East Prussia (German: Ostpreußen, pronounced [ˈɔstˌpʁɔʏsən]; Polish: Prusy Wschodnie; Lithuanian: Rytų Prūsija; Latin: Borussia orientalis; Russian: Восточная Пруссия) was a province of the Kingdom of Prussia from 1773 to 1829 and again from 1878 (with the Kingdom itself being part of the German Empire from 1871); following World War I it formed part of the Weimar Republic's Free State of Prussia, until 1945. Its capital city was Königsberg (present-day Kaliningrad). East Prussia was the main part of the region of Prussia along the southeastern Baltic Coast.
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. After the conquest the indigenous Balts were gradually converted to Christianity. Because of Germanization and colonisation over the following centuries, Germans became the dominant ethnic group, while Poles and Lithuanians formed minorities. From the 13th century, East Prussia was part of the monastic state of the Teutonic Knights. After the Second Peace of Thorn in 1466 it became a fief of the Kingdom of Poland. In 1525, with the Prussian Homage, the province became the Duchy of Prussia. The Old Prussian language had become extinct by the 17th or early 18th century.
Because the duchy was outside of the core Holy Roman Empire, the prince-electors of Brandenburg were able to proclaim themselves King beginning in 1701. After the annexation of most of western Royal Prussia in the First Partition of the Polish-Lithuanian Commonwealth in 1772, eastern (ducal) Prussia was connected by land with the rest of the Prussian state and was reorganized as a province the following year (1773). 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. However, the Treaty of Versailles following World War I granted West Prussia to Poland and made East Prussia an exclave of Weimar Germany (the new Polish Corridor separating East Prussia from the rest of Germany), while the Memel Territory was detached and was annexed by Lithuania in 1923. Following Nazi Germany's defeat in World War II in 1945, war-torn East Prussia was divided at Joseph Stalin's insistence between the Soviet Union (the Kaliningrad Oblast in the Russian SFSR and the constituent counties of the Klaipėda Region in the Lithuanian SSR) and the People's Republic of Poland (the Warmian-Masurian Voivodeship). The capital city Königsberg was renamed Kaliningrad in 1946. The German population of the province was largely evacuated during the war or expelled shortly thereafter in the expulsion of Germans after World War II. An estimated 300,000 (around one fifth of the population) died either in war time bombings raids, in the battles to defend the province or through mistreatment by the Red Army.