Complex sentence: The Soviet Union invaded Eastern Poland on September 17, its path cleared by the lack of British and French military activity against the Nazis in Western Poland and a desire to build a cordon against German military strength west of the Soviet Union's national border.
Split into simpler sentences: The Soviet Union invaded Eastern Poland on September 17, its path cleared by the lack of British and French military activity against the Nazis in Western Poland. It wanted to build a cordon against German military strength west of the Soviet Union's national border.