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 Davies 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 further 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 repeated 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, reaching 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. Worse for the garrison of Harper’s Ferry, McClellan decided to make his main attack at Turner’s and Fox’s Gaps (Battle of South Mountain). Success here would bring McClellan up against Lee’s smaller section of the Confederate army.
A smaller force, 12,000 men under Major-General William B. Franklin, was sent to Crampton’s Gap, further south. This force might not have been big enough to defeat Jackson’s entire force around Harper’s Ferry, but it was easily big enough to deal with that part of Jackson’s force that had remained north of the Potomac, which was no more than 8,000 strong, and thus to rescue the garrison. However, Crampton’s Gap was also defended. First, Franklin would need to fight his way through the pass.
This should not have been a problem. He was opposed by three brigades from the force that had been sent against Harper’s Ferry, a total of 2,200 men. Even by his own account, Reynolds was able to get 6,500 men into action at Crampton’s Gap. Despite this numerical advantage it took Reynolds most of 14 September to fight his way through the pass. He suffered 533 casualties during the battle (113 dead, 418 wounded and 2 missing), and probably inflicted twice that many (he captured 400 prisoners). However, the victory came too late in the day to achieve its aim. The next morning, when Reynolds made a tentative move towards Harper’s Ferry, he decided that he was too weak to attack the Confederate forces north of the river. In any case it was by then too late. Harper’s Ferry surrendered early on the morning of 15 September.