Schleswig Plebiscite

The Schleswig plebiscites were two plebiscites, organized according to section XII, articles 109 to 114 of the Treaty of Versailles of June 28, 1919, in order to determine the future border between Denmark and Germany through the former duchy of Schleswig. The process was monitored by a commission with representatives from France, the United Kingdom, Norway and Sweden.

The plebiscites were held on 10 February and 14 March 1920, and the result was that the larger northern portion (Zone I) voted to join Denmark, while the smaller southern portion (Zone II) voted to remain part of Germany.

The Duchy of Schleswig had been a fiefdom of the Danish crown since the Middle Ages, but it, along with the Danish-ruled German provinces of Holstein and Lauenburg, which had both been part of the Holy Roman Empire, was conquered by Prussia and Austria in the 1864 Second War of Schleswig. Between 1864 and 1866, Prussia and Austria ruled the entire region as a condominium, and they formalised this arrangement in the 1865 Gastein Convention. The condominium was terminated due to the Austro-Prussian War in 1866.

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 idea of a plebiscite had been presented earlier. During the early phases of the First Schleswig War, the secessionist government of Schleswig-Holstein had unsuccessfully suggested a plebiscite in parts of Schleswig, but this had been rejected by the Danish government, and during the 1863 London Conference's attempts to defuse the Second Schleswig War, one of the suggestions of Prussian Prime Minister Otto von Bismarck was a plebiscite in North Schleswig. Bismarck's initiative was not adopted by the conference, primarily since the option had not been included in the instruction to the Danish delegation. The inclusion of the promise of a plebiscite in the 1866 Austro-Prussian Peace of Prague was a diplomatic concession to Austria, but was not implemented. The reference to it was subsequently dropped in the 1877 Gastein Convention between Austria and Germany. 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.

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