Schleswig (shlĕsˈvĭkh) [key], Dan. Slesvig, former duchy, N Germany and S Denmark, occupying the southern part of Jutland. The Eider River separates it from Holstein. German Schleswig forms part of Schleswig-Holstein. Danish Schleswig, known as North Schleswig (Dan. Nordslesvig or Sønderjylland ) includes the cities of Åbenrå, Haderslev, Sønderborg, and Tønder, and was incorporated with Denmark following a plebiscite in 1920.
The duchy of Schleswig, created in 1115, was a hereditary fief held from the kings of Denmark. King Waldemar III (who had been duke of Schleswig as Waldemar V) conferred Schleswig on his uncle, Gerhard, and granted a charter forbidding the union of Schleswig and Denmark under a single overlord. In 1386 the count of Holstein received Schleswig as a hereditary fief. His descendant, Christian I of Denmark, inherited (1460) both Schleswig and Holstein, but he was obliged to recognize the inseparability of the two territories and to affirm that they were bound to the Danish crown by a personal union only.
In the 16th cent. Schleswig and Holstein (which had also become a duchy) underwent complex subdivisions, although theoretically the principle of the inseparability of the two duchies was not violated. The three main divisions were: a ducal portion, including parts of both duchies, which was conferred on Adolphus, duke of Holstein-Gottorp, younger brother of Christian III of Denmark, and on his descendants, the dukes of Holstein-Gottorp; a royal portion, including parts of both duchies, ruled directly by the Danish kings; and a common portion, ruled jointly by the Danish kings and the dukes of Holstein-Gottorp.
By the Treaty of Roskilde (1658) the Danish crown renounced its suzerainty over ducal Schleswig; the resulting quarrels between Denmark and the duke of Holstein-Gottorp were a major factor in the Northern War (1700–1721), which ended with the dispossession of Duke Charles Frederick of Holstein-Gottorp and the union of the ducal portion of Schleswig with the Danish crown. Grand Duke Paul (later Emperor Paul I), renounced (1773) the ducal portion of Holstein, yielding it to the Danish crown, in exchange for Oldenburg. Thus all Schleswig and Holstein were once more united under the Danish kings. The events related in the article Schleswig-Holstein led to the annexation (1866) of both duchies by Prussia.
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