In 1864, the Prussian army, commanded by Herwarth von Bittenfeld, invaded Als following a night attack planned by Chief of Staff, Feltmarschall Leonhard Count Blumenthal. This attack ended the 2.nd Slesvig War between Denmark on the one side and Prussia/Austria on the other side. The Battle of Als was the last major battle of the war.
Als was defended by 9,000 Danish troops including the Danish garrison at Dybbøl, which had retreated to the island of Als. In the night of the 29th June 1864, 2500 Prussian troops attempted to cross the Straight of Alssund in small boats. The modern, armoured warship Rolf Krake anchoring in Augustenborg Fjord, moved to the Straight of Alssund, where she created major difficulties for the Prussian troops and for a while stopped the invasion.
However, following a misunderstood order, she left suddenly and sailed away, leaving the Prussians to complete their crossing. Two o’clock at night the Prussian army landed on the peninsula of Arnkil and overran the Danish trenches under heavy fire.
The Prussians then built a pontoon bridge across the Straight of Alssund.
The 5th Regiment of the Danish army withdrew to Sønderborg while the 18th Regiment had another skirmish with the Prussians near the village of Kær. The Danish army finally withdrew to Kegnæs – some troops surrendered and others were evacuated by ships. Denmark lost almost 3,000 troops (dead, wounded and captured) in the Battle of Als.
Following the Danish defeat by the Austrian/Prussian army, Als became a part of Prussia and later of Germany itself.