Lee had about 50,000 soldiers to McClellan's 85,000. But McClellan thought Lee had 100,000 troops and was extremely reluctant to engage him. McClellan now had a chance to destroy Lee's Army of Northern Virginia while it was weak and divided, if he acted quickly. This unexpected piece of valuable information led the usually overcautious McClellan to commit his forces on September 14 resulting in the Battle of South Mountain.