Eric Foner is a professor of history at Columbia and the author, most recently, of “The Fiery Trial: Abraham Lincoln and American Slavery.”

ONE hundred and fifty years ago, on Jan. 1, 1863, Abraham Lincoln presided over the annual White House New Year’s reception. Late that afternoon, he retired to his study to sign the Emancipation Proclamation. When he took up his pen, his hand was shaking from exhaustion. Briefly, he paused — “I do not want it to appear as if I hesitated,” he remarked. Then Lincoln affixed a firm signature to the document.

Like all great historical transformations, emancipation was a process, not a single event. It arose from many causes and was the work of many individuals. It began at the outset of the Civil War, when slaves sought refuge behind Union lines. It did not end until December 1865, with the ratification of the 13th Amendment, which irrevocably abolished slavery throughout the nation.

But the Emancipation Proclamation was the crucial turning point in this story. In a sense, it embodied a double emancipation: for the slaves, since it ensured that if the Union emerged victorious, slavery would perish, and for Lincoln himself, for whom it marked the abandonment of his previous assumptions about how to abolish slavery and the role blacks would play in post-emancipation American life....