Complex sentence: His parents returned to Poland under a repatriation agreement between Stalin and the Polish government-in-exile, and later--finding the post-war environment there to be dangerously anti-Semitic--the family immigrated to France.
Split into simpler sentences: His parents returned to Poland under a repatriation agreement between Stalin and the Polish government-in-exile. Later, finding the post-war environment in Poland to be dangerously anti-Semitic--the family moved to France.