After his success at Second Manassas, Gen. Robert E. Lee led the Army of Northern Virginia on an invasion of Maryland in September of 1862. Lee divided his army, sending a portion of it into western Maryland while Stonewall Jackson's wing attempted to capture the Federal arsenal at Harper's Ferry. The bold plan was jeopardized on September 13 when a mislaid copy of Lee's orders revealing the Confederates' plans and positions was given to Union commander Maj. Gen. George B. McClellan. Emboldened by this intelligence, McClellan tried to force his army through three passes in the South Mountain range on September 14, 1862. Intense fighting drew increasing numbers of troops of both armies to Fox's, Turner's, and Crampton's gaps. Though the Federals ultimately gained control of all three passes, stubborn resistance on the part of the Southerners bought Lee precious time to begin the process of reuniting his army, and set the stage for the Battle of Antietam.