Second Schleswig War (1864): Prussia and Austria vs Denmark

The Second Schleswig War was fought from February to October 1864 between Denmark and the allied German powers Prussia and Austria. It was triggered by a dispute over the duchies of Schleswig and Holstein and ended in a heavy Danish defeat. Historians usually count it as the first of Otto von Bismarck’s three wars that led to the unification of Germany.

Background: the Schleswig-Holstein question

Schleswig and Holstein were two duchies on the Jutland peninsula ruled by the King of Denmark, but not fully part of the Danish kingdom. Holstein, with a largely German-speaking population, belonged to the German Confederation; Schleswig, with a mixed Danish and German population, did not. The British statesman Lord Palmerston is often credited with the joke that only three people had ever understood the question, though the remark is not reliably documented.

A first war over the duchies in 1848–1851, part of the wider Revolutions of 1848, ended with the London Protocol of 1852. It confirmed the duchies under the Danish crown but forbade Denmark to tie Schleswig more closely to the kingdom than Holstein.

In November 1863 King Frederick VII died. His successor, Christian IX, signed the so-called November Constitution, which in effect incorporated Schleswig into Denmark. German nationalists protested, and a rival claimant, Frederick of Augustenburg, demanded the duchies. Federal troops from Saxony and Hanover occupied Holstein in December 1863.

The road to war

Bismarck, the Prussian minister-president since 1862, did not want a new small German state under Augustenburg. Instead he persuaded Austria to act jointly with Prussia as signatories of the London Protocol. On 16 January 1864 the two powers demanded that Denmark withdraw the constitution within 48 hours. The Danish government under Ditlev Gothard Monrad refused, expecting British or French help that never arrived.

On 1 February 1864 Prussian and Austrian troops crossed the River Eider into Schleswig. The allied commander was the 79-year-old Prussian field marshal Friedrich von Wrangel.

Course of the war

Denmark’s main defence was the Danevirke, an ancient earthwork line across southern Schleswig. Fearing encirclement in winter conditions, the Danish commander Christian de Meza abandoned it on the night of 5–6 February. The retreat saved the army but shocked public opinion in Copenhagen, and de Meza was dismissed.

The Danes fell back to the fortified position at Dybbøl, opposite the island of Als. Prussian forces under Prince Friedrich Karl began a siege and bombarded the redoubts for weeks with modern rifled artillery. On 18 April 1864 Prussian infantry stormed the line and took it within a few hours. Prussian losses were around 1,200 killed and wounded; Danish losses, including prisoners, were several times higher.

At sea, a Danish squadron fought Austrian ships off Heligoland on 9 May, forcing them to withdraw. A peace conference met in London from April to June with an armistice in force, but it failed. When fighting resumed, Prussian troops crossed to Als on 29 June and later advanced through Jutland. Denmark asked for peace in July.

Key people

• Otto von Bismarck: Prussian minister-president who used the crisis to enlarge Prussia’s influence.
• Christian IX: Danish king whose signing of the November Constitution started the crisis.
• Ditlev Gothard Monrad: Danish prime minister during the war.
• Friedrich von Wrangel and Prince Friedrich Karl: Prussian commanders.
• Ludwig von Gablenz: commander of the Austrian corps.

Consequences

The Treaty of Vienna, signed on 30 October 1864, forced Denmark to cede Schleswig, Holstein and Lauenburg to Prussia and Austria jointly. Denmark lost roughly two-fifths of its territory and a large share of its population, including many Danish speakers in northern Schleswig.

Joint rule soon caused friction. Under the Gastein Convention of August 1865 Prussia took over the administration of Schleswig and Austria that of Holstein. Disputes over Holstein became Bismarck’s pretext for the Austro-Prussian War, decided at the Battle of Königgrätz in 1866, after which Prussia annexed both duchies.

Legacy

In Denmark, 1864 became a national trauma that turned the country away from great-power ambitions and towards domestic reform. The phrase often linked with this period, “what is lost outwardly must be won inwardly”, expressed the mood of rebuilding at home. Dybbøl remains a site of remembrance, commemorated every 18 April.

After Germany’s defeat in the First World War, a plebiscite in 1920 returned northern Schleswig to Denmark, creating the border that still exists. In Germany the war was remembered as a step on the road to the Unification of Germany in 1871.

Timeline

• 1863, 18 November: Christian IX signs the November Constitution.
• 1864, 16 January: Prussia and Austria issue an ultimatum to Denmark.
• 1864, 1 February: Allied troops cross the Eider into Schleswig.
• 1864, 5–6 February: The Danish army abandons the Danevirke.
• 1864, 18 April: Prussian troops storm the Dybbøl redoubts.
• 1864, 9 May: Naval battle off Heligoland.
• 1864, 29 June: Prussian forces cross to the island of Als.
• 1864, 30 October: Treaty of Vienna ends the war.

Why it matters

The Second Schleswig War showed the effectiveness of the reformed Prussian army and Bismarck’s skill at exploiting a diplomatic crisis. It reshaped Denmark’s borders and identity and set Prussia and Austria on the path to their own war two years later.