The Duke of Slesvig was a title held by the rulers of the historical region of Slesvig, located in present-day southern Denmark and northern Germany. This title was part of the larger title of the Duke of Schleswig and was associated with the control of the duchy, which played a significant role in the political landscape of Northern Europe. The title became particularly important during the 19th century, especially during the conflicts between Denmark and Germany. The Duchy of Schleswig was a focal point in the Second Schleswig War (1864), which ultimately led to its annexation by Prussia and Austria, altering the balance of power in the region.