Oldenburg

Oldenburg was occupied several times during the Napoleonic Wars and became a grand duchy in 1829.

In 1866 the grand duke Nicolaus Friedrich Peter sided with Prussia against the Austrian Empire and joined the North German Confederation, and in 1871 the grand duchy became a state of the German Empire.

After German Emperor Wilhelm II was forced to abdicate following the exhaustion and defeat of the German Empire in World War I, monarchic rule ended in Oldenburg as well with the abdication of Grand Duke Frederick Augustus II of Oldenburg (Friedrich August II von Oldenburg) on 11 November 1918. The Grand Duchy of Oldenburg now became the Free State of Oldenburg (Freistaat Oldenburg) and was part of the Weimar Republic.

(Source: Wikipedia.org)