When Lincoln declared the slaves in rebel hands “shall be, then, thenceforward, and forever free,” he had carefully exempted from his proclamation vast territories then under Union control. These included not only the four loyal border states—Missouri, Kentucky, Maryland, and Delaware—but also occupied areas of Tennessee, Louisiana, Florida, Virginia, and North Carolina. The London Spectator sneered that the only principle in the proclamation was that a man may not own another unless he is loyal to Lincoln’s government. But the Spectator then (and cynical Lincoln critics ever since) misunderstood the basis of Lincoln’s action. Because he was a constitutional leader and not a despot, Lincoln could only free the slaves as a war measure to suppress rebellion. The confiscation of enemy property during wartime is recognized as legitimate under the rules of war.
Thus, it is not true to say he freed the slaves where he had no power and left in bondage those over whom he exercised control. Lincoln had no constitutional authority to free the slaves in the loyal border states. And he knew he could not emancipate slaves in those areas where Union arms had quelled the rebellion. The practical effect of the Emancipation Proclamation was that the Union army became an army of liberation. Wherever it moved, thousands of slaves swarmed into its ranks, because the soldiers of the United States carried freedom in their haversacks.
Some British observers were more perceptive than the ruling aristocracy. Philosopher John Stuart Mill showed an acute understanding of what Lincoln had achieved: