Ten earthworks stood here — seven large redoubts and three smaller ones — in a three-kilometre half-circle from Vemmingbund to the Als Sound, begun in 1861 and still unfinished when the war arrived. Most of the army's fortification effort had gone instead into the Danevirke to the south, and when that old rampart line was given up the army fell back on a position lacking even safe shelter in its forward trenches. Artillery had also moved on. Long-range rifled guns could now reach across the southern inlet and rake the Danish line along its whole length, so between assaults most of the defending crews were pulled far to the rear. The men holding the redoubts on 18 April were exhausted and too few, and they carried M1822 percussion muskets converted to Minié rifling against a Prussian army with the Dreyse needle-gun, which loaded at the breech and could be reloaded lying flat.

The Prussians committed 11,000 men in the first wave, with 26,000 in reserve and 126 guns. The Danes had roughly 5,000 at the defences, 6,000 behind them, 66 guns and 11 mortars. They also had one thing Prussia could not answer: the ironclad Rolf Krake, whose turret-mounted eight-inch guns shelled the shore from the Alssund at will. No Prussian warship in those waters could drive her off, and her presence drained enough Prussian morale that some Danish generals persuaded themselves a frontal assault would never come. The Prussian columns took position at two in the morning and waited eight hours in the dark while the artillery worked. When the guns lifted at ten, the infantry went forward. Fire from Rolf Krake was not enough to stop them.

What saved the Danish army from destruction was a counterattack with almost no chance of succeeding. The 8th Brigade went into the collapsing centre and held the Prussian advance long enough for the rest of the army to withdraw across the Alssund to the island of Als. It cost the brigade 1,399 men out of 3,000 — close to half, in a matter of minutes. They were conscripts: farmers, apprentices and labourers sent to hold ground their own commanders had privately judged untenable. Danish memory has honoured the 8th Brigade ever since, and the honour is earned. It should not obscure what they were spending their lives to cover.

The casualty lists disagree with one another, which is itself part of the story of a rout. The Danish army's official tally recorded 671 dead and 987 wounded, 473 of the wounded taken prisoner where they lay, plus 3,131 unwounded men captured or gone from the ranks — 4,789 in all. A later Danish study of the war gives 808 dead, 909 wounded, 2,872 captured and 215 missing. Prussian losses came to 1,201, including 263 killed. Aboard Rolf Krake one man died and ten were hurt. A truce held from 12 May to 26 June while negotiators in London tried to draw a new border; Denmark asked for the line of the River Eider and was refused. The war resumed, the Prussians crossed to Als in late June, and Bismarck took another step toward the empire he was assembling.

No crowds met the army when it returned to Copenhagen. Newspapers did not blame the soldiers, but the silence carried its own verdict. The battlefield now lay inside Germany and out of Danish reach, and the Germans built on it: a 24-metre column, the Düppel-Denkmal, raised on what had been Danish redoubt 4, with a hotel nearby by 1885 and the Kaiser visiting in 1890. Danes had instead the Dybbøl Mill — wrecked in 1849, rebuilt, wrecked again in 1864, rebuilt once more by owners of firmly Danish sympathies. After the poet Holger Drachmann came in 1877 and wrote of the place as a Danish Thermopylae, the mill became a national symbol and has stayed one.

The plebiscites that followed the First World War returned Northern Schleswig to Denmark. Some 50,000 people gathered on the hill on 11 July 1920, with Christian X standing in the tenth redoubt, known ever since as Kongeskansen, and a veteran of the battle handing the king an old Dannebrog. Donations from across the country bought the battlefield, which was given to the state as a national park in 1924. The Düppel-Denkmal was destroyed in May 1945, presumably by members of the resistance; nobody was ever identified, and it was never rebuilt. An offer of a joint anniversary with Germany was declined in 1966. German soldiers first joined the commemoration in 1998 and marched alongside Danes for the first time in 2011. Every 18 April, Danish soldiers in period uniform form up here, and the country goes on disagreeing about whether the day was a catastrophe, a warning, or the beginning of the smaller, stubborner Denmark that grew out of it.