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.

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.

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.

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.

[4] 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.

[11] 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.

[11] 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"[12]).

Alexander tended to favor a pro-Prussian line provided that Bismarck gave assurances that Prussia would not annex Denmark proper, and limit its ambitions to the two duchies.

[13] 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.

[14] In July 1863, British prime minister, Lord Palmerston, had given a speech saying: "I am satisfied with all reasonable men in Europe, including those in France and Russia, in desiring that the independence, the integrity and rights of Denmark may be maintained.

[16] Palmerston's rejection of having Britain attending the projected Congress of Paris was taken as a slight by Napoleon III, who notably refused British appeals during the war to co-operate against Prussia.

The same applied to foreign powers, such as Great Britain, France and Russia, who would not accept a weakened Denmark in favour of Germany, nor a Prussia that had acquired Holstein with the important naval harbour of Kiel that controlled the entrance to the Baltic.

The King died in 1863 at a particularly critical time; work on the November Constitution for the joint affairs of Denmark and Schleswig had just been completed, with the draft awaiting his signature.

This order to retreat without combat caused adverse comments among some Danish private soldiers,[17] but the military circumstances made it wise to shorten the frontier that needed to be defended.

Also, as the administrations of Holstein and Lauenburg were members of the German Confederation, not pulling back might have caused a severe political crisis and perhaps war with Great Britain, a guarantor of the London Protocol.

The political dimension of this strategy was to draw out the war and hence give time and opportunity for the "great powers" to intervene diplomatically—it was assumed that such an intervention would be to the advantage of Denmark.

The first attempt to bypass the position failed near Missunde, but eventually the Germans appeared in force in the Danevirke's rear, compelling the Danish high command to order the line abandoned.

But the political level did not appreciate the gravity of the situation, insisting on maintaining military presence in Schleswig and at the same time refused more modest German demands of peace.

All the inland waters (Eider River, Treene, Schlei, and the marshes east of Husum and around the Rheider Au) that the Danes were relying on as defence to guard the flanks of the Dannevirke, were frozen hard and could be crossed easily.

In the Battle for Königshügel (Danish Kongshøj, translated 'King's Hill') near Selk on 3 February 1864, Austrian forces commanded by General Gondrecourt pushed the Danes back to the Dannevirke.

Near Stolk-Helligbek, about 10 kilometers north of Schleswig, pursuing Austrians reached them, and in heavy fighting near Oversø, the 9th and 20th regiments of the 8th Brigade lost 600 men dead, injured and captured.

The loss of the Dannevirke without a fight, which in the 19th century played a big role in Danish national mythology due to its long history, caused a substantial psychological shock in Denmark and, as a result, de Meza had to resign from supreme command.

On 18 February 1864, some Prussian hussars, in the excitement of a cavalry skirmish, crossed the north frontier of Schleswig into Denmark proper and occupied the town of Kolding.

Christian IX's negotiations were not publicly known until published in the 2010 book Dommedag Als by Tom Buk-Swienty, who had been given access to the royal archives by Queen Margrethe II.

[25] The Peace of Prague in 1866 confirmed Denmark's cession of the two duchies, but promised a plebiscite to decide whether north Schleswig wished to return to Danish rule.

From a Danish perspective, perhaps the most grievous consequence of the defeat was that thousands of Danes living in the ceded lands were conscripted into the Imperial German Army in World War I and suffered huge casualties on the Western Front.

This is still (but waning in time as the children of the conscripted men are dying out) a cause of resentment among many families in the southern parts of Jutland and the direct reason why a German offer of a joint centenary anniversary in 1966 was rejected.

[30] After his death in 1898, Bismarck came to be the subject of an immense personality cult in Germany, being portrayed as a super-human larger-than-life figure, the "great redeemer" who had finally achieved German unification.

In his novel The Riddle of the Sands (1903), Irish novelist Erskine Childers refers to the Dybbøl, when protagonists Davies and Carruthers encounter the (then present) German victory monument during a stop at Sønderborg on their Baltic yachting expedition.