Britannica –
“Issued by U.S. Pres. Abraham Lincoln on January 1, 1863, that freed the slaves of the Confederate states in rebellion against the Union (see original text). Before the start of the American Civil War many people and leaders of the North had been primarily concerned merely with stopping the extension of slavery into western territories that would eventually achieve statehood within the Union.
With the secession of the Southern states and the consequent start of the Civil War, however, the continued tolerance of Southern slavery by Northerners seemed no longer to serve any constructive political purpose. Emancipation thus quickly changed from a distant possibility to an imminent and feasible eventuality. Lincoln had declared that he meant to save the Union as best he could—by preserving slavery, by destroying it, or by destroying part and preserving part. Just after the Battle of Antietam (Sept. 17, 1862) he issued his proclamation calling on the revolted states to return to their allegiance before the next year, otherwise their slaves would be declared free men. No state returned, and the threatened declaration was issued on January 1, 1863.
Two months before the war ended—in February 1865—Lincoln told portrait painter Francis B. Carpenter that the Emancipation Proclamation was ‘the central act of my administration, and the greatest event of the nineteenth century.’ ”
Encyclopaedia Britannica Online, (accessed November 13, 2008)