Countless school children have been taught that Abraham Lincoln was the Great Emancipator. Others have been taught — and many have concluded — that the Emancipation Proclamation, which Abraham Lincoln announced on Sept. 22, 1862, has been overemphasized, that it was inefficacious, a sham, that Lincoln’s motivations were somehow unworthy, that slavery was ended by other ways and means, and that slavery was on the way out in any case. The truth is that Lincoln’s proclamation was an exercise in risk, a huge gamble by a leader who sought to be — and who became — America’s great liberator. Since...