Abraham Lincoln's Emancipation Proclamation was issued on January 1, 1863. Originally, Lincoln was slow to make slavery an issue in the Civil War, knowing that if he did so, states that supported the Union, but also supported slavery, would join with the Confederacy (the South). In this case, Lincoln knew they would lose the war, which would not serve the abolition of slavery. Slavery was something that Lincoln could find no way to reconcile in his own mind: he detested the practice. Slavery was:

...a moral and constitutional paradox that lay at the heart of the American national experience.

In 1860, there were more than four million slaves in the United States, representing approximately one seventh of the total population. Change was in the wings;

Slavery was 'the sleeping serpent' waiting to be awakened.

Ultimately, the slaves of the South made the decision for the Union, and so, too, for President Lincoln. The slaves realized that as the Union army advanced southward, the opportunity presented itself for their escape. Approximately 600,000, or fifteen percent of the total number of slaves, escaped from their plantations and traveled North. Many sought out the Union camps. Union leaders did not know what to do: return the escaped slaves to their owners or allow them to stay? Most decided to allow the slaves to remain, eventually identifying them as "contraband" (or property) seized during a military encounter. Under this premise, "seizing" slaves supported the "military effort," not the emancipation of slaves.

There was criticism of the proclamation because it only freed slaves in Confederate states that were captured by the Union's military. It did not free slaves that were in areas already captured, or with states that were supportive of the Union in the war. A president's power was limited by the Constitution, and the only real power Lincoln had was as "commander-in-chief" from a military standpoint.

Eventually, the Emancipation Proclamation liberated two-thirds of the slaves in this country as part of the war effort. States that were supportive of the Union's cause freed their slaves, and finally, the 13th Amendment, passed in 1865, outlawed the keeping of slaves in the United States.

The major significance of the Emancipation Proclamation is that it made it clear that the Civil War was about slavery and the abolition of slavery.  That had not been clear before.  This helped improve the image of the North in the eyes of foreign audiences, especially those in Britain.  However, it did also create more opposition to the war in the North.

Before the issuing of the proclamation, it was not at all clear what would happen to slavery in the South if the North won.  The North was fighting to force the South back into the Union.  But on what terms?  The proclamation made it clear that slavery would end.

This made many foreigners support the North more than they had.  This was especially true in anti-slavery Britain.  However, many Northerners were not in favor of abolition.  This led to more opposition to the war among those who did not support abolition.  This can be seen in things like the New York Draft Riots of 1863.