Following his tactical success in the Battle of Second Manassas (Second Bull Run, August 28–30, 1862), General Robert E. Lee led his Army of Northern Virginia into western Maryland to secure supplies—and in the vain hope of winning its people to the Confederate cause. Under Special Orders No. 191 Lee divided his army during the Maryland Campaign, creating a vulnerability that Union general George McClellan discovered after a lost copy of Lee’s orders had been found by a Union private. Despite boasting of the trap he would set, McClellan moved too cautiously to make full use of the intelligence, giving Lee time to reunite his forces. What could have been a decisive Union victory instead left the Union simply holding the field at Antietam after the costliest single day of combat during the Civil War.