Oldenburg was a county resp. (after 1773) a duchy of the Holy Roman Empire. After the dissolution of the empire (1806) and the episode of Napoleonic occupation Oldenburg was made an independent country by the Congress of Vienna in 1815. In 1871, it joined the German Empire, and in 1918, it became a state within the Weimar Republic. After World War II, its main part was merged into the newly founded state of Lower Saxony, the district of Eutin was added to the state of Schleswig-Holstein, and the district of Birkenfeld was added to the state of Rhineland-Palatinate.
Oldenburg had an area of 5,375 kmē, and 580,000 inhabitants (1939).
For more details about the regional history see Oldenburg.