McClellan's concentrating his attack on the Confederates on South Mountain meant he did not help the garrison at the Battle of Harpers Ferry. From having a copy of Lee's Special Order 191, McClellan knew Jackson was attacking the Union Forces at Harpers Ferry. What little help he did send arrived too late to prevent the surrender. It also gave Lee the time he needed to gather his forces together at Sharpsburg, Maryland. Had McClellan moved faster and more decisively, the costly Battle of Antietam might never have taken place. Confederate losses for the day amounted to 2,700 soldiers. The Union losses were 2,300 dead or wounded. The Confederates were able to delay McClellan's forces by fiercely defending the gaps. This gave Lee precious time to regroup and move on to Sharpsburg where the Battle of Antietam pitted the two armies against each other again. The battle of South Mountain, however, forced Lee to give up his plans for invading the North and caused him to have to defend his army.