It's one of the iconic photographs of American history: in 1963 Birmingham, a policeman uses a German shepherd to threaten a black teenager. The image captures the climax of the Southern Christian Leadership Conference's pivotal 10-week campaign to end segregation in the city, and in telling this story, Paul Kix provides a window into the minds of the four extraordinary men who led the campaign—Martin Luther King Jr., Wyatt Walker, Fred Shuttlesworth, and James Bevel—and reflects on what it means for 21st-century America.