A town divided by history and split by water

Wars & Conflicts · Published 16 October 2026 Swipe more like this →

In short: Sønderborg represents a complex geographic puzzle where history and water collide. Straddling both sides of the Alssund strait, this Danish town was shaped by centuries of territorial friction between Denmark and Germany. Its very boundaries reflect a past defined by military fortifications and shifting borders in the Schleswig-Holstein region.

The town's physical layout reflects its complex geopolitical past. While much of Sønderborg sits on the island of Als, its western suburbs expanded onto the mainland of Jutland, specifically into what was formerly the interior of the fort of Dybbøl. This expansion highlights how military fortifications and shifting borders have dictated urban growth over centuries. The area also features a castle-like barracks constructed by German forces in 1906. This structure stands centrally by the Als Fjord.

The city's identity was deeply contested during the 19th and 20th centuries. Prior to the Second Schleswig War in 1864, Sønderborg was part of the Duchy of Schleswig, a Danish fief. This placed it at the heart of the long-standing friction between Denmark and Germany over the Schleswig-Holstein region. The tension reached a climax during the 1920 Schleswig Plebiscite, where the local population was asked to choose their allegiance. Sønderborg Castle currently serves as a museum dedicated to local culture and history. It remains open for visitors throughout every year.

The results of that vote reveal a community nearly split down the middle: 43.8% of Sønderborg's inhabitants voted for cession to Denmark, while 56.2% voted to remain part of Germany. Today, the town has transitioned from a contested border zone into a modern economic hub for trade, tourism, and high-tech industry. The town is now recognized by UNESCO as a Global Network of Learning City. It also hosts several institutions like the University of Southern Denmark.

Modern infrastructure now bridges these historical divides. Two distinct road bridges connect the city across the Alssund strait: the King Christian X's Bridge, built between 1925 and 1930, and the much longer Alssund Bridge, constructed from 1978 to 1981. The Alssund Bridge measures exactly 682 meters in length. Meanwhile, King Christian X's Bridge spans a distance of 331 meters.

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