Of course, the end of slavery had only practical effect where Union forces were in control, and this was not the case in most of the 750,000 square mile Confederacy. The plantation economy was already severely strained with cotton markets cut off by military blockade. And, the loss of slave labor would change the economics of cotton, rice and tobacco production.
But, there was still a war to win, and Lincoln was determined to press aggressively to destroy Robert E. Lee's Army of Northern Virginia. Lincoln sacked McClellan after Antietam when the general refused to follow up on his repulse of Lee's invasion. Ambrose Burnside (he of the grandiose sideburns) replaced McClellan and in December marched south to the environs of Fredericksburg VA, on the Rappahannok River only 60 miles from the Confederate capitol at Richmond.
Lee was entrenched across the river in a nearly unassailable position. (It seemed at times that Union commanders forgot that Lee was a superb military engineer). Burnside hurled his army of 100,000 against Lee and was thrown back with heavy losses. The Army of the Potomac fell back, having been routed by the rebels, and retreated toward Washington.
Many thought this terrible defeat would cancel out the effect of Antietam and cause Lincoln to reconsider his decision. But on January 1, 1863 Lincoln signed and issued the Proclamation. It immediately freed 50,000 slaves and would apply as Union armies advanced. It did not make slavery illegal - that would be left to the passage of the 13th Amendment to the Constitution in 1865.
