As Lincoln would later explain, "the moment came when I felt that slavery must die that the nation might live." With the moral weight of a righteous cause behind the Union, the threat of foreign intervention on behalf of the Confederacy would dwindle to insignificance, and better still, Northerners of all stripes would begin to throw additional support behind a war with a more concrete purpose.
On the advice of Seward, Lincoln decided to wait for a major Union military victory before releasing a preliminary version of the Emancipation Proclamation, in order to give it a more substantial cast. But the Battle of Antietam was as close as the Union would come to a "victory" in the late summer of 1862, and on the strength of this mixed result, Lincoln issued the preliminary version to Congress anyway on September 22.
The final draft, which was officially released and put into effect on January 1, 1863, provided for the emancipation of all slaves in rebel territory. Such a decree was, by any honest analysis, patently impracticable, as the Union forces were lagging severely, and powerless to enforce such legislation. Further, the proclamation did not apply to border states (where insurgents had lost the right to hold slaves, while loyalists retained the right!), or even to the Union-held western portions of the Confederacy.
Thus, in paradoxical fashion, but nevertheless to his eventual success, Lincoln refused to act in the areas where he had jurisdiction, and made a sweeping pronouncement that applied to an area where he had no authority at all! Ultimately, it was only the power of war that would see the Proclamation through. But in early 1863, such power was by no means conclusive. On the very day that the Emancipation Proclamation was issued, a bloody, wasteful battle was fought at Mumfreesboro, Tennessee. After 20,000 casualties, no clear winner emerged. With the tactics of the two sides as confused as ever, Lincoln's strong words were somewhat undermined by yet another failure in the field.
In addition to being potentially unfeasible, the Emancipation Proclamation was of doubtful legality, standing as a striking extension of the president's war powers. Lincoln himself was unsure of its legality, and privately wondered if it would function simply as a war measure to be repealed later. Opposition to the policy was certainly strong. Several regiments in Illinois and Indiana came close to mutiny. Many of Lincoln's contemporaries criticized him heavily for his presumption and usurpation of power.
Radical abolitionists, on the other hand, chose to decry him for his refusal to initiate a more concerted emancipation policy in the border states. In the event, Lincoln had merely opted for a plan by which a state referendum in support of repeal would be rewarded by federal compensation. None of the border states took the bait.