Complex sentence: After 1945, most of the Polish population of the area east of the new Soviet-Polish border fled or was expelled to the newly annexed German territories and the area today is almost entirely Belarusan (in the north) or Ukrainian (in the south).
Split into simpler sentences: After 1945, the Polish population of the area east of the new Soviet-Polish border was in general confronted with the alternative either to accept a different nationality or to emigrate. The area today is almost entirely Belarusan (in the north) or Ukrainian (in the south).