Despite its shocking casualty figures, the most important consequence of Antietam was off the field. From the outset of the war, slaves had been pouring into Federal camps seeking safety and freedom. Early in the war, Lincoln had slapped the wrists of commanders who tried to issue emancipation edicts in areas under their control. Trying to balance political and military necessity against moral imperatives, Lincoln believed that keeping the slave-owning border states—Maryland, Delaware, Missouri, and particularly Kentucky—in the Union was critical and that making any move toward freeing slaves could incite those states to secede. Moreover, the Constitution protected slavery in several ways, most importantly through its defense of property rights. Finally, Lincoln believed for the first year or so of the war that a significant number of Unionists existed in the seceded states and that, given time, those people would rise up and revolt against the Confederate government.

As early as August 1861, though, slaveholders’ claims to property rights had begun to erode when Congress passed its First Confiscation Act, which allowed Union troops to seize rebels’ property, including slaves who fought with or worked for the Confederate military. One Union general, Benjamin Butler, a prominent attorney and politician in civilian life, read up on military law and used confiscation laws to the Union’s benefit by turning the slave owner’s claim to property rights on its head. Armies had always been able to confiscate property of military value, Butler argued, and slaves were instrumental in supporting the Confederate cause. With so many slaves manning factories and working fields, about 80 percent of eligible white Southern men wound up serving in the military. Butler declared slaves who came into his lines to be “contrabands” of war and therefore not liable for return to their masters. The name contrabands was used for the remainder of the war to describe slaves who ran from their masters to the Union army.

In April 1862 Congress abolished slavery in the District of Columbia and paid owners in the district about $300 on average for each slave. Three months later Congress passed the Second Confiscation Act, which mandated that any Confederate civilian or military official who did not surrender within 60 days would have his slaves freed. Two days after that, Congress banned slavery from the territories.

Lincoln, meanwhile, was meeting with men from the border states, especially Kentucky, hoping to persuade them to agree to a compensated emancipation. Over the course of these encounters, it became clear to him that the broad Unionist sentiment he thought existed in the South was a chimera. When talks with the Kentucky delegates broke off in July, Lincoln immediately sat down and drafted the Preliminary Emancipation Proclamation. In its final form, the Emancipation Proclamation would free the slaves in areas that were not under Union control as of January 1, 1863, when it went into effect. This meant it did not apply in the border states or places such as New Orleans, which were already under Union military occupation by that time. Lincoln realized that such a move would strike a serious blow militarily to the Confederates, who relied on bondsmen for the bulk of their labour during the war, by both demoralizing white Southerners and giving additional incentive to slaves to run away.

However, the summer of 1862 had been a bleak one for Federal forces, and Lincoln did not want to issue the proclamation when the North appeared to be losing. He did not want other countries to consider it an act of desperation. So he put the document in his desk drawer and waited for a victory. Antietam, while technically a draw, was close enough that Lincoln claimed it as a Union win and announced the proclamation. This was an important turning point. The war was now a contest not just about saving the Union but also about freeing four million bondsmen and bondswomen. This new moral element to the war persuaded the British and French to stay out of the conflict and to never offer the Confederates the diplomatic recognition they desperately sought.