General Lee had wanted Europe to notice and help the Confederates. Only did they not know this would be the bloodiest single day in the civil war. At first, Gen. Joseph Hooker's union soldiers launched a powerful attack on Lee's left side, which began the Battle of Antietam. Lee wanted to advance North because he was confident that he had won Bull Run.
By taking up a defensive position west of Antietam creek, Gen. Lee challenged McClellan to attack him. But McClellan had always been cautious when attacking, especially coming to the Confederates. But as time went on, McClellan had the upper hand with the streams assistance as a defensive moat to aid him in the counter attacks of the talented Lee.
By mid-morning Gen. Lee's confederate troops were crouched behind the high banks of a county lane. They fired at the advancing union troops, but Gen. McClellan held an advantage, having sent a scout who had discovered a copy of the enemies plans.
An overwhelming number of northerns broke through the confederate lines. Union bullets rained the lane causing multiple casualties. After hours of bloody battle, Lee began to retreat across the Potomac river. Lee planned to find new recruits and supplies in Maryland, because they were a slave state and the confederates fight for slavery. But they didn't prevail.
The south lost approximately 10,500 men that day. It was the bloodiest day in the civil war history. This battle, driving the confederates out of Maryland and away from the border, gave Lincoln the hold he needed to declare the Emancipation Proclamation. This battle was a turn around and kept McClellan's job a little long before Lincoln demoted him for not being able to capture the confederate capital. However this was a numerous loss for Lee, losing around a quarter of his men and barely making a dent in the union troops(they only lost about 12,000 out of 87,000 men, but that's nothing compared to the 10,500 out of 45,000 men). However, there was no clear winner in this battle. This battle was a draw from military perspective.