The Second Schleswig War, also known as the Dano-Prussian War, began on February 1, 1864, as Prussian and Austrian forces crossed the border into the Danish Duchy of Schleswig. This conflict arose from the Schleswig-Holstein Question, a long-standing dispute over control of the duchies of Schleswig, Holstein, and Lauenburg. Although a similar struggle had been resolved in favor of Denmark in the First Schleswig War (1848–1851), tensions reignited due to succession disputes and nationalistic ambitions, culminating in this renewed conflict.
The roots of the war lay in the passing of the November Constitution in 1863, which aimed to integrate Schleswig more closely with Denmark. The German Confederation viewed this move as a breach of the 1852 London Protocol, which stipulated that the duchies should remain separate from Denmark. Upon the death of Danish King Frederik VII in 1863, King Christian IX ascended the throne and faced immediate challenges to Danish authority over the duchies. The German-speaking population of Holstein and Schleswig supported the House of Augustenburg, a German-friendly cadet branch, while Denmark sought to solidify its control over Schleswig, leading to a clash of nationalist sentiments on both sides.
In January 1864, tensions mounted as Prussian and Austrian forces positioned themselves south of the Eider River, facing Danish defenses to the north. By February 1, Prussia and Austria declared war, and hostilities began. The Danish army, numbering around 38,000 men, faced a combined Prussian and Austrian force of over 60,000. The Danes initially defended the Dannevirke fortification, an ancient defensive line and a symbol of Danish national pride. However, on February 5, Danish Commander-in-Chief Lieutenant General Christian Julius De Meza ordered a retreat from the Dannevirke to avoid being encircled, a decision that caused great shock and demoralization in Denmark.
Military clashes in Schleswig/Slesvig. @ Maximilian Dörrbecker
As Danish forces withdrew to the nearby town of Flensburg, the harsh winter weather took its toll, resulting in the deaths and capture of several hundred soldiers. Prussian and Austrian troops pursued the retreating Danish army, leading to intense battles at Sankelmark and other locations. By mid-April, Danish forces had fortified themselves at Dybbøl, but on April 18, the Prussian army launched a decisive assault, leading to the fall of Dybbøl fortifications after a six-hour bombardment and frontal attack. This defeat proved to be a turning point, and Danish defenses continued to crumble.
The London Conference, held from April to June 1864, attempted to negotiate a peaceful resolution, but these efforts failed, and fighting resumed. On June 29, Prussian forces launched a successful amphibious assault on the Danish island of Als, marking the last major engagement of the war. By July 3, the last Danish forces were defeated at the Battle of Lundby, and the German occupation extended across all of Jutland.
On October 30, 1864, the Treaty of Vienna was signed, officially ending the conflict. Denmark ceded control of Schleswig, Holstein, and Lauenburg to Prussia and Austria. This resulted in a significant loss for Denmark, as it forfeited 40% of its land and nearly 1 million of its population, reducing its influence and territorial integrity.
The Second Schleswig War had far-reaching consequences. It marked the end of Danish ambitions over the duchies, strengthened Prussia's influence in German affairs, and foreshadowed the eventual unification of Germany. For Denmark, the loss was a national trauma, and the country adopted a more cautious foreign policy, avoiding military conflicts until the late 20th century. The conflict's legacy also persisted, as thousands of Danes conscripted into the German army during World War I suffered heavy losses, leaving a lasting sense of bitterness among families in southern Jutland. The Schleswig plebiscite of 1920, held after World War I, saw North Schleswig returned to Denmark, finally addressing some of the territorial disputes that had sparked the war.