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By: Randall Lesaffer

In 2014, while the centenary of the commencement of the First World War drew massive attention in most of Europe, Denmark had another war to commemorate: the Schleswig-Holstein war of 1864. In the fall of 2014, the Danish national broadcasting company, DR, launched a television series about the war, entitled ‘1864’. The series focuses on the siege and battle of Dybbøl (Düppel) in Eastern Schleswig, where the Danish army made its stand against the Prussian invaders. Following the adventures of a few individuals, the series gives a vivid picture of the carnage the war wrought and the lives it destroyed, both at and behind the front. But it does little to explain the political background to the war, except for a stereotyped opposition of the cold calculations of great power diplomacy to the nationalist frenzy of some misguided, overtly religious, and/or naive Danes. In the grand narrative of 19th-century European history, the war of 1864 normally only receives a cursory mention as the first and smallest of the three wars the Prussian Prime Minister, Otto von Bismarck (1815–1898), plotted and through which he achieved German unification. For Denmark, it was a national tragedy.

The war of 1864 indeed turned on nationalism, but from both sides. It found its direct cause in the clash of the Danish and German nationalists within the Duchy of Schleswig. The former sought incorporation of the Duchy in the Kingdom of Denmark and were supported by the Eider Danes, a nationalist faction in Denmark which considered the river Eider between Schleswig and Holstein the natural border of the realm.  The latter wanted incorporation in the German Confederation, thus securing its unity with the German-speaking Duchy of Holstein, which was already a member. They enjoyed support from the liberal-nationalist Diet of the German Confederation in Frankfurt and from many supporters of German unity, including in Prussia.

The tension between the Danish and German factions within Schleswig had first reared its head and erupted in an international crisis in the late 1840s. The nationalist agenda stood at odds with the constitutional settlement of the Duchies, which had its roots in feudalism and old-fashioned dynastic interests. The two duchies had been ruled by the same princes since the 14th century. When the Danish King Christian I (r. 1448–1481) was accepted as duke in 1460, he had to agree to a charter which stated the unity of the two duchies in perpetuity. This remained part of the constitutional settlement of Schleswig-Holstein all through the period of its personal union with Denmark. This unity of the two duchies persevered, even though they straddled the border of the Holy Roman Empire (until its dissolution in 1806) and later the German Confederation (after its foundation in 1815), with the Southern Duchy of Holstein belonging to the Confederation and the Northern Duchy of Schleswig outside of it. Since 1815, the Danish kings also held the Duchy of Lauenburg, another member of the German Confederation to the south of Holstein, in personal union.

In the 1840s, dark clouds gathered. The likelihood that the royal house of Denmark would become extinct in the male line threatened to sever the personal union between the realm and the Duchies of Schleswig-Holstein. As the laws of inheritance in the duchies excluded accession to the throne through the female line, the succession would pass to different branches of the royal family in Schleswig-Holstein and in Denmark. To the Danish King Christian VIII (r. 1839–1848) and his government, this was unacceptable in as far as the northern Duchy, with its sizeable Danish-speaking population, was concerned. In 1846, the king announced the future incorporation of Schleswig into Denmark. After his accession to the throne in January 1848, Frederick VII (r. 1848–1863) promulgated the imminent incorporation through a new Danish constitution. On 21 March 1848, formal annexation followed. 

This proved unacceptable to the German Assembly at Frankfurt. The German nationalists formed a provisional government in Schleswig and the German Confederation voted the accession of Schleswig. Meanwhile, Prussian troops, acting on behalf of the Confederation, attacked and invaded Schleswig and Denmark. However, pressure from the great powers, and especially Russia, stayed the hand of Berlin and Frankfurt. After the Armistice of Berlin of 10 July 1849 (103 CTS 207), the great powers and Denmark reached a settlement through a conference in London in 1851–1852. By the Treaty of London of 8 May 1852 (108 CTS 121), the great powers guaranteed the personal union between Denmark, Schleswig-Holstein, and Lauenburg, thus ignoring the rules of succession within the duchies. This settlement, however, by restoring the personal union also prevented Schleswig’s incorporation into the Kingdom of Denmark. Its incorporation into the German Confederation was also undone.

When a new crisis erupted in 1863, at the death of Frederick VII and his succession by a prince from the House of Schleswig-Holstein-Sonderburg-Glücksburg as Christian IX (r. 1863–1906), the international situation had changed. This time, Russia did not intervene. The Russian alliance with Austria, Prussia’s competitor for leadership within the German Confederation, had not survived the Crimean War (1853–1856). Moreover, Bismarck had secured Russia’s good will through his support for the tsarist regime during the Polish revolt in early 1863. France for its part was engaged in an imperialist adventure in Mexico whereas Britain had its eyes first and foremost on the Civil War in North America. 

Three days after his accession to the throne, on 18 November 1863, Christian IX promulgated a constitution which extended the border of the Kingdom of Denmark to include Schleswig, but German nationalists within and without the duchies were now able to play the card of legitimacy by invoking the rules of succession of the duchies and supporting Duke Frederick of Augustenburg (1829–1880), who staked his claim to Schleswig-Holstein and Lauenburg. On 7 December, the German Diet at Frankfurt issued an executive order to force Denmark to abide by the 1852 London Treaty and undo the incorporation of Schleswig. Troops from the Kingdoms of Hanover and Saxony were summoned and invaded Holstein.

For Bismarck, who had only become Prime Minister in 1862 and whose grip on power was far from secure, the crisis was an opportunity to prove the effectiveness of his leadership to king and country and to weaken the liberal opposition who held the majority in the Prussian parliament. His ulterior goal was the annexation of both duchies, which would strengthen Prussia’s position in its long-term bid for hegemony in Germany against Austria. But as King William I (r. 1861–1888) proved more than hesitant to oppose what he considered the legitimate claims of the Duke of Augustenburg, Bismarck first chose the path of cooperation with Austria. This at least would allow Prussia to appear as one of two leaders of the Confederation and give it a front seat at the table where the outcome of the crisis would be brokered.

On 16 January 1864, Austria and Prussia issued a joint ultimatum to Denmark demanding the retraction of the November constitution. After the expiration of the deadline, Austrian and Prussian troops moved into Schleswig and quickly overran the Duchy. Whereas Austrian troops performed admirably, the Prussian army initially made less of an impression. Although Bismarck did not want to allow a full invasion of Danish Jutland for fear of provoking a Russian, French, or British intervention, he craved a resounding military success. The retreat of the Danish army, which had been easily unsettled at the Danevirke, its line of fortifications to the south, to the fortress at Dybbøl in Eastern Schleswig offered an opportunity. After a siege and bombardment of two weeks, the Prussian troops attacked on 18 April 1864. The ensuing slaughter caused around 3,000 casualties, leaving the Prussian army victorious. After further operations over the summer, the Danish government sued for peace.

Meanwhile, the great powers had begun to take action. Since 24 April 1864, six days after the battle at Dybbøl, a great power conference had convened at London. Although the other three great powers would have preferred some form of return to the status quo ante bellum, for them Schleswig-Holstein was in the end a sideshow. When Austria and Prussia raised the stakes at the conference by stating that they considered the 1852 London Treaty to have lapsed, none of the other powers showed much inclination to unhinge the two German powers who were now in factual possession of the duchies. After Bismarck had taken Duke Frederick of Augustenburg out of the equation by offering him an impossible deal he had duly rejected, the way to an Austrian-Prussian annexation of the duchies lay open. 

On 1 August 1864, an armistice as well as a preliminary peace agreement was signed at Vienna between Austria, Prussia, and Denmark. In the agreement, Denmark ceded its rights to Schleswig-Holstein and Lauenburg and turned those territories over to joint Austrian-Prussian administration. The agreement was confirmed by the Treaty of Peace of Vienna of 30 October 1864. After almost a year of troubles between the two administrating powers in the duchies and of tedious negotiations, Bismarck succeeded in arm-wrestling Vienna into an agreement on Schleswig-Holstein. On 14 August 1865, at Bad Gastein, Austria and Prussia signed a convention to split up the effective administration of the duchies, while the unity of the joint administration of the two duchies was vaguely confirmed in principle (Article 1; 131 CTS 343). Whereas Austria obtained Holstein, Schleswig went to Prussia. Moreover, Prussia was allowed to buy the Duchy of Lauenburg, which was geographically contingent to its own territory, from Austria. 

For the Danish Monarchy, this meant a severe truncation of its territory as well as the population it ruled. After the First World War, Denmark would regain a northern portion of the Duchy of Schleswig. For Bismarck, the war was a resounding success. It improved his stance with the king and strengthened his hold on the reins of government. It also gave him a source of continuous tension with Austria to exploit at will, in a theatre far from the heartland of the Austrian Monarchy.



Christopher Clark, Iron Kingdom. The Rise and Downfall of Prussia, 1600-1947 (London: Allen Lane 2006).

Theodor Fontane, Der Schleswig-Holsteinsche Krieg im Jahre 1864 (Helmuth Nürnberger ed., Berlin: Ullstein 1981) (1866).

Emanuel Halicz, Russia and Denmark, 1856-1864: A Chapter of Russian Policy towards the Scandinavian Countries (Copenhagen: C.A. Reitzel’s 1990).

W.E. Mosse, ‘Queen Victoria and her Ministers in the Schleswig-Holstein Crisis 1863-1864’, English Historical Review, 78 (1963) 263–83.

Jonathan Steinberg, Bismarck. A Life (Oxford: Oxford University Press 2011).

Waldemar Westergaard, Denmark and Slesvig, 1848-1864 (London: Cumberlege 1946).