To begin with, the Emancipation Proclamation did not free a single slave. It applied only to slaves in the states “in rebellion” – the Confederacy – where the Union had no power or authority, thus having about as much real effect as the famous (putative) papal bull against Halley’s comet. And it did not apply to the Border states where there still was slavery (Delaware, Maryland, Kentucky, Missouri). In other words, as Secretary of State William Seward remarked ironically at the time, “We show our sympathy with slavery by emancipating slaves where we cannot reach them and holding them in bondage where we can set them free.”
But then, it was not really intended to liberate anybody. It was, as Lincoln took care to spell out, a “military measure,” issued under the President’s authority as commander-in-chief, and its real purpose was to undermine the Confederate Army. This would be accomplished, Lincoln hoped, first by encouraging slaves to take over the plantations so that they would no longer be supplying foodstuffs for the soldiers, and second by causing plantation owners serving in the army to desert and rush home to protect their wives and children from the presumably dangerous freedmen.
via The Truth About the Emancipation Proclamation by Kirkpatrick Sale.