Lincoln's issuance of the Emancipation Proclamation after Antietam is not coincidental. Lee's purpose in invading Maryland was to encourage European nations to recognize (and hopefully assist) the Confederacy; similar to French recognition of the United States following the Battle of Saratoga. Lincoln apparently was aware, and by issuing the Emancipation Proclamation, placed European nations (which opposed slavery) in a moral dilemma. If they recognized the South, they would be supporting an institution which they abhorred. The Proclamation also gave the North the perceived moral high ground, and also was intended to promote slave rebellions in the South. There were several arguments against Emancipation, even in the North. The "border states" in which slavery was still legal would be alienated, even though they were excluded. Racial prejudice in the North was endemic. Most abolitionists only wanted to prohibit slavery in the West; they could live with it where it previously existed. Also, if Lincoln truly believed that the southern States had not left the Union, he wasn't sure he had the authority to issue the proclamation. Odious as slavery was to Lincoln, he believed it to be constitutionally protected. Obviously, others agreed with him, as it was only ended by Constitutional amendment. Interestingly, several counties in Virginia and parishes in Louisiana were exempted from the proclamation. These areas were under Union control. So the Emancipation Proclamation--which freed no slaves--was primarily a tactical move to aid the northern war effort.
Following the Battle of Antietam and the Union "victory" there, Abraham Lincoln issued the Emancipation Proclamation in September of 1862, to take effect January 1, 1863. It was one of his more brilliant moments as President.
The proclamation freed no actual slaves, as it applied only to those slaves living in states that had seceded from the Union, or to put it another way, in states Lincoln did not control. The four border states, Maryland, Delaware, Kentucky and Missouri, slavery remained untouched, as Lincoln did not want to give them another reason to secede, especially Maryland.
At the same time, the Emancipation Proclamation was a promise. Slaves would be freed in the South if the Union won the war. This got abolitionists like William Lloyd Garrison and Frederick Douglass off Lincoln's back, and got the abolitionists solidly behind the war effort.
So in one day, Lincoln had kept the border states, made the abolitionists happy, and promised to do the right thing and free the slaves after the war.