In the aftermath of his great victory at Second Bull Run, Robert E. Lee was determined to launch an invasion of the North. He hoped that a similar victory on northern soil would weaken the North’s resolve, and possibly encourage Maryland to rise and join the Confederacy. Lee convinced Jefferson Davis to approve his plan, and at the start of September Lee’s victorious army crossed the Potomac.
Once in the north, Lee became concerned about the 13,000 strong Federal garrison of Harper’s Ferry. He decided that he could not risk leaving that garrison in his rear. To capture it he took the decision to split his army. Two thirds of the army, under Stonewall Jackson, was sent to capture Harper’s Ferry, while he remained farther north with the rest of the army. Lee was taking a massive risk. He assumed that the Federal army defeated at Bull Run would take weeks to be recover, especially with George McClellan restored to command. He had repeatedly demonstrated a slow, cautious attitude during the Peninsula Campaign, and Lee expected more of the same.
He was wrong. McClellan had taken over a beaten army, but not a demoralised or unorganised one. McClellan soon had an army 70,000 strong on the move towards Lee. He also had a stroke of luck when a copy of Lee’s order for the move against Harper’s Ferry was discovered on 13 September. McClellan received this piece of luck at Frederick, less than twenty miles from Harper’s Ferry, where the garrison was still holding out.
Even with this information in hand, McClellan still did not move quickly. He was nearly always convinced that whatever army he commanded was badly outnumbered – here he was convinced that Lee had at least 100,000 men, twice the real number. Accordingly, he did nothing on 13 September other than issue orders for a movement on the following day.
The main barrier that faced McClellan was South Mountain. This mountain runs north from the Potomac, from the river just east of Harper’s Ferry. McClellan’s men would have to force their way through Confederate held passes before they could engage Lee or go to the relief of Harper’s Ferry. McClellan decided to move his main force through Fox’s gap and Turner’s gap. This would bring him out at Boonsborough, where the intercepted order suggested he would find Longstreet’s corps. If the move succeeded it would put McClellan’s army between the two main wings of Lee’s army, and give him a very good chance of capturing Lee, who would be isolated to his north.
Unfortunately, but hardly unexpectedly, McClellan’s advance was appallingly sluggish. For most of 14 September McClellan was held up by D.H. Hill’s single division. Two entire Federal army corps were held up for long enough for reinforcements to arrive. Eventually Federal troops captured other routes through the mountain, but Hill had held McClellan back for long enough for Lee to get his supply trains safely away. The Federal army was to remain largely inactive for the next two days, while Lee pulled his army back together behind the line of the Antietam Creek, and prepared for battle.