Dybbøl had fallen on 18 April, and the Danish garrison there had withdrawn onto the island of Als. Peace talks opened in London on 12 May. The Prussians offered Denmark a border close to where the modern one runs, and Copenhagen refused it. They then proposed settling Schleswig's future by referendum, and Copenhagen refused that too. The government of Prime Minister D. G. Monrad went on insisting that the frontier be drawn at the Eider, a demand the military situation could no longer support.

On 26 June the ceasefire ran out. Prussia and Austria were free to resume, and with Jutland already taken, the next move was across water to the Danish islands. The Prussian command wanted Als and Funen simultaneously. Otto von Bismarck decided that Als would go first.

The Danish defence of Als was not casual. Trenches and artillery positions ran the length of the coast from Sønderborg in the south to Arnkilsøre at the tip of the Kær peninsula. Warships and gunboats patrolled the surrounding water, ready to intervene. The 1st Division held the shoreline from south of Sønderborg up to Arnkilsøre, with headquarters at Ulkebøl; the 7th Brigade and 1st Battery covered the stretch north of Stolbro Næs; the 3rd Cavalry Brigade watched the eastern coast down to Kegnæs.

And there was a warning. The owner of Sandbjerggård, whose land included the forest running along the Alssund from Sandbjerg to Sottrupskov, had at one point offered the Danish Ministry of War the right to fell the trees facing the water. The Ministry declined. It did not believe the Prussians would use that ground. Those were precisely the woods in which 2,500 men assembled unseen on the night of 28 June.

General Eberhard Herwarth von Bittenfeld issued the crossing orders on 28 June, with the plan itself the work of his chief of staff, Leonhard Graf von Blumenthal. The Manstein Division would go over first and seize Arnkil and Rønhave, then push toward Ulkebøl and Hørup to stop the Danes from getting away by sea. The Wintzingerode Division would follow. Boarding was set for one to two in the morning, the crossing to follow without delay.

For a short while it looked as though the plan would fail. The Danish armoured monitor Rolf Krake, lying in Augustenborg Fjord, came round into the Alssund and opened on the boats, causing severe difficulties and stopping the crossing outright. Then heavy Prussian shelling damaged her, her bridge included, and her captain — believing that Prussian troops had already landed south of his position and that he risked being cut off — turned the ship about and sailed away. The boats went on. At 2:15 the leading troops came ashore at Arnkil, took the Danish entrenchments under heavy fire, and cleared the way for a pontoon bridge of thirty-two segments to be thrown across the strait.

What followed on the island was not a battle so much as a collapse under weight. Roughly 10,000 Danish troops faced some 23,000 Prussians who now had a bridge. Regiment No. 5 fell back north toward Sønderborg. Regiment No. 18 fought the advancing Prussians around the village of Kær. General Peter Frederik Steinmann's men gave ground toward Kegnæs, at the island's southern tip, where some were taken off by ship and many were not.

The arithmetic of that morning, as most accounts break it down: 216 Danish soldiers killed, 462 wounded, 1,878 captured, 536 missing. Sources put total Danish losses at somewhere around 3,100 men (estimates range from about 3,090 up to 3,150, against a Danish force variously given as ten to twelve thousand), most of them not dead but marched into captivity. Prussian losses came to 365 killed or wounded and 7 missing — 372 in all, a figure on which the sources agree. It was, in the language of staff colleges, an exemplary amphibious operation. For the men in the trenches at Arnkil, woken by boats in the dark, it was a few hours in which the war and their country's southern third both ended.

Als passed to Prussia, and then to the German Empire, and it stayed German until the plebiscite of 1920 returned northern Schleswig to Denmark — the referendum Copenhagen had turned down in London in 1864, held at last, after two wars and two generations.

The victors marked the day in their own way. King Wilhelm I struck the Alsenkreuz, awarded to every Prussian combatant and non-combatant who took part on 29 June. Johann Gottfried Piefke, the famous German march composer, dedicated "Der Alsenströmer" to the crossing. On the Danish side the reckoning took longer and went deeper; 1864 remains the hinge year in the national memory, revisited most recently in Tom Buk-Swienty's 2010 account *Dommedag Als* — Doomsday Als. Stand today on the Sundeved shore at Sottrupskov and the Alssund is a narrow green channel with pleasure boats on it. It is a very short distance. That was the point.