 While Major General George B. McClellan's 87,000-man Army of the Potomac was moving to intercept Lee, a Union soldier discovered a mislaid copy of the detailed battle plans of Lee's army, on Sunday 14 September.The order indicated that Lee had divided his army and dispersed portions geographically (to Harpers Ferry, West Virginia, and Hagerstown, Maryland), thus making each subject to isolation and defeat in detail - if McClellan could move quickly enough.However, McClellan waited about 18 hours before deciding to take advantage of this intelligence and position his forces based on it, thus endangering a golden opportunity to defeat Lee decisively.