How did the Emancipation Proclamation change the purposes of the Civil War?

Americans understandably thought that the Civil War would be brief. The early battles were fought in Maryland and Virginia, not far from the two sides' capitals, in Washington, D.C. and Richmond, Virginia. Either side might have surrounded its opponent's capital and forced its surrender. The war's first major battle, at Manassas (or Bull Run) Virginia, resulted in a Confederate victory. The loss of this battle worried Lincoln, so he appointed a new commander of the Union forces, Gen. George B. McClellan. McClellan, unfortunately, was not an aggressive commander, and repeatedly squandered opportunities to defeat Lee's army.

At the war's beginning Lincoln's goal was to preserve the Union. He wrote in a letter to New York newspaper editor Horace Greeley in August 1862, "My paramount goal in this struggle is to save the Union, and it is not either to save or destroy slavery."

On September 17, 1862, in a battle at Antietam, Maryland, McClellan's army fought Lee's Confederates to a draw. The fighting at Antietam was fierce. 5,000 men were killed and 17,000 wounded, making it the single bloodiest day in American history. When the battle was over, Lee's army retreated across the Potomac River into Virginia, away from Washington, D.C. In Washington, Lincoln breathed a sigh of relief upon learning that the U.S. Army had pushed the Confederates away from the U.S. capital.

Five days after the Battle of Antietam, Lincoln issued the Emancipation Proclamation. This order declared that on January 1, 1863, all slaves in Confederate states "shall be then, thenceforward, and forever free." The Proclamation did not free slaves in the border states, because Lincoln was desperate to keep these states in the Union. He did not want to anger slave owners in those states. Still, the proclamation declared what most Americans had always known: the Civil War was a war not only to save the Union, but a war over slavery.