Ostpreußen Ostpreussen flag 90 x 150 cm
East Prussia (German: Ostpreußen) refers to the main part of the region of Prussia along the southeastern Baltic Coast from the 13th century to 1945.[1] From 1772–1829 and 1878–1945, the Province of East Prussia was a province of the German state of Prussia. The capital of East Prussia was Königsberg.

East Prussia enclosed the bulk of the ancestral lands of the Baltic Old Prussians. During the 13th century, the native Prussians were conquered by the crusading Teutonic Knights. The indigenous Balts who survived the conquest were gradually converted to Christianity. Because of Germanization and colonisation over the following centuries, Germans became the dominant ethnic group, while Poles and Prussian Lithuanians formed minorities. From the 13th century on, East Prussia was part of the monastic state of the Teutonic Knights, which became the Duchy of Prussia in 1525.[2] The Old Prussian language became extinct by the 17th century[3] or early 18th century.

Upon the death of Hohenzollern Albert of Brandenburg Prussia, Duke of Prussia (1525-1568) the prince-elector Kurfürst of Brandenburg Joachim II Hector became co-inheritor of Ducal Prussia. Since 1577 House of Hohenzollern co-regents took over administration from Albert's only son Albert Friedrich. In 1618 the Duchy of Prussia was again inherited and in personal union with the Hohenzollerns of Brandenburg and the territory was called Brandenburg-Prussia. The territories of the House of Hohenzollern were scattered in Franconia, Brandenburg, eastern Prussia and elsewhere.

Because the duchy was outside of the core Holy Roman Empire (yet Prussia was under HRE adminitration by the Teutonic Order grandmasters) the prince-electors of Brandenburg were able to proclaim themselves kings in Prussia beginning in 1701. After the annexation of most of western Royal Prussia in the First Partition of the Polish-Lithuanian Commonwealth in 1772, East Prussia was connected with the rest of the Prussian state and reorganized into the Province of East Prussia the following year. Between 1829 and 1878, the Province of East Prussia was joined with West Prussia to form the Province of Prussia.

The Kingdom of Prussia became the leading state of the German Empire after its creation in 1871. The Treaty of Versailles following World War I made East Prussia an exclave of Weimar Germany, while the Memel Territory was added to Lithuania. Following Nazi Germany's defeat in World War II in 1945, war-torn East Prussia was partitioned between Soviet Union (the Kaliningrad Oblast), the People's Republic of Poland (the Warmian-Masurian Voivodeship), and the Lithuanian SSR (the constituent counties of the Klaipėda Region).[4] The capital city Königsberg was renamed Kaliningrad in 1946. The German population of the province largely evacuated during the war, during the years 1944–46, but an estimated 300,000 (around one fifth of the population) died due to war circumstances[citation needed]and the remainder were subsequently expelled.
source: Wikipedia