Complex sentence: By the time of the Potsdam conference, Poland and eastern Germany were already occupied by Russian troops and a puppet government, loyal to Moscow, was well advanced with the systematic consolidation of control over Poland as defined according to the new de facto frontiers.
Split into simpler sentences: By the time of the Potsdam conference, Poland and eastern Germany were already occupied by Russian troops. A puppet government, loyal to Moscow, was well advanced with the systematic consolidation of control over Poland as defined according to the new de facto frontiers.