austrian armies.
The Danish government now had little alternative but to seek peace.
2.11 The Treaty of Vienna, 30 October 1864
Negotiations eventually produced the Treaty of Vienna, signed on 30 October 1864.
Denmark renounced its claims to Schleswig, Holstein and Lauenburg, transferring the duchies to Prussia and Austria.
The war was over, but the future of the territories remained uncertain.
The two victorious powers now faced the difficult task of deciding how the duchies should be governed.
Their victory had been achieved through military cooperation, but their political interests were very different.
Prussia wanted to strengthen its position in northern Germany and extend its influence over the territories. Austria, meanwhile, wished to maintain its standing within the German Confederation and prevent Prussia from becoming the dominant German power.
The dispute over the administration of Schleswig-Holstein would soon become a serious source of conflict between the former allies.
3. The Consequences of the Wars
The 1864 campaign revealed the changing character of nineteenth-century warfare.
Prussia’s success rested on an integrated system: railways moved troops and supplies; the general staff coordinated movements; artillery prepared positions for infantry; and breech-loading rifles increased infantry firepower. Victory came not from one invention but from the combination of these elements.
Denmark’s defeat rested on a different combination: numerical inferiority, diplomatic isolation, a frontier that was difficult to defend, and the need to preserve its field army rather than risk it in a single decisive battle. Danish troops fought effectively at Mysunde, during the retreat from the Danevirke, and in the fighting around Fredericia and Als. But effective local resistance could not compensate for the allies’ strategic advantage.
The war also gave Prussia a rehearsal for 1866. The same ingredients that defeated Denmark, including rapid mobilisation, staff planning, railway movement, and concentrated force, would be used on a far larger scale against Austria.
3.1 Denmark: The Loss of the Duchies
The consequences for Denmark were severe.
The loss of Schleswig, Holstein and Lauenburg removed a substantial portion of the Danish monarchy's territory and population. The defeat also forced Denmark to reconsider its position as a European power.
The country had entered the war hoping to preserve its constitutional arrangements and national interests. It emerged substantially reduced in size and influence.
The loss of Schleswig was particularly painful because the duchy contained communities with strong Danish cultural and linguistic connections.
The defeat also had a lasting effect on Danish national consciousness. The memory of 1864 became closely associated with the loss of territory, the failure of political leadership and the sacrifices made by Danish soldiers.
Dybbøl became one of the most important symbols of the conflict, representing both national defeat and the determination of the Danish army to resist.
In the longer term, Denmark increasingly concentrated on consolidating itself as a smaller national state rather than attempting to maintain its former composite monarchy.
ABOVE: Map Showing the New Reality Following the War
3.2 Prussia: A Major Increase in Power
For Prussia, the war represented an important political and military success.
It had demonstrated its ability to defeat a European power in cooperation with Austria, while securing greater influence over the future of the duchies.
The acquisition of Schleswig was particularly important because it strengthened Prussia's position in northern Germany and provided greater control over the approaches to the Baltic Sea.
However, Prussia's ambitions extended beyond the duchies themselves.
Bismarck wanted to establish Prussia as the leading political power within Germany. The Danish victory helped advance that objective, but it also brought Prussia into a more direct struggle with Austria.
The two powers had cooperated because they shared an immediate objective. Once Denmark had been defeated, their competing ambitions became more difficult to reconcile.
3.3 Austria: An Alliance That Would Not Last
Austria had participated in the war partly to preserve its influence within the German Confederation and prevent Prussia from gaining exclusive control over the German national movement.
Its military contribution had been significant, and Austrian troops had fought alongside the Prussians throughout the campaign.
However, Austria and Prussia disagreed over the future administration of the duchies.
The Convention of Gastein, signed on 14 August 1865, attempted to resolve the dispute by dividing administrative responsibilities.
Prussia was given responsibility for Schleswig, while Austria administered Holstein. Lauenburg was transferred to Prussia in return for financial compensation.
The arrangement appeared to offer a temporary solution, but it did little to resolve the deeper rivalry.
Prussia wanted to extend its political influence, while Austria sought to preserve its position as an equal German power.
The administration of Holstein became a particular source of tension. Disagreements over the future of the duchies, including the position of Frederick of Augustenburg, created further opportunities for conflict.
The Danish Wars had therefore produced an unexpected result. Prussia and Austria had defeated a common opponent, but the settlement created conditions that would bring them into direct conflict.
3.4 The Augustenburg Question
One issue that deserves particular attention is the position of Frederick of Augustenburg.
He represented a dynastic claim to the duchies and enjoyed considerable support among German nationalists, who believed that Schleswig-Holstein should become an independent state under his rule.
Many supporters of the German national movement had expected the defeat of Denmark to result in the establishment of an independent Schleswig-Holstein.
Bismarck, however, was reluctant to accept such an arrangement unless the new state became closely dependent upon Prussia.
He did not want an independent duchy that might restrict Prussian influence in northern Germany or become a political obstacle to his wider ambitions.
This disagreement demonstrated that the interests of German nationalists and the Prussian government were not always identical.
For many nationalists, the war had been fought to achieve the unity and independence of Schleswig-Holstein. For Bismarck, the most important consideration was the strengthening of Prussian power.
The distinction is important because it illustrates the difference between the broader German national movement and the policies of the Prussian state.
3.5 The Road to the Austro-Prussian War of 1866
The unresolved Schleswig-Holstein question became one of the immediate causes of the Austro-Prussian War of 1866.
By 1865, relations between Prussia and Austria had deteriorated considerably.
The Convention of Gastein had divided administrative control of the duchies, but neither power was satisfied with the arrangement.
Prussia wanted greater influence over the northern German states, while Austria wished to prevent Prussia from displacing it as the leading power within the German Confederation.
Bismarck increasingly viewed the rivalry with Austria as an obstacle to Prussian leadership of Germany.
In 1866, the dispute escalated into open confrontation. Prussian troops entered Holstein, and the Austrian government sought the support of the German Confederation.
The resulting war lasted only seven weeks.
Prussia's victory at Königgrätz on 3 July 1866 decisively altered the political organisation of Germany. Austria was excluded from German affairs, the old German Confederation was dissolved, and Prussia established the North German Confederation under its leadership.
The conflict of 1864 had therefore played an important part in setting the stage for the events of 1866.
4. Conclusion: The First Steps Towards German Unity
The Danish Wars were about much more than the fate of two duchies.
They exposed the difficulties of reconciling national identity, historical rights and political sovereignty in a region where Danish and German communities lived alongside one another.
The First Schleswig War demonstrated the strength of the German national movement, but also revealed its inability to achieve lasting political unity through the revolutionary and constitutional methods of 1848.
The Second Schleswig War produced a different outcome. Prussia and Austria used their combined military strength to defeat Denmark, acquire the duchies and strengthen their positions within the German political world.
Yet their victory also created a new rivalry. The former allies disagreed over the future of Schleswig-Holstein and over the wider question of who should lead Germany.
For Prussia, the war was an important step towards political dominance. For Denmark, it was a devastating national defeat. For Austria, it marked the beginning of a period in which its influence over German affairs would increasingly be challenged.
The next stage in this story would be decided not on the battlefields of Schleswig, but in the struggle between Prussia and Austria for leadership of Germany.
In 1866, that struggle would be settled by war.
The first step towards German unification had been taken. The decisive contest for control of Germany was still to come.
Updated: 2 NOVEMBER 2026
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