Complex sentence: The eastern Polish territories which the Soviet Union had occupied in 1939 (minus the Bialystok region) were permanently annexed, and most of their Polish inhabitants expelled: today these territories are part of Belarus, Ukraine and Lithuania.
Split into simpler sentences: The eastern Polish territories which the Soviet Union had occupied in 1939 (minus the Bialystok region) were permanently annexed, and most of their Polish inhabitants expelled. Today these territories are part of Belarus, Ukraine and Lithuania.