Reno had a reputation as a "soldier's soldier" and often was right beside his troops without a sword or any sign of rank. On September 12, 1862, Reno's IX Corps spent the day in Frederick, Maryland, as the Army of the Potomac under Maj. Gen. George McClellan advanced westward in pursuit of the Confederate Army of Northern Virginia under Gen. Robert E. Lee. Elements of Lee's army defended three low-lying "gaps" of South Mountain—Crampton's, Turner's, and Fox's—while concentrating at Sharpsburg, Maryland, to the west, the location of the subsequent Battle of Antietam (September 17, 1862). In the Battle of South Mountain on September 14, Reno stopped directly in front of his troops as he reconnoitered the enemy's forces advancing up the road at Fox's Gap. He was shot in the chest by a rookie Union Soldier from the 35th Massachusetts who mistook him for Rebel Cavalry at dusk. The manuscript of Union Officer Ezra A. Carman, published in The Maryland Campaign of September 1862, Vol. 1: South Mountain, Edited and annotated by Thomas G. Clemens, , document Jesse's death by men of General John Bell Hood who were in and fired from the woods that the 35th Massachusetts skirmishes had just retreated from.