Battle of South Mountain (northern field of battle) (Photo credit: Wikipedia)
This book is about the Civil War battle of South Mountain, on September 14, 1862. This was part of Lee’s first invasion of the North with the Army of Northern Virginia, and it marks the time when the operation went from bold stroke to a scramble for survival for Lee.
If you look at this map of the area of Frederick, MD, you can follow the strategic positon. Early in September Lee had moved across the Potomac to Frederick after driving the Union army back to Washington DC after the battles of Second Bull Run and Chantilly. His initial plan was to move further north, rallying southern sympathizers in Maryland and garnering political points to possibly gain European intervention.
As he rested at Frederick, the Union garrison at Harper’s Ferry began to draw his attention. It sat behind him, on his line of communications south. He decided to take it out. This was the first mistake – if the goal is to move north, taking your eyes off the main army and scattering yours is a dangerous mistake. Lee was counting on the Union army to remain inactive.
His plan was to break his army into five parts and use three of them to surround Harper’s Ferry. The other two would cover the rear – D. H. Hill‘s division at South Mountain and Longstreet at Hagerstown against anything coming from the north. This was another mistake – the position at South Mountain was the more critical and most exposed to danger. It should have had the larger force.
The Army of the Potomac was quickly reorganized under McClellan who showed his usual energy and efficiency that was always present in him except when in contact with the enemy. He marched them out to meet Lee and arrived at Frederick on September 12 to find him gone. Now at this point, McClellan had already made the risk of any move North by Lee too dangerous. From Frederick, he could move on interior lines or cut his lines of communication. If Lee attacked him directly, the hills to the west of town made a good defense.
But then McClellan had a stroke of luck – soldiers found a misplaced copy of Lee’s entire march plan. McClellan thus knew that Lee’s army was scattered and that he was closer to the forces attacking Harper’s Ferry or holding the center than Lee’s other forces were. Lee himself knew nothing of this, because his cavalry under Stuart were doing an unbelievably poor job of scouting and delaying the Union forces. So on the 13th, when Hill saw two-thirds of the Union army approaching his positon, it was a shock indeed.
South Mountain is a considerable hill, so the defense had an advantage.man for man, but that was not nearly enough to overcome the weight of Union force. The defenders of Crampton’s Gap to the south were routed, the forces at Fox’ Gap were crushed and pushed back, and the main pass at Turner’s Gap was closely pressed and flanked. Several of the defending units suffered severe losses. Lee had to fall back at night to avoid a worse defeat the next day. McClellan had broken the center of his position, and Lee retired towards the Potomac.
Lee then found out that Harper’s Ferry was about to surrender and decided to unite his army for a final battle at Sharpsburg. This was his last mistake of the campaign, because there was no real chance of a decisive victory coming out of it – even a victory would let the Union fall back out South Mountain and defend there. And the poor condition of his own army made that victory unlikely, where a defeat with his back to the Potomac could be a disaster.
But Little Mac managed to take three days to advance to the battle, and though he managed to grind up Lee in the bloodiest day of the entire war and defeat him, he could not crush him, and the Army of Northern Virginia retired to fight again.
To the Union Army, South Mountain was a tonic – it was the first victory they had had in months and gave them the confidence that on their home turf they could match the Confederates and drive them off. This confidence carried on to Sharpsburg and to Gettysburg the next year, which ended the invasions.