Nicholas Lemann, a Washington Monthly contributing editor, is dean of Columbia University's Graduate School of Journalism and the author of "Redemption: The Last Battle of the Civil War."

Children in elementary school often come home with the idea that the purpose of the Civil War was to end slavery—but if that were true, then why did it take Abraham Lincoln so long to issue the Emancipation Proclamation, and why was it less than universally popular in the Union states? If you see the movie Lincoln, you get a much fuller picture of the contingency of emancipation, and of the difficulty of passing the Thirteenth Amendment to the Constitution, which abolished slavery completely—but why didn’t Lincoln and the Congress think to address at the same time the obvious question of what status the freed slaves would have after that? After Lincoln’s assassination, Congress and the state governments settled that matter by passing the Fourteenth and Fifteenth Amendments, which gave the former slaves full civil rights and voting rights—but why was it necessary for exactly the same rights to be reenacted, after enormous struggle, nearly a century later, during the civil rights era?

The answers to all these questions are essentially the same: for most of American history, white America has been highly ambivalent, or worse, about the idea of full legal equality for black Americans. Emancipation itself was a forced move, an obvious consequence of the war only in retrospect; it happened because in war zones in the Confederate states, slaves left their plantation homes and appeared at Union army encampments (they were known at the time as “contraband”), and somebody had to decide what to do about them; sending them back to their owners would be both morally suspect and a form of material aid to the enemy. There has always been a debate about what kind of Reconstruction regime Lincoln would have instituted after the war, had he lived; his racial impulses were generous, but he was not an abolitionist until he actually abolished slavery. Reconstruction—the tumultuous decade or so that followed the Civil War—was an enormous shaping force in American history, and not just in the area of race relations. It’s worth recounting in basic outline, because it’s a far less familiar story than that of the Civil War itself, but far more relevant today.

The word “Reconstruction” is somewhat misleading in the American case, because it implies that the main challenge was managing the tension between punishing the South for seceding and getting it back on its feet economically and politically. In this instance the more pressing question was what the lives of the millions of freed slaves in the South would be like. Would they be able to vote? To hold office? To own property? To sue white people? Would government undertake an active, expensive effort to educate them and put them on the way to economic self-sufficiency? Merely to say that former slaves were now free turned out to resolve remarkably little....