PBS:

Early in the Civil War, President Abraham Lincoln was hard pressed by the Radical Republicans -- the party's abolitionist wing -- to abolish slavery by proclamation. Lincoln was opposed. He said that his main concern was preserving the union and he subordinated his feelings about slavery to that goal: "My paramount object is to save the Union, and not either to save or destroy slavery." Moreover, he knew that if he decreed emancipation at the beginning of the war, Missouri, Kentucky, and probably Maryland, all of which technically remained on the Union side, would have joined the South. As the war gloomily dragged on in 1862, and things looked bleak for the Union cause, Lincoln realized that he would have to end slavery. He was willing to issue an Emancipation Proclamation but he felt that the people of the North were not yet ready for it.
Lincoln called the proclamation "the central act of my administration, and the greatest event of the 19th century."         Lincoln
He needed a military victory. In September of 1862, he barely got one. Five days after the Confederate Army's northward march was stopped at the battle of Antietam on September 17, 1862, Lincoln issued a preliminary proclamation.

The key paragraph of the Proclamation read: "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." Lincoln gave the Confederate states until the end of the year to return to the Union if they wanted to maintain slavery. They ignored him. On January 1, 1863, Lincoln issued the final proclamation and slavery was officially abolished, affecting about three million enslaved blacks. The proclamation did not apply to the border states, which were not in rebellion against the Union, and it could not be enforced in the regions held by Confederate troops. Critics charged that the Proclamation ended slavery in areas where it did not exist and was unable to end slavery in areas where it existed. But the Proclamation proved effective as Northern armies penetrated deeper into the South, freeing those who had been enslaved. Lincoln called the proclamation "the central act of my administration, and the greatest event of the 19th century." Later, the Thirteenth Amendment incorporated the abolition of slavery into the U.S. Constitution.