The Duchy of Schleswig and Holstein refers to two historical regions in northern Europe, located in what is now Germany and Denmark. These duchies were part of the Holy Roman Empire and later became significant in the conflicts between Germany and Denmark during the 19th century. Schleswig was predominantly inhabited by Danish speakers, while Holstein had a majority of German speakers. The differing national identities led to tensions, culminating in the Second Schleswig War in 1864, after which both duchies were annexed by Prussia. This event played a crucial role in shaping the national boundaries in the region.