Last week marked the 49th anniversary of one of the most important events in American history. It began on March 7, 1965, when Alabama state troopers routed peaceful demonstrators on the Edmund Pettus Bridge in Selma, which was dedicated on last Monday as a national landmark. The violence that engulfed the nonviolent, overwhelmingly black cadre of marchers helped inspire a national outcry against police brutality, institutional racism and segregation.

Georgia Rep. John Lewis, then the young chairman of the Student Nonviolent Coordinating Committee (SNCC), suffered severe head wounds, and breaking news of the violence interrupted a network TV broadcast of “Judgment at Nuremberg,” a film about trials for Nazi war crimes. Martin Luther King Jr., who had led a voting-rights campaign in Selma, led a face-saving demonstration two days later that turned around at the bridge, lest demonstrators be subjected to another violent police attack like the previous episode, which would be forever known as “Bloody Sunday.”

Leading up to the march, 26-year-old activist Jimmie Lee Jackson had been shot as he tried to shield his mother from a police beating after police and state troopers broke up an earlier voting-rights march. The Selma campaign made Alabama — the citadel of Dixie — ground zero in the grass-roots struggle to pressure the federal government into passing voting-rights legislation.

Bloody Sunday's violence reverberated far enough into American political culture to inspire a diverse group of politicians and civil rights activists. Stokely Carmichael, the young SNCC organizer prone to challenging conventional wisdom, joined Selma's demonstrations with renewed vigor. Carmichael would use the pageantry of the March 21-25 demonstration from Selma to Montgomery to plough fertile organizing ground in Lowndes County, Ala.

President Lyndon Johnson responded with both words and deeds. Johnson's televised address to a joint congressional session on Monday, March 15, would prove historic. Arguing that “the dignity of man and the destiny of democracy” required black voting rights, Johnson channeled King's poetic eloquence by placing Selma's demonstrators in the pantheon stretching back to the American Revolution.

Johnson concluded his speech with a capstone that was as succinct as it was stunning: “And we shall overcome,” he proclaimed.

King, who had refused the White House invitation to attend Johnson's speech out of his commitment to the movement, wept as he heard these words. Not only had the president of the United States endorsed the movement's long-suppressed quest for the vote, but he had openly embraced the movement's anthem before the entire nation.

Less than five months later, Johnson signed the Voting Rights Act, handing one of the commemorative pens to King, whose smile signaled just how much the world's most powerful elected leader had come to rely on him.

The Bloody Sunday marchers' eventual victory now seems almost predestined — a fait accompli enshrined as the high point of the civil rights struggle's heroic era. Yet this is too facile.

The movement struggled to find its way in the immediate aftermath of that terrible Sunday's violence. From the outside, King's voting-rights crusade seemed poised for defeat. Internally, activists debated and fought among themselves over which direction to go. The government struggled to respond and react to events on the ground that seemed to outpace bureaucratic vision and political imagination.

Almost a half-century later, we remain more comfortable discussing historical landmarks that ultimately serve as signposts for political victories and defeats.

The movement's internal challenges and the nation's collective ambivalence about — if not outright hostility toward — the very ideal of racial equality is a part of the civil rights story that deserves additional reflection. This past weekend, which fell between Bloody Sunday and LBJ's “Moral Monday” speech, gives us the opportunity do more than simply commemorate; we can also contemplate the struggles that led up to the game-changing historical moments that we more frequently study, discuss and debate.

These struggles exemplify the way in which, contrary to popular belief, social movements do not move in a straight line. More often they proceed in fits and starts, lurching inelegantly toward victories and defeats in ways that confound supporters and opponents alike. But in this chaotic atmosphere of social change lies the beauty of political struggle and resistance, even when — like those valiant, nonviolent soldiers in Selma — their future remains unknown.

Peniel E. Joseph, a contributing editor at The Root and Tufts University history professor, is founding director of the Center for the Study of Race and Democracy, the Caperton fellow for the W.E.B. Du Bois Research Institute at Harvard and author of “Dark Days, Bright Nights: From Black Power to Barack Obama” and the newly released “Stokely: A Life. “