You're wrong on this one. if the secessionist states had returned to the Union, they could have kept their slaves. This is what the Emancipation Proclamation was intended to do. First, it was intended to tell the states who had seceded that if they returned, they could keep their slaves. This is why the Proclamation stated that those slaves in states currently in rebellion against the United States would be freed. As has been noted, many of the union states held slaves (20%, to be exact, 5 of the union states, not counting Washington DC which also allowed slavery0. The Emancipation Proclamation purposely did not free slaves held by these states. So by declaring the slaves of the secessionist states free, Lincoln hoped to lure them back into the Union. If they laid down their weapons, they could keep their slaves. If they continued in "rebellion", they would lose them. Lincoln was not the "great liberator" he has been portrayed to be. If he wanted ot free the slaves, why didn't he free them in the union states as well?
The second purpose of the Emancipation Proclamation was geopolitical. By making the war about slavery, something it had not been about for the previous 2 years of the war, he insured that England and France would not openly support the South.
It was a brilliant political move, but that's what it was, i.e. political. Limcoln was not concerned with black civil rights. He was concerned with keeping the union intact. period. As for why this was important, I will bow to those with more knowledge on this side of things than I.
Further support of Nacho's claim is the fact that the Emanicpation Proclamation started riots in New York. Several black people were killed. The reason for the riots? As the protestors proclaimed, they didn't want to fight to free black people.
The Civil War wasn't about slavery. It was about the rights of the States to determine their own destiny. The South wasn't fighting to keep them, and the North wasn't fighting to take them away.