Excerpts from the article:
Union commander George B. McClellcm won the Battle of South Mountain in 1862. So why was it such a strategic disaster?
"In the annals of warfare, it is beyond rare that the commander of an army is given the enemy's battle plans. Yet that is precisely what happened in September 1862, when a copy of Confederate general Robert E. Lee's Special Orders No. 191 fell into the hands of the commander of the Union's Army of the Potomac, Major General George B. McClellan. It was, in the words of historian Bruce Catton, "the greatest security leak in American military history," and for a moment it gave McClellan the opportunity to end the Civil War--an opportunity that was, tragically, squandered."
"Ultimately, though, the responsibility rests with McClellan. With superior forces and the intelligence in hand to bring the war to an end, he failed to take advantage of the opportunity. Catton, in his timeless Army of the Potomac Trilogy, faults McClellan for a fatal lack of urgency: "With everything in the world at stake, both for the country and for McClellan personally, why couldn't the man have taken fire just once?"
"Although the Battle of South Mountain caused Lee to rethink his strategy, a far bloodier confrontation lay just ahead before Lee would abandon his Maryland Campaign. By failing to destroy the Army of Northern Virginia after South Mountain, McClellan gave Lee enough time to solidify his position and ready his forces for the major battle that would follow within days.""The Rebels would remember it as the Battle of Sharpsburg; to the people of the North, it was Antietam, and it would claim some 23,000 casualties, making it the bloodiest single day of combat in the nation's history. And once again, despite possessing superior numbers, McClellan--through an excess of caution bordering on timidity--would fail to seize a second opportunity to destroy Lee's army. The war was destined to last another two and a half years, and to tally a butcher's bill of three-quarters of a million lives. "
"The Battle of South Mountain is generally viewed as a tactical Union victory. Late on September 14, Lee himself stated matter-of-factly, "The day has gone against us." It was, however, a strategic disaster for McClellan. Through a series of inexcusable delays, the Army of the Potomac's commanders--having squandered so much precious time--failed to follow up their success in the South Mountain passes with a decisive move against Lee"
"When Lee was first informed that Crampton's Gap had fallen, he ordered Fox's and Turner's Gaps abandoned, intending to lead his men back into Virginia. But word soon reached him of the surrender of Harpers Ferry, and--with the reunification of his army--he instead determined to confront the Army of the Potomac in open battle a short distance from South Mountain"
Full Text Source: MHQ: Military History Quarterly.29:2, Winter, 2017,
Image Source: Battle of South Mountain, Civil War Trust