The Duke of Sleswig was a title held by the rulers of the historical region of Sleswig, located in present-day Denmark and northern Germany. This title was part of the larger Duchy of Sleswig, which was often associated with the Duchy of Holstein. The dukes played a significant role in the political landscape of northern Europe, especially during the Middle Ages. The title of Duke of Sleswig has its roots in the feudal system, where dukes governed territories on behalf of a king. Over time, the duchy became a point of contention between Denmark and Germany, leading to conflicts such as the Second Schleswig War in 1864.