Part 1 of 2. This new Frontline documentary, presented just days before the inauguration of Donald Trump as the 45th President of the United States, tries to explain, and explore, why our country seems so starkly polarized as the new chief executive is about to take office. Fittingly, for such a divisive and divided subject, this Divided States of America installment of Frontline is presented in two parts. It concludes tomorrow night. Check local listings.