Leading to the Emancipation Proclamation.
The Battle of Antietam was one of the bloodiest in American history. The war was photographed and many letters were sent home of the horrors seen at the battle.
The battle itself was a stalemate but the Union claimed victory. The battle kept Confederate forces in Maryland and prevented the war from entering the Union states. This convinced Lincoln to issue his Emancipation Proclamation which freed slaves in Confederate states. The proclamation was symbolic more than it had any actual effect. It gave the bloody war a moral reason to continue the fight. From that point the war became about unifying the Union--a Union that would be without slavery.