The Second Schleswig War: A Pivotal 19th-Century Conflict and Its European Ramifications

Known by various names, including the Second Schleswig War, the Dano-Prussian War, the Prusso-Danish War, or, in its original languages, Krigen i 1864 (Danish: "The War of 1864") and Deutsch-Dänischer Krieg (German: "German-Danish War"), this conflict represented a critical moment in 19th-century European history. It was the second major military engagement arising from the complex and long-standing Schleswig-Holstein Question, a deeply intricate national and dynastic dispute concerning the future of three strategically important duchies that had simmered for decades.

The Genesis of Conflict: The Enduring Schleswig-Holstein Question

The roots of the 1864 war lay in the ongoing contention over the duchies of Schleswig, Holstein, and Lauenburg. While all three were historically linked to the Danish Crown, their political and demographic realities were vastly different. Holstein and Lauenburg were predominantly German-speaking, culturally German, and crucially, members of the German Confederation, though they were ruled by the Danish king in personal union. Schleswig, conversely, had a mixed population of Danes, Germans, and Frisians and was a Danish fief, but also possessed historical ties and a constitutional relationship with Holstein. The First Schleswig War (1848–1852), also known as the Three Years' War, had previously attempted to resolve these tensions between German nationalist sentiment and Danish claims, culminating in the internationally brokered London Protocol of 1852. This significant treaty aimed to preserve the integrity of the Danish monarchy under the new Glücksburg line of succession, while crucially stipulating that Schleswig would not be formally incorporated into Denmark, thereby maintaining its separate constitutional status from the Kingdom.

The Immediate Catalyst: The November Constitution of 1863

Despite the international agreements of 1852, the underlying tensions persisted and intensified. The immediate trigger for the Second Schleswig War was the controversial passing of the November Constitution of 1863 by Denmark. This new constitution was a direct and blatant violation of the London Protocol, as it sought to fully integrate the Duchy of Schleswig into the Danish kingdom. This unilateral act was swiftly interpreted as a grave provocation by the German Confederation, particularly by the rising power of the Kingdom of Prussia under its astute Minister-President Otto von Bismarck, and the Austrian Empire. Compounding the already volatile situation was the death of King Frederick VII of Denmark in 1863 without a direct male heir, which reignited complex dynastic succession disputes over the duchies. While Christian IX, Duke of Glücksburg, ascended the Danish throne in accordance with the London Protocol, his claim to the duchies was vigorously challenged by Frederik, Duke of Augustenborg, who was strongly supported by German nationalists. Denmark's unilateral move to incorporate Schleswig, coupled with the unresolved succession, provided Prussia and Austria with the legitimate casus belli they sought to intervene and assert German interests in the duchies.

The War Commences: February to October 1864

On 1 February 1864, forces from Prussia and Austria crossed the Eider River, marking the formal commencement of hostilities. The combined might of the German powers, particularly the technologically advanced Prussian army, quickly overwhelmed Danish defenses. Despite fierce resistance, notably during the Siege of Dybbøl, where Danish fortifications were subjected to intense bombardment and eventual assault, the Danes were ultimately outmatched by the superior numbers and organization of the Prussian and Austrian forces. The conflict underscored the burgeoning military strength of Prussia and its strategic ambitions under Bismarck to reshape the German states.

Resolution and Lasting Impact: The Treaty of Vienna

The Second Schleswig War concluded less than a year after it began, on 30 October 1864, with the signing of the Treaty of Vienna. This definitive treaty formally ended the conflict and imposed significant territorial concessions on Denmark. Under its harsh terms, Denmark was forced to cede the duchies of Schleswig, Holstein, and Saxe-Lauenburg entirely to Prussia and Austria. An exception was made for the island of Ærø, which remained Danish. This outcome dramatically reshaped the map of Northern Europe and marked a definitive end to Denmark's aspirations as a major European power. For Prussia, the victory was a crucial and strategic step in Otto von Bismarck's grand unification strategy for Germany, as it brought the valuable and strategically located duchies under direct German influence. The joint administration of Schleswig and Holstein by Prussia and Austria, however, proved to be an uneasy alliance and soon led to new disputes over their future. These disagreements directly paved the way for the decisive Austro-Prussian War of 1866, further demonstrating the complex interplay of power, nationalism, and ambition that defined 19th-century Europe and ultimately led to the unification of Germany under Prussian leadership.

Frequently Asked Questions about the Second Schleswig War

What was the primary cause of the Second Schleswig War?

The primary cause was Denmark's passing of the November Constitution in 1863, which aimed to fully integrate the Duchy of Schleswig into the Danish kingdom. This act directly violated the London Protocol of 1852 and was perceived as a grave challenge by the German Confederation, especially Prussia and Austria, amidst an ongoing succession dispute for the duchies following the death of King Frederick VII.

Who fought in the Second Schleswig War?

Denmark fought against a coalition of the Kingdom of Prussia and the Austrian Empire.

What was the significance of the London Protocol of 1852?

The London Protocol of 1852 was an international treaty signed by major European powers (including Great Britain, France, Russia, Sweden, Norway, Austria, and Prussia) that aimed to resolve the Schleswig-Holstein Question by confirming the integrity of the Danish monarchy and settling the succession. Crucially, it stipulated that Schleswig, while remaining under the Danish Crown, should not be formally incorporated into the Kingdom of Denmark, preserving its separate status.

What was the outcome of the Second Schleswig War?

The war ended with Denmark's decisive defeat and the signing of the Treaty of Vienna on 30 October 1864. Denmark was forced to cede the duchies of Schleswig, Holstein, and Saxe-Lauenburg entirely to Prussia and Austria. This outcome significantly enhanced Prussian power and set the stage for later conflicts, particularly the Austro-Prussian War of 1866, which further advanced German unification.

Why is it also called the Dano-Prussian War or Prusso-Danish War?

While Austria was a key participant in the coalition against Denmark, Prussia was the dominant German power and arguably the main architect of the conflict's outcome and subsequent territorial gains. The alternative names "Dano-Prussian War" or "Prusso-Danish War" highlight Prussia as the principal belligerent from the German side and acknowledge its leading role in the military campaign and the strategic implications of the victory for its own unification ambitions.