The Dec. 25 release of the movie Selma showcases not only one of the key chapters in the civil rights movement but also one of the most important episodes in American history, one that paved the way for more recent social-justice struggles.

Directed by Ava DuVernay and starring David Oyelowo as Martin Luther King Jr., Selma tells the epic story of how disenfranchised black Americans won the right to vote by organizing in the streets, lobbying in Washington, D.C., and being brutalized on the Edmund Pettus Bridge in a shocking act of state violence that helped win the support of President Lyndon Johnson.

In the late winter of 1965, Selma became a battleground for a voting-rights campaign that featured local activists, King’s Southern Christian Leadership Conference, and the young movement leaders John Lewis and Stokely Carmichael of the Student Nonviolent Coordinating Committee.

On Sunday, March 7, Lewis was among a group of several hundred peaceful demonstrators who were routed on the Edmund Pettus Bridge in Selma. Lewis was severely beaten, and the carnage from the event was depicted in iconic photos and news footage broadcast that evening. The brutal images from the day—which became known as Bloody Sunday—stunned Americans, emboldened young civil rights organizers and turned the voting-rights demonstrations into a global phenomenon.

Two days later, King led a second demonstration that, by prearrangement, turned around before crossing the Edmund Pettus Bridge. “Turnaround Tuesday” frustrated and upset young SNCC organizers, who vowed to participate in voting-rights efforts that they had earlier decried as a top-down strategy from movement leaders out of touch with the grass roots...