In March 1965, 600 people set out from Selma to Montgomery, Alabama, in a nonviolent march for voting rights. State troopers attacked the protesters at the edge of the city, and the violence galvanized activists. A few weeks later, President Lyndon Johnson introduced the Voting Rights Act, which guaranteed the right to vote regardless of race or ethnicity. Filmmaker Ava DuVernay interprets these and other events in her epic drama “Selma.”