The Second Schleswig War or German-Danish War, also sometimes known as the Dano-Prussian War or Prusso-Danish War, was the second military conflict over the Schleswig–Holstein question of the 19th century. The war began on 1 February 1864, when Prussian and Austrian forces crossed the border into the Danish fief Schleswig. Denmark fought troops of the Kingdom of Prussia and the Austrian Empire representing the German Confederation.
Like the First Schleswig War (1848–1852), it was fought for control of the duchies of Schleswig, Holstein, and Lauenburg. Succession disputes concerning the duchies arose when Frederick VII, King of Denmark died without an heir acceptable to the German Confederation. The war started after the passing of the November Constitution of 1863, which tied the Duchy of Schleswig more closely to the Danish kingdom, which was viewed by the German side as a violation of the London Protocol.
The war ended on 30 October 1864, with the Treaty of Vienna and Denmark's cession of the Duchies of Schleswig (except for the island of Ærø, which remained Danish), Holstein and Saxe-Lauenburg to Prussia and Austria.
Background
In 1848, Denmark received its first liberal constitution. At the same time, and partly as a consequence, the secessionist movement of the large German majority in Holstein and southern Schleswig was suppressed in the First Schleswig War (1848–51), when the Germans in both territories failed in their attempt to become a united, sovereign and independent state: at the time, King Frederik VII of Denmark was also duke of the duchies of Holstein and Schleswig. The movement continued throughout the 1850s and 1860s, however, as Denmark attempted to integrate the Duchy of Schleswig into the Danish kingdom, while liberal proponents of German unification expressed the wish to include the Danish-ruled duchies of Holstein and Schleswig in a Greater Germany. Holstein was completely ethnically German, had been a German fief before 1806 and was a part of the German Confederation from 1815. Schleswig was a Danish fief and was linguistically mixed between Low German, Danish and North Frisian.
Before the Middle Ages, the people of Schleswig spoke Danish and Frisian, and as late as the 18th century, many rural areas of southern Schleswig still spoke Danish. In the early 19th century, the northern and middle parts of Schleswig spoke Danish, but the language in the southern half had shifted to German. German culture was dominant among the clergy and nobility; Danish had a lower social status and was spoken mainly by the rural population. For centuries, while the rule of the king was absolute, these conditions had created few tensions. When liberal and egalitarian ideas spread and nationalist currents emerged about 1820, identification was mixed between Danish and German: the German elites in Schleswig wished to be a part of Germany, while the Danes wanted Schleswig to be more firmly integrated into Denmark proper. Furthermore, there was a grievance about tolls charged by Denmark on ships passing through the Danish Straits between the Baltic Sea and the North Sea. To avoid that expense, Prussia and Austria planned to construct the Kiel Canal, which could not be built as long as Denmark ruled Holstein.
Much of the dispute focused on the heir of King Frederik VII of Denmark. The Germans of Holstein and Schleswig supported the House of Augustenburg, a cadet branch of the Danish royal family, but the average Dane considered them too German and preferred the rival Glücksburg branch with Prince Christian of Glücksburg as the new sovereign. Prince Christian had served on the Danish side in the First Schleswig War (1848–1851). Both Britain and the Russian Empire wanted the Danish Straits, linking the North Sea to the Baltic Sea, to be controlled by a relatively weak power such as Denmark to allow their respective navies to either enter the Baltic, in the case of Britain, or exit the Baltic, in the case of Russia. During the First Schleswig war in 1848–49, Russia had threatened twice to enter the war on the side of Denmark, which proved to be the deciding factor in the outcome of the war.
The peace treaty that had ended the war in 1851 stipulated that the duchy of Schleswig should be treated the same as the duchy of Holstein regarding its relations with the Kingdom of Denmark. However, during the revisions of the 1848 constitution in the late 1850s and early 1860s, Holstein refused to acknowledge the revision, creating a crisis in which the parliament in Copenhagen ratified the revision but Holstein did not. In 1863, Frederik VII died, and the new Danish monarch, King Christian IX, ordered that the new constitution should apply to Schleswig and Denmark, but not to Holstein. This was a clear breach of the 1851 peace treaty and the London Protocol of 1852 and gave Prussia and the German Confederation a casus belli against Denmark. The German position was considerably more favourable than it had been thirteen years before, when Prussia had to give in due to the risk of military intervention by Britain, France and Russia on behalf of Denmark: France had colonial problems, not least with Britain. Otto von Bismarck had succeeded in obtaining cooperation from the Austrian Empire, which underlined its great power status within the German union, while Britain was upset that Denmark had violated the London Protocol.
To understand the Danish resolve in this question one must understand that the Danes regarded Schleswig as an ancient core region of Denmark. The southern part of Schleswig contains the ruins of the old Danish Viking "capital" Hedeby and the Danevirke fortification; its first sections were built around 400–500 CE, possibly to protect Denmark from migrating tribes during the age of migration. Before the Danes took possession of the area, around 500 CE, Schleswig was the home of the Angles, of which many migrated to Britain, where they later formed the Anglo-Saxon kingdoms; the remaining Angles are believed to have assimilated with the Danes, indeed the Angles and the Danes seem to have had a very close relationship as attested by the shared sagas of the early English and Danes. Thus, to suggest that the region did no longer fully belong to Denmark was seen as a great provocation to the Danes' ancestral claim to Schleswig.
The Germans, on the other hand, referred to medieval history: Already in 1326 and 1448, the Danish kings had accepted the almost complete independence of Schleswig from the Danish Crown. The Germans argued that the duchy had therefore not been part of Denmark proper for 400 years, but instead was "forever inseparable" (up ewich ungedeelt) from the German duchy of Holstein, something the Danish king had promised as early as 1460.
In short, the Danes considered Schleswig to be an integral part of Denmark and wished to make this clear by enacting a new constitution that excluded Holstein, while the Germans thought that Schleswig was inseparable from Holstein: If the Danish crown wished to treat the two Duchies differently, the only solution was, in their eyes, to get rid of Danish rule altogether. Both sides thus saw the other as the aggressor.
The international situation
The Prussian minister-president, Otto von Bismarck, had been appointed to that position in 1862 with orders from the king to resolve a crisis caused by the unwillingness of the liberal lower house of the Landtag of Prussia to vote for increased taxes to pay for increased military spending. The refusal of the Landtag to vote for the taxes that the war minister, General von Roon, had insisted were absolutely necessary to pay for a program of military modernization had caused a profound political crisis. In effect, the liberal-dominated Landtag were making a bold bid to be co-equals along with the king in governing Prussia by asserting their "power of the purse" over the military. The kings of the House of Hohenzollern always jealously guarded their prerogatives to have sole control of the military, and King Wilhelm I was completely against making any concessions to the Landtag in exchange for the Landtag voting for the taxes to pay for military modernization.
The diplomat Bismarck had been appointed minister-president largely because the king believed that he was a man capable of having the Landtag vote for the Army Law taxes without making any concessions over the king's control of the military. Bismarck had a reputation of someone who was a gruff bully who liked to push other people around, but also a man of considerable charm and grace when he wanted to be. Finally, Bismarck was perceived as a man who was very clever in resolving apparently intractable problems while being very committed to upholding the existing system, and it was for this reason the king had appointed him minister-president, believing that he was the best man to resolve the crisis.
Bismarck's "blood and iron" speech, in which he stated that the problems of Germany would be solved by "blood and iron" instead of talks, was an effort to win over the support of the liberals for his policy of increased taxes to pay for higher military spending. The Landtag refused to vote for the requested taxes. Bismarck solved the crisis simply by ordering the Prussian state to collect the taxes without the consent of the Landtag by claiming there was "a hole in the constitution" by saying if the Landtag and the king were deadlocked then the state had the right to collect taxes without the permission of the Landtag, a claim that he privately admitted to being nonsensical.
Bismarck's actions in having collecting taxes without the permission of the Landtag was manifestly illegal and unconstitutional, and made him unpopular. The liberals in Prussia also tended to be German nationalists who supported including the two duchies of Schleswig-Holstein into a projected unified German state, and Bismarck saw launching a war in the name of German nationalism as a way to bring around the liberals into supporting the Prussian state, all the more so as a war would demonstrate the value of a stronger Prussian army and thus justify the illegal taxes.
In the First Schleswig War, the possibility of Russian intervention on the side of Denmark had proved decisive in deciding the outcome of the war. The Crimean War had changed the entire posture of Russian foreign policy. Before the Crimean War, Russia had been the most reactionary of the European states and the one most committed to more or less upholding the status quo established by the Congress of Vienna in 1815. After the Crimean War, Russia was now a revisionist power out to challenge the European status quo, and any developments likely to change the European power structure were now welcome in St. Petersburg. Furthermore, the Crimean War and its aftermath made it extremely unlikely that Russia would work together with Britain or France, which established a room for maneuver for Prussia that did not exist in 1848–1850.
Tsar Alexander II saw the possibility of a stronger Prussia as a way of weakening France. During the Polish Uprising of 1863–1864, Napoleon III had taken a strongly pro-Polish line, which increased the already considerable mistrust and dislike of France in St. Petersburg (Kissinger lists this as a prime example of "strategic frivolity"). Alexander tended to favour a pro-Prussian line provided that Bismarck gave assurances that Prussia would not annex Denmark proper, and limit its ambitions to the two duchies.
Britain was the power most committed to supporting Denmark, but while Britain had the world's most powerful navy, the relatively small size of the all-volunteer British army led London to need a continental ally to provide the necessary military force on land. The Crimean War had so poisoned Anglo-Russian relations that it proved impossible for London and St. Petersburg to work together during the crisis. Additionally, in the American Civil War, the United States Navy blockaded the South, causing the so-called "cotton famine" that gravely damaged the British economy. Although Britain had found an alternative source of cotton in the form of Egypt, the "cotton famine" and the efforts of blockade runners to smuggle cotton out of the Confederate States of America to Britain had led to acute Anglo-American tensions with many Americans perceiving Britain to be supporting the Confederacy. The continual mistrust between Washington and London posed constraints on British foreign policy and limited London's options during the Second Schleswig War with much of the British Army being stationed in what is now Canada to guard against a possible American invasion.
Finally, after the Indian Mutiny of 1857–58, the British army in India was vastly increased. The fact that much of the British army was garrisoning India and Canada led to a shortage of British troops being free for operations in Europe.
In July 1863, British prime minister Lord Palmerston had given a speech saying: