What obstacles kept most black citizens from being able to vote

Selma: The Bridge to the Ballot is the story of a courageous group of Alabama students and teachers who, along with other activists, fought a nonviolent battle to win voting rights for African Americans in the South. Standing in their way, a century of Jim Crow, a resistant and segregationist state, and a federal government slow to fully embrace equality. By organizing and marching bravely in the face of intimidation, violence, arrest and even murder, these change-makers achieved one of the most significant victories of the civil rights era.