Ger. Ostpreussen, former province of Prussia, extreme NE Germany. The region of East Prussia has low rolling hills that are heavily wooded, and it is dotted by many lakes (especially in Masuria) and drained by several rivers including the Nemen (Nieman). Its Baltic coast is deeply indented by the Vistula Lagoon (Frisches Haff) and by the Gulf of Kursh (Kurisches Haff). In the 13th cent. the Teutonic Knights conquered the region from the Borussi, or Prussians (a people related to the Liths), displaced the original population, and secured the territory as a fief for their order. In 1309, Malbork became the headquarters of the grand master of the Teutonic Knights.
In 1466, by the Peace of Torun, the knights ceded Pomerelia (see Pomerania; later a part of West Prussia) and Ermeland to Poland and accepted Polish suzerainty over the rest of their domain. Grand Master Albert of Brandenburg, after secularizing the Teutonic order, took the title “duke of Prussia” in 1525, remaining under Polish suzerainty. The duchy was inherited (1618) by the elector of Brandenburg. Frederick William, the Great Elector, won full sovereignty over the duchy at the Peace of Oliva (1660), and in 1701 his son, Frederick III, had himself crowned “king in Prussia” as Frederick I at Königsberg (Kaliningrad).
East Prussia, as the original Prussia came to be called, from 1701 to 1945 shared the history of Prussia. It remained the stronghold of the Prussian landowning and military aristocracy—the Junkers—whose immense estates took up a large part of the province. From 1919 to 1939 it was separated from the rest of Germany by the Polish Corridor and the Free City of Danzig (Gdańsk). Königsberg was the capital. East Prussia bordered on Poland and Lithuania in the south and east and stretched to Memel and the Baltic Sea in the north and northeast.
In 1945, at the end of World War II, East Prussia was overrun by Soviet troops and about 600,000 of its inhabitants were killed. Most Germans who had not left by the end of the war were expelled by the Polish and Soviet governments shortly after its end. At the Potsdam Conference (1945), East Prussia was divided by two transfers; the transfers were made permanent by treaties between West Germany and Poland and the USSR that were signed and ratified between 1970 and 1972. The northern part was assigned at Potsdam to the USSR; it includes the cities of Kaliningrad, Sovetsk (Tilsit), Chernyakhovsk (Insterburg), Gusev (Gumbinnen), and Baltiysk (Pilau). The rest was incorporated into Poland as Olsztyn province; this part includes the cities of Olsztyn (Allenstein), Malbork (Marienburg), and Elbląg (Elbing).