Duchy of Schleswig
From the Middle Ages till 1864 Fehmarn formed part of the Danish Duchy of Schleswig. When the duchy was partitioned in 1544, it formed part of the duchy of John the Elder. Upon his death without heirs in 1580, Fehmarn became part of the Duchy of Schleswig-Holstein-Gottorp. After the Great Northern War Fehmarn along with the rest of Schleswig was united with the Danish crown. In 1864 Schleswig passed to Prussia as a result of the Second Schleswig War.