Alternate History

Battle of Sønderborg (Magnam Europae)

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Battle of Sønderborg



Als, near and in Sønderborg harbor


Viking victory. Franks are repelled from Jutland. Founding of Sønderborg fortress.


Carolingian Union
Oriflamme du Irene Carolingian Union

  • Oriflamme du Irene Frankish Empire

No flag Danes
No flagSwedes


No flag Ragnar Lodbrok




Casualties and Losses




The Battle of Sønderborg was a major battle in the First Viking Wars that resulted in the expulsion of Frankish forces from Jutland until their second invasion in 856. The period following the Battle of Sønderborg saw devastation along the Rhine river in Frankish territory. The Frankish loss was likely due to attrition, exhaustion, and lack of supplies to the faltering war effort.


Following the Battle of Ravning Bridge, which resulted in a Frankish defeat, the Franks were rapidly expelled from Jutland due to Swedish reinforcements and attrition during a particularly harsh winter. While the Franks were able to achieve a number of Pyrrhic victories, the war effort was largely and catastrophically faltering. Multiple ambushes and counterattacks pushed the Frankish war effort to the Frankish-Danish border.

At the time of the battle, Sønderborg was a largely unpopulated area inhabited mostly by commoners. When the Franks invaded, however, they established a small, hastily-constructed fortress on Als, near Sønderborg harbor. While the other Frankish-held towns and forts were retaken or destroyed by the Vikings, Sønderborg remained in Frankish hands. Upon receiving word of the existence of this fortress, the Danes and the Swedes prepared for an attack in the autumn of 847.


The Vikings had established a presence near Als to prepare for an attack, though they made no attempt to invade until a foggy morning. Early that morning, ships surrounded the island while raiders landed in Sønderborg Harbor. The raiders surrounded the fort in preparation to attack. The first wave of Vikings attacked while many Frankish troops were still asleep, allowing for an element of surprise. By the time the second wave arrived, the Frankish numbers had been devastated.

The battle was described to have been short and bloody, with the Vikings in full control of the fort and the Franks on the run by noon. A very low number of Franks successfully escaped the island. The rest, including those that surrendered, were slaughtered by the Vikings.


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