America’s Constitution was meant to unify the new nation and help avert a civil war over the thorniest of divisions: slavery. Oops! In retrospect, that charter proved much too ambiguous, lending itself to both proslavery and abolitionist causes. In this season’s premiere, historian Liz Varon discusses the deep roots of polarization in the United States — with Will, Siva and an auditorium full of their students. The Union may have survived, Varon tells us, but its bloodiest war still echoes.