The Western Allies refuse to vacate parts of Germany assigned to the Soviet occupation zone, while Stalin undermines the four powers Berlin division. As early as 1946, Germany is increasingly divided between east and west. Stalin, knowing the GDR is unviable without Silesian coal deposits, allows Silesia and Pomerania to be repopulated by refugees.
In 1949, the Federal Republic of Germany is formed, including Saxony, Thuringia, and Saxony Anhalt. Shortly afterwards, the GDR is formed including all of Berlin, Silesia, and Pomerania. All of Upper Silesia is annexed into Poland.
East Prussia is made an ASSR within the Lithuanian SSR; Stalin stages a referendum giving this popular legitimacy. The Polish parts of southern East Prussia are annexed into Poland. Also, the German population deported to Siberia are re-deported to East Prussia. Danzig is annexed into the East Prussian ASSR to more tightly monitor Polish seaward trade. However, East Prussia is intentionally kept deindustrialized with low levels of education; ironically Stalin allows the Lutheran Church (infiltrated by NKVD) to retain its influence to keep East Prussians superstitious.
During the Khrushchev thaw, riots erupt as East Prussians demand their own SSR. The riots are crushed by the Soviet and Polish Army, claiming the rioters are fascist agitators. Rioters not sent to Siberia are sent to West Germany.
The GDR government collapses in 1989. It is swiftly annexed into the Federal Republic within a year. East Prussia remains under Soviet rule.
During the USSR"s final months, violence between Germans and Lithuanians erupt, played up by the KGB to force a crackdown. The East Prussian ASSR declares independence from the Lithuanian SSR. Although officially disowned by the German government, private German assistance flows in.
The Soviet Union collapses. Desperate to avoid war, Germany brokers a deal where East Prussia holds a referendum on its future while Lithuania receives generous financial assistance. East Prussians narrowly choose annexation to Germany over independence. To allay fears of German militarism, Kohl promises to permanently declare East Prussia demilitarized. The Danzig corridor issue is solved by a transit treaty with Poland. When Poland joins the EU, the issue becomes irrelevant.
There. I solved the challenge.