As soon as the armistice in the war of 1864 ended at noon on Sunday, 26th June all the Prussian batteries immediately opened fire (1) on the proposed crossing points in preparation for their attack against the island of Als on June 29th.

The army corps charged with this operation (2) was under the commands of General Herwarth (Prince Frédéric Charles having become general in chief). The force was composed of: 24 battalions - 19,200, 8 squadrons - 1,200 and 14 batteries (three Horse) - 2,600 making a total of 23,000 men (full order of battle for both sides is found here - coming).

In support was a strong reserve including 50 heavy and light calibre guns distributed along the length of the coast, which were intended to silence the fire of the Danish batteries established at Arnkil, Rønhave, etc. (3)

General Steinmann who had been charged with the defence of the island, had six regiments of infantry two squadrons of dragoons and, three light and three medium field batteries under his command, in all about 10.000 men. In addition, there were some 64 heavier guns distributed along the length of Als Sund.

With an army Corps of 10.000 men and a secure refuge in the nearby peninsula of Kegnæs (4), it was hoped that the enemy could be made to pay dearly for any attempt to cross the Sound. However this possibility was to some extent voided by the danger posed by the necessity of spreading the available forces thinly along the coast.

A situation, which usually proved fatal to the forces, involved. Making matters worse was the fact that many of the troops lacked experience and training. (5)

To reduce these risks as much as possible, some regiments were sent to Nordborg and Hardeskøj (6). Unfortunately this had the opposite effect of putting them at even greater risk of being trapped. This deficient distribution explains why such a great number of Danes were taken prisoner.

The orders were given for the crossing of the Als Sound to begin at 2am on Wednesday 29th June. At midnight, the troops were gathered for the assault; without packs and wearing the fieldmutz instead of a helmet. The Commander-in-Chief had selected the narrow straight of East Schnabeck as the place to cross that enabled the Storeskov woods to be used to mask preparations from the Danes. Several demonstrations were also to be made along the coast from Sandberg, in front of the small bay of Kjär-Westermark, to the point where the Sound broadened as it emerged into the gulf of Augustenborg. The assault force consisted of (7):

• Manstein's division, composed of the Röder and Göben brigades, Jagers, a regiment of Ziethcn hussars and four batteries.
• Winzingerode's division, composed of the Schmid and Canstein brigades (8).
  Manstein's troops would lead the attack supported by Winzingerode.

At 2 am the boats were launched, the men embarked and what would be one of the only amphibious assaults of the late 19th Century began. The Danes were caught completely by surprise. They had received no intelligence of the attack (9) and their guards failed to notice what was happening until the Prussians were almost upon them. Upon seeing the Prussians the sentries retired spreading the alarm as they went. The batteries from Als immediately opened fire but were soon overwhelmed.

The crossing was well under way (10) when the valiant Rolf Krake (11) appeared. She had come at full steam through a hail of heavy projectiles, into the middle of Sound, in order to sink the boats and shell the enemy positions.

In an effort to protect the Rolf Krake's bridge as much as possible it had been reinforced by two rows of sand bags (12). At first this protection was successful but the hail of shells was so fierce that in less a quarter of hour all the bags were torn to pieces.

This and other damage (13) forced the monitor to withdraw and seek shelter behind a tongue of land leaving the Prussians to resume their crossing (14). With all threat of intervention gone the crossing now began in earnest from six different points from Schnabeck to Sandberg (refer map).

After arriving on the shore the troops advanced into the woods which fringe the coast without meeting any real resistance. As they advanced toward the south the Prussians easily drove back the few defenders who opposed them.

Now that both shores of the Sound were under their control the Prussians moved men and equipment across without interference enabling them to assemble twelve battalions of Manstein's Division on the Danish shore in five and a half hours. These troops quickly moved south with the focus of this movement on the left in the hope of cutting off the retreat of the troops scattered on the Island.

The Danes, surprised by this sudden attack, experienced great difficulty in assembling their forces. Lacking any plan or direction, they marched toward the firing in small detachments and were repulsed everywhere with loss. A regiment of unsupported infantry (15) valiantly attacked the advancing Prussians but, finding them strongly established in the village of Kjær, were forced to retire after suffering some losses.

In the meantime, General Steinmann, who had succeeded in assembling the available troops, hurriedly marched toward the firing and met the aforementioned regiment which was retiring in good order.

He rallied it and along with his own column launch a new attack, but had no more success than before. With the Prussians constantly receiving reinforcements and confusion among his own troops he decided it was necessary to order a fighting retreat. This was conducted in good order with the Danes defending every inch of ground until they reached the area between Ulkebøl and Vollerup.

Here Steinmann, seeing that the retreat of the detachments spread out in the island was threatened by the advance of the Prussian left wing, halted the retreat and turned on the advancing Prussians in order to give these datachments time to escape.

He launched his columns in a vigorous counter attack and for half an hour held his position against overwhelming numbers. Having achieved his goal, and seeing his own left in danger of being overwhelmed and his line of retreat threatened, he resolved to withdraw towards Hørup and from there to the isthmus of Kekenis taking with him a large number of his wounded.

The Prussians immediately began spreading over the whole island troubling the Danish retirement with shelling which inflicted heavy losses on the retreating troops. Winzingerode's division, which had been advancing on the left, received the order to modify his movement in order to cut off the retirement to the troops retreating on Nordborg, Augustenborg, etc while the main column continued advancing toward Horüp.

Meanwhile Goeben's brigade (Manstein division), on the right wing, seized the so-called windmill height, close to Sonderborg, and then the city itself. Here a regiment of Danish infantry was surprised and made prisoner. However a large number who refused to surrender succeeded in carving a way out at the point of the bayonet and rejoined the column of General Steinmann; who continued a slow fighting retreat in good order, preceded by his artillery, munition, baggage and wounded. Towards noon, this column cleared the isthmus of Kekenaes.

Outposts were established in order to observe the enemy while the majority of troops were kept under arms, in the expectation of an attack. Embarkation of everything salvaged from disaster; materials, horses, etc., etc. was begun immediately. This was achieved without Prussian interference. It is not until 1st July that the Prussians crossed the gulf in some boats which they had taken from Høruphav.

The small Danish flotilla, being already embarked, immediately set sail. It was a sad spectacle; the energetic defenders kept a bitter silence as they watched the last shred of the duchy, which they had valiantly defended and which a centuries-old possession made them love like their homeland, vanish over the horizon. The embarked troops arrived at Funen on the evening of the same day.

The Danes losses included 216 dead, 462 wounded, 1878 prisoners and 536 missing. The Prussians suffered 372 casualties including seven missing. They seized thirty pieces of artillery and several flags (16).

Some month Latter the Prussian Monitor published a royal decree declaring that commemorative monuments would be constructed on the field of battle of Düppel and on the beach of Als "in order to perpetuate through all times the memory of this glorious war. "