On the face of it, this question might seem a bit obtuse, because annexations were not uncommon in 19th-20th century European conflicts. However, I was just curious about eastern Germany. Looking at old maps, it looks as though these regions (Eastern Pomerania, Frankfurt, Liegnitz, Breslau, Oppeln, Ostprussen) belonged to Germany/Prussia for quite some time, some parts since 1700.
I'm more curious as to why these annexations happened, what the justifications were, what kind of opposition Germans put up against it, etc. Did these regions have ethnically polish majorities?
Starting around 1944 the German population in these areas began to flee back west. The first to begin fleeing were the German settlers who had settled in the Baltic area and the areas annexed by Poland. Many found their way to East Prussia. The Wehrmacht and what was left of the Kreigsmarine fought an extremely desperate rearguard action to allow these refugees to be evacuated via boat from the port of Pillau. It was relatively successful but some Soviet submarines got into the harbor and destroyed a few of the ships killing around 8000 German civilians.
Now after the war the Russians and other allied communist states began to systematically remove the remaining German population from Czechoslovakia, Hungary, and East Germany. As a result these areas annexed by Russia and the Eastern European satellite states had next to no ethnic Germans living in them any more. The allies also felt they need to compensate the states that had suffered under Nazi rule.
Source:
Second World War by: John Keegan