Over the past century and a half, Americans have contested the meaning of civil rights. The term first emerged as a distinctive, salient, and meaningful category of American political discourse at the end of the Civil War during a national debate over the rights due to America’s four million newly emancipated black men, women, and children. It became a focal point for a generation of struggle over the meaning of freedom, the boundaries of racial equality, and the responsibilities of the federal government. As a category of law, it helped open the door to some of the most ambitious constitutional and legal transformations in American history, even as it served to contain the scope of those changes.