1920 Schleswig plebiscites

Article 5 of the Austro-Prussian Peace of Prague (1866) stipulated that a plebiscite should be held within the ensuing six years in order to give the people of the northern part of Schleswig the possibility of voting for the region's future allegiance by allowing regions voting for Danish rule to be restored to Danish administration.

The 1864 border was confirmed in the 1907 German-Danish Optant Treaty, but Danish North Schleswigers continued to argue for a plebiscite citing the 1866 Peace of Prague.

Danes campaigning for implementation of the plebiscite promise, in the hope that it would result in the area being restored to Danish rule, often made a comparison with the French demand for return of Alsace-Lorraine.

Clausen's first line delineated a coherent territory that he expected would vote Danish in a future plebiscite, and the second line (about 10–20 km or 6–12 miles further south) included a thinly-populated rural region in Central Schleswig, which Clausen believed had potential for assimilation into Denmark, as the population of Central Schleswig was pro-German in allegiance, but also Danish-speaking.

[5] At the same time, he effectively abandoned his second line, as the population of Central Schleswig remained pro-German, and as he considered it vital that the future border should be based on the self-determination of the local populace.

When in doubt, Clausen primarily relied on the wishes of the rural communities, which he considered autochtone (indigenous), in contrast with the North Schleswig towns, which he considered largely irrelevant due to their smallness and their less-autochtone population, notably the demographics of the town of Sønderborg (Sonderburg) which had become strongly influenced by the presence of a large German naval base.

In 1918 and early 1919, the leading Danish political parties argued that the future allegiance of North Schleswig should be decided by a plebiscite, in which the entire region should be counted as one indivisible unit, i.e. vote en bloc.

As a compromise, it was decided to request the Allied Powers to extend the plebiscite until Clausen's second line, which was adapted to include Flensburg and Glücksburg.

The Allied Powers granted this request but extended it, consequently entitling anyone who had previously lived in the region to vote in the plebiscite.

The Allied Powers had offered to include this region in the plebiscite, but the Danish government had expressly asked for Zone III to be excluded.

[8] Tiedje's Line was strongly criticised in Denmark, as it would have transferred a large number of pro-Danish communities south of the future border, and was refused by all parties in the Danish parliament.