On this date in 1862 President Abraham Lincoln issued a proclamation, in effect threatening the rebellious states:
“That on the 1st day of January, A.D. 1863, all persons held as slaves within any State or designated part of a State the people whereof shall then be in rebellion against the United States shall be then, thenceforward, and forever free; and the executive government of the United States, including the military and naval authority thereof, will recognize and maintain the freedom of such persons and will do no act or acts to repress such persons, or any of them, in any efforts they may make for their actual freedom.
Reknowned Civil War historian James M. McPherson provides us this background in the Reader’s Companion to American History:
Most Republicans had become convinced by 1862 that the war against a slaveholders’ rebellion must become a war against slavery itself, and they put increasing pressure on Lincoln to proclaim an emancipation policy. This would have comported with Lincoln’s personal convictions, but as president he felt compelled to balance these convictions against the danger of alienating half of the Union constituency. By the summer of 1862, however, it was clear that he risked alienating the Republican half of his constituency if he did not act against slavery.
Moreover, the war was going badly for the Union. After a string of military victories in the early months of 1862, Northern armies suffered demoralizing reverses in July and August. The argument that emancipation was a military necessity became increasingly persuasive. It would weaken the Confederacy and correspondingly strengthen the Union by siphoning off part of the Southern labor force and adding this manpower to the Northern side. In July 1862 Congress enacted two laws based on this premise: a second confiscation act that freed slaves of persons who had engaged in rebellion against the United States, and a militia act that empowered the president to use freed slaves in the army in any capacity he saw fit—even as soldiers.
By this time Lincoln had decided on an even more dramatic measure: a proclamation issued as commander in chief freeing all slaves in states waging war against the Union. As he told a member of his cabinet, emancipation had become “a military necessity…. We must free the slaves or be ourselves subdued…. The Administration must set an example, and strike at the heart of the rebellion.” The cabinet agreed, but Secretary of State William H. Seward persuaded Lincoln to withhold the proclamation until a major Union military victory could give it added force. Lincoln used the delay to help prepare conservative opinion for what was coming.
The battle at Antietam on September 17, which while not a decisive Union victory, had forced the Confederate army to retreat into Virginia. It gave Lincoln the emphasis he needed.
The Emancipation Proclamation itself was issued on January 1, 1863. Like this Preliminary Proclamation, it abolished slavery only in those places outside Union control (that is, the Confederacy).
It was the Thirteenth Amendment to the Constitution that abolished slavery in the United States. It was ratified in December 1865.