McClellan's battle plan was to open the battle with an attack on Lee's left flank, followed by a second blow on the Confederate right flank. When either of these two assaults made progress, he would follow up with a decisive blow with his reserves wherever the enemy had suffered the most. McClellan almost immediately lost control of the strategic situation, however, and the Union offensive degenerated into a series of uncoordinated attacks. Wave after wave of Union divisions moved onto the battlefield, uncoordinated and unsupported by other divisions. Lee, on the other hand, superbly handled Confederate forces by personally directing their movements from one area of the field to wherever they might be needed. As a result, Lee, who was greatly outnumbered, engaged fully 100% of his forces, while McClellan engaged about 50% of his own.
At first light on the morning of September 17th, McClellan opened the battle on Lee's left flank with his I Corps. The Union XII Corps was camped about a mile behind, but did not arrive on the battlefield until two hours later; by that time, the I Corps had been nearly exhausted. The XII Corps virtually single handedly carried the battle for another hour until the Union II Corps arrived. By this time, the I Corps commander, Joe Hooker, was wounded and off the field, and the XII Corps commander, J.K.F. Mansfield, had been mortally wounded. Therefore, the II Corps commander, E.V. Sumner, received field control of the battle, but was totally uninformed as to the disposition of Union and Confederate forces in the area. In the biggest blunder of the battle, Sumner recklessly moved his lead division into the West Woods where Confederate reenforcements completely surprised and routed them. Sumner also failed to leave directions for his other two divisions, which accidently wandered not toward the West Woods, but toward Lee's center. By 10:00 AM, McClellan's pincer movement had degenerated into a mere frontal assault.
Lee's center occupied a sunken farm lane, a natural trench excellent for defense. For several hours, Union forces, often one brigade at a time, made many assaults against the sunken road. Just as the Confederates were driven out, the senior Union division commander, I.B. Richardson, was wounded and taken from the field. Again, with no strong presence of leadership on the field, Union resolve to drive the Confederates dried up.
Meanwhile, on the southern region of the battlefield, the Union IX Corps was supposed to take a stone bridge over the Antietam creek and drive the Confederate right flank. Because of a poor chain of command structure, this action did not start until 10:00 AM, four hours after the battle started. For two hours, Union commanders repeated the now familiar pattern of feeding troops piecemeal into their attempt to take the stone bridge. Finally, the Confederates were driven back and Union troops began to gather on the west bank of the creek. At this critical moment, however, strong Union leadership on the battlefield was absent and it took another two hours to get the attack going again. Finally, at 3:00 PM the final drive on the Confederate right flank began. The object was to sweep west, and turn north and enter Sharpsburg from the south. Initially, the Union drive went without a hitch, until, just like in the West Woods, Confederate reenforcements surprised very poorly deployed Federal troops; in failing daylight, the Union line collapsed and retreated back to the creek.
Lee waited all day on the 18th for the attack that never came. During the night of the 18th/19th, Lee slipped back over the Potomac, ending his invasion of the North.
Because of the overall pattern of the Union attack, the battle of Antietam is traditionally broken into three geographic phases: Northern, Middle, and Southern.
The sections that follow below detail troop movements during each phase of the battle. In general, these maps are fairly high level, showing movements on the division level. Because only a limited treatment of events will be given, many details have been omitted. The Campaign Histories should be consulted for a thorough synopsis of the battle; Sears's book contains nice maps at the brigade level, while Priest's maps go down to the regimental level. The maps presented here are derived primarily from Priest's.