 Prior to the Second Schleswig War of 1864, Sønderborg was situated in the Duchy of Schleswig, a Danish fief, so its history is properly included in the contentious history of Schleswig-Holstein.In the 1920 Schleswig Plebiscite returned Northern Schleswig to Denmark, 43.8% of the city of Sønderborg's inhabitants voted for the cession to Denmark and 56.2% voted for remaining part of Germany.The town of Sønderborg lies on both sides of Alssund; the narrow strait between these two sides is called Als Strait ("Alssund").