The Ku Klux Klan is active in my part of the U.S., and the film helped me understand its bitter legacy — “The Birth of a Nation” was “the most effective and long-lasting source of racial propaganda in American history,” Timothy Egan rightly argues in his new book about the spread of the Ku Klux Klan beyond the South, A Fever in the Heartland (Viking, 2023). Roger Ebert has written perhaps the…