German-Danish War, also called Second Schleswig War, (1864), the second of two conflicts over the settlement of the Schleswig-Holstein question, a complex of problems arising from the relationship of the duchies of Schleswig and Holstein to Denmark, to each other, and to the German Confederation. Involved in it were a disputed succession, a clash of Danish and German nationalism, and a threat to the international balance of power.

Origins of the conflict

Located on the Jutland Peninsula in northern Europe, the duchies of Schleswig and Holstein had been ruled separately by Danish kings since the 15th century but had been united at various points in their history (notably from 1386 to 1460). The death of King Frederick VI of Denmark in 1839 triggered a crisis, as the succession laws of Denmark conflicted with those of Holstein, and the status of Schleswig was uncertain. A royal proclamation that the law had to be reckoned the same in all three places because the monarchical realm was indivisible was undermined by nationalist passions. The population of Schleswig was Danish in the northern countryside, German in the south, and mixed in the northern towns and the centre. The Eider-Danes, a Danish nationalist political movement, claimed the Eider River—the traditional frontier between Schleswig and Holstein—to be Denmark’s national boundary and aspired to detach Schleswig from Holstein for incorporation in Denmark. German nationalists conversely sought to confirm Schleswig’s association with Holstein and to detach Schleswig from Denmark.

The crisis in the duchies came to a head in 1848. In January the absolutist Danish King Christian VIII was succeeded by Frederick VII, and, just days into his reign, Frederick published a proposed constitution that would have recognized Denmark, Schleswig, and Holstein as autonomous bodies within a single governmental structure. German sympathies predominated in Schleswig and Holstein at the time, however, and the duchies responded to Frederick’s initiative by requesting admission to the German Confederation as a single state. With the outbreak of revolution in Germany in March, the duchies rebelled against Denmark, and on March 24 a provisional government at Kiel proclaimed their independence, offered support for German freedom and unity, and renewed their appeal for admission to the German Confederation. On March 27 Frederick published a draft constitution linking Schleswig more closely to Denmark than to Holstein. The Danish crown dispatched troops to Schleswig, but initial gains were soon checked by Prussia, which had dispatched forces to the duchy in support of the rebels. Thus began the First Schleswig War. The Prussian armies continued into Denmark proper but withdrew in June; on August 26 representatives of Denmark and Prussia met at Malmö, Sweden, and signed a seven-month truce. Hostilities were renewed upon the treaty’s expiration in 1849, and once again Prussian forces advanced into Denmark. Eager to preserve the balance of power in northern Europe, Russia, France, and Britain pressured a settlement. A six-month truce and a preliminary peace treaty were signed on July 10, 1849. That accord, which was finalized in Berlin on July 2, 1850, largely restored the status quo, although rebels in the duchies continued to resist Danish authority. Virtually all the great powers of Europe were signatories to the Treaty of London (May 8, 1852), which declared the integrity of the Danish realm to be a permanent element in the European balance of power.

Danish constitutional reforms and German backlash

Efforts to craft a constitutional solution for the Schleswig-Holstein crisis continued through the 1850s, but the short-term result was the achievement of the Eider-Danes’ ideal, a Danish policy that brought Schleswig and Denmark together, with Holstein left out. Meetings of the regional governments in Holstein in 1859 and 1861 and in Schleswig in 1860 failed to settle the constitutional dispute in a conclusive manner. In autumn 1862, when Otto von Bismarck came to power in Prussia, the question was thus still open. Danish nationalism was then on the ascent; ideas of a common Scandinavian front against Germany were being canvassed; and Eider Denmark seemed to have been attained. Frederick’s decree of March 30, 1863, appeared to confirm Holstein’s separation: it allowed legislation enacted for the whole monarchy to be valid for Denmark and Schleswig but subject to veto within Holstein. The German Confederation asked Denmark to withdraw the March decree, but many developments encouraged Danish resistance. Prussia was immersed, from May, in an internal constitutional struggle, and it was contesting with Austria for supremacy within the confederation. On July 23, 1863, British Prime Minister Henry John Temple, 3rd Viscount Palmerston, warned those who threatened the integrity of Denmark that it would not be Denmark alone with whom they would have to contend. In August Prussia’s antagonism to Austria was demonstrated by the Prussian refusal to take part in the congress of princes that Austria had convened to consider the reconstruction of Germany. Great Britain and France had sought to assert their influence over Russia’s treatment of Poland in the wake of the January Insurrection, but in November the British rejected Napoleon III’s proposal for a congress to settle the outstanding problems of Europe.

Prussia and Austria declared war upon Denmark. Their action was governed by a request on January 16, 1864, for Denmark to rescind its November 1863 constitution; within days, Denmark refused to do so. The Austro-Prussian demand provided for a joint settlement of the Schleswig-Holstein question and excluded the broader German Confederation. On February 1 the Austro-Prussian army, ignoring the federal troops in Holstein, invaded Schleswig. The move marked Prussia’s separation from federal policy, as Bismarck hoped to achieve the annexation of the duchies to Prussia, with Kiel to serve as a Prussian naval base. Within days of the initial contact between the opposing armies, the Danes were in retreat. Much of the Danish strategy hinged on mounting a frontal defense at the Danewirk, a band of earthen fortifications that had guarded against incursions into Jutland since the 7th century ce. The Danes abandoned the Danewirk without a struggle on February 6, however, when the Prussians threatened to turn the flank of the whole Danish defensive line. That inauspicious start would set the tone for the campaign to follow, as Danish defenders found themselves outnumbered and outgunned—especially in terms of artillery—at virtually every turn.

As their armies advanced into Schleswig, the broader European political situation was favourable for Austria and Prussia. Napoleon III had drawn toward Prussia. Russia, unassertive after the Crimean War, was no longer inclined to support Denmark and now backed Austro-Prussian conservatism. Although Great Britain claimed that the Treaty of London was valid whether Denmark fulfilled the edict of December 1851 or not, neither Palmerston nor fellow statesman John Russell could win the support of Queen Victoria or the cabinet for a pro-Danish policy. Under these circumstances, Bismarck signed a fresh alliance with Austria on March 6, 1864, to cover the invasion of Denmark proper. The alliance was unusual, given the intense rivalry that existed between Prussia and Austria, and Austrian participation was inspired, at least in part, by the desire to thwart Prussian ambitions in the duchies. It aimed at a settlement that would discard the edict of December 1851 but still—to meet Austria’s wishes—achieve a personal union of the duchies with Denmark. The Prussian proposal, which aimed at independence for Schleswig-Holstein under Frederick of Augustenburg, was rejected by the Austrians. Fighting was sporadic but intense, and the Prussians won a significant victory when they captured the Danish stronghold at Dybbøl on April 18, following a two-week siege. Denmark fared better at sea, maintaining a successful blockade of German Baltic ports and repulsing a small Austro-Prussian flotilla at Helgoland on May 9, but those small triumphs amounted to little in the broader context of the war. Before they could fully realize their military goals, Austria and Prussia were faced with a European conference.

The London Conference, which convened on April 25, 1864, was never master of the situation that it tried to regulate. The meeting was delayed by unavailing efforts to bring about an armistice first (the armistice eventually took effect from May 12), by the difficulty of securing the representation of the German Confederation (which was not a signatory of the Treaty of London), and by Bismarck’s deliberate effort to postpone the opening until Prussia and Austria were in possession of all of Denmark. Napoleon’s proposal to consult the wishes of the populations was altered by Bismarck to a proposal for separating Holstein and southern Schleswig from Denmark while letting the populations decide the frontier line and the question of independence or union with Prussia. That marked the first time that the last possibility had been proposed internationally. Though he could not publicly advocate for that without violating the March alliance with Austria, Bismarck privately began to work toward that end.

The principle of Danish integrity was tacitly abandoned at the London Conference, and the Danes rejected the proposal of personal union that was central to the March alliance. That left independence for the duchies under Augustenburg as the only solution still in the field. The conference dispersed on June 25, 1864, without conclusion: Denmark rejected independence within the frontier proposed, and Austria rejected Bismarck’s stipulation that the duchies be militarily subordinated to Prussia. A fresh spell of fighting from the end of June to July 20, 1864, left the whole of Jutland in Austro-Prussian hands and enabled Bismarck to free the question from international discussion. Fresh peace negotiations, directed toward the cession of the duchies to Austria and Prussia jointly, achieved their purpose in the Treaty of Vienna of October 30, 1864.

From the beginning, Austrian and Prussian attitudes toward the duchies were in opposition, and conflict seemed inevitable. The Convention of Gastein (August 14, 1865) therefore modified the condominium by assigning the internal administration of Holstein to Austria and that of Schleswig to Prussia. That arrangement was short-lived, however, as the wider conflict over supremacy within Germany led to the Seven Weeks’ War between Austria and Prussia. The Peace of Prague (August 23, 1866) ceded the duchies to Prussia, and after Prussia’s formation of the German Empire in 1871, the Schleswig-Holstein question was narrowed to a contest between Denmark and Germany over North Schleswig. Over subsequent decades, the Danes of North Schleswig bristled under German rule, and the conscription of Danes into German armies in World War I—a conflict in which Denmark was neutral—strengthened the movement for reunion with Denmark. The postwar Paris Peace Conference decreed that a plebiscite be held to determine the final disposition of the territory. In February 1920 the northern zone voted for union with Denmark, and the following month central Schleswig voted for union with Germany. North Schleswig as far south as Tønder and the northern bank of Flensburg Fjord became part of Denmark, and the resulting boundary ceased to be a matter of contention.