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. 
German Flag History:
The current German flag was adopted on May 9, 1949. The German flag has actually been used at least three times in German history. It was adopted in 1848 and was abolished in 1852. It was then readopted as the flag of the Weimar Republic on August 11, 1919, and again abolished on March 12 1933, when it was replaced by the flag of the Third Reich. The German flag was also used by the German Democratic Republic (East Germany) until 1959, but a coat of arms to it from 1959 to 1989. Germany separated into two countries after the Second World War and reunited on October 3, 1990, and this flag was chosen.