Complex sentence: In the post-war period, the Communist government of the People's Republic of Poland (1944--90) created monuments at the extermination camp sites, that mentioned no ethnic, religious, or national particulars of the Nazi victims.
Split into simpler sentences: In the post-war period the Communist government of the People's Republic of Poland (1944--90) created monuments at the extermination camp sites. The monuments mentioned no ethnic, religious, or national particulars of the Nazi victims.