The origins of the Danish-German War of 1864 lay in an issue whose complexity is best illustrated by an aphorism attributed to Britain's Lord Palmerston. He declared that only three men ever truly understood the Schleswig-Holstein question: a Danish politician who was dead, a German professor who went mad, and Palmerston himself—who had forgotten it.
Essentially the provinces were legally joined together under the personal rule of the Danish crown. Holstein was also part of the German Confederation created in 1815. When in 1848 Denmark proposed to integrate Schleswig into its administrative structure, the confederation resisted. The resulting compromise steadily eroded until in 1863 Denmark announced a new constitution including Schleswig.
German nationalists accused Denmark of plotting the annexation of Schleswig in defiance of international law. The German Confederation voted armed sanctions with ruffles and flourishes. And Otto von Bismarck, the Prussian minister-president (prime minister), saw an opportunity to propel his state into a position of leadership in Germany. He began by persuading Austria to cooperate in taking the military initiative against Denmark. Each state dispatched an army corps, and in January 1864 the newly minted allies crossed into Holstein.
Most of the 35,000 to 40,000 Danes called to arms for the crisis were reservists, including married men in their thirties and volunteers with neither training nor experience. The bulk of them were deployed in Schleswig, manning improvised fieldworks they had no chance of holding for long against the forces of two Great Powers. But withdrawing without even token resistance was to concede the war.
While the Prussians were slow off the mark, a series of dashing Austrian attacks compelled the Danes to evacuate Schleswig, retreating into Jutland and to the permanent fortifications at Düppel (Dybbøl in Danish). Prussia and Austria faced the simultaneous challenges of convincing the other powers that further military action was not aimed at the territorial and political integrity of Denmark, and of overcoming Düppel's formidable landward defenses, which offered few possibilities for anything but a time-consuming formal siege.
Helmuth von Moltke, the Prussian chief of the general staff, argued that stalemate in the field was the best guarantee of great-power intervention. The bulk of the Prussian contingent remained around Düppel. The Austrians advanced toward Denmark proper as Bismarck convinced their dubious government they were in too deep to withdraw. The allies approved a plan based on capturing Düppel and Alsen (or Als) Island, lying immediately behind it, and mounting a full-scale advance into Jutland for the purpose of screening those operations against a Danish counteroffensive.
Austria's agreement freed Bismarck to respond positively to increasingly uncompromising British and Russian demands for an international conference on the question of the duchies. That, however, was only one of the cards Bismarck was playing. Simultaneously, he began to prepare public, political, and royal opinion for the direct annexation of Schleswig and Holstein to Prussia.
Both initiatives depended on a Prussian victory. As men and equipment moved into position to begin the slow-motion ballet of a siege, Bismarck put increasing pressure on his generals to finish off Düppel by assault. As an alternative to a lengthy siege and a costly storm, a junior staff officer proposed an amphibious operation—a surprise attack in force on Alsen Island to bypass the Danes' defenses and force them into the open. Moltke regarded the risks as too high. For six weeks the Prussians brought up guns and dug trenches. For two weeks more they bombarded the Danish fortifications. On 18 April the assault finally went in. At the cost of a thousand casualties the defenses were overrun.
The Danes responded by evacuating their last mainland fortress of Fredericia. Moltke saw that as a sign that Denmark no longer had any intention of undertaking large-scale land operations. Despite Austrian reluctance, he oversaw the overrunning of the Jutland peninsula as representatives of the Great Powers met in London, seeking to resolve the Schleswig-Holstein question in an international context. Bismarck took full advantage of continuing Danish refusal to compromise, rejecting even Franco-British initiatives for dividing the duchies and British proposals for arbitrating Denmark's new frontier.
On 29 June Moltke supervised the crossing of the Alsen Fjord by 25,000 Prussians. They encountered no significant resistance from an army by now demoralized by three months of inactivity. Alsen marked the end of the serious fighting. As Moltke busied himself with plans for an attack on Fünen (Fyn in Danish) and an invasion of Zeeland, Denmark's King Christian IX (r. 1863–1906) decided further resistance was impossible. Denmark's French and British patrons had no leverage remaining. On 1 August Christian ceded all rights to the duchies to Prussia and Austria.
Bucholz, Arden. Moltke and the German Wars, 1864–1871. Basingstoke, U.K., 2001. Contains the best account of the military operations.
Carr, William. Schleswig-Holstein, 1815–1848. Manchester, U.K., 1963. The best English-language source on the background of the duchies' status.
Showalter, Dennis. The Wars of German Unification. London, 2004. Covers the diplomacy as well as the campaign.