Als lies to the east of the Jutland peninsula, across from the Danish town of Sønderborg, and north of the coast of Southern Schleswig, Germany.

King Christian III's son, Duke John, came in possession of the island as a titular Duchy, and he bought the other noblemen out.

The Duchy was taken over by the Danish Crown after the last Duke of Augustenborg to live at the palace, Christian August II, had sided with the Schleswig-Holstein pro-German nationalist movement against Denmark.

That same year during the First Schleswig War (1848–1851), the Danes directed their main attack against Field Marshal Friedrich Graf von Wrangel's Austro-Prussian army from the lighthouse on the peninsula of Kegnæs at the southwest end of Als.

Sønderborg castle is located in the centre of the town, and houses a museum focusing on the history and culture of the area.

Sandbjerg Estate, which had belonged for many years to the Dukes of Sønderborg, and then to the Reventlow family, was donated to Aarhus University in 1954.

The writer Herman Bang was born in 1857 Asserballe on Als and was a child during the Second Schleswig War when the island came under Prussian (later German) rule.

That traumatic war forms the background of Bang's novel Tine (1889), which tells the tragic love story of a young girl on the island of Als.