Following the Austro-Prussian War in 1866, which ended in Austrian defeat, Schleswig and Holstein were annexed by decree of William I on 12 January 1867.

Following the defeat of Imperial Germany in World War I, the Allied powers organised two plebiscites in Northern and Central Schleswig on 10 February and 14 March 1920, respectively.

The remainder of Schleswig remained part of Schleswig-Holstein, now a province of the Free State of Prussia.

After World War II, Schleswig-Holstein was part of the British occupation zone, although some municipalities of Schleswig-Holstein east of Ratzeburg were exchanged for municipalities of Mecklenburg in the Soviet occupation zone (Barber Lyashchenko Agreement).

The British-occupied section became the new German state of Schleswig-Holstein on 23 August 1946, which joined the Federal Republic of Germany on 23 May 1949.