The historian who drafted an open letter, signed by more than 1,000 scholars, calling for Donald Trump's impeachment explains why experts on the past should weigh in on the present. A must-listen, as the Senate prepares for the fourth presidential impeachment trial -- in this case, of an ex-President. There seems little doubt Donald Trump will be acquitted. Since Watergate, Americans politics have become more polarized and the media more partisan. Historian David Greenberg weighs in on the changed political and media landscape.