The communist German Democratic Republic was established in the historic "Mitteldeutschland" ( Middle Germany ). Former German territories east of the Oder and Neisse rivers, mainly the Prussian provinces of Pomerania , East Prussia , West Prussia , Upper Silesia , Lower Silesia , the eastern Neumark of Brandenburg , and a small piece of Saxony were thus detached from Germany. To compensate Poland for the USSR's annexation of its eastern provinces, the Allies provisionally established Poland's post-war western border at the Oder–Neisse line at the Yalta Conference (1945). As a result, most of Germany's central territories became the Sowjetische Besatzungszone (SBZ, Soviet Occupation Zone). All other lands east of the Oder–Neisse line were put under Polish administration, with the exception of historic northern East Prussia , which went to the USSR. 
On December 1, 1989, the People's Chamber removed the section of the East German Constitution granting the SED a monopoly of power—thus ending Communist rule in East Germany. Before the month was out, the SED transformed from a Leninist cadre party into a democratic socialist party, renaming itself first to Socialist Unity Party — Party of Democratic Socialism and later in the same year, to Party of Democratic Socialism (PDS). Hence, the party's subsequent leaders were no more leaders of East Germany than the leaders of other parties.