By September 1862, the American Civil War had raged for about a year and a half. It had not gone well for the Army of the Potomac, the Union army in the east. On the fourth of that month, Confederate General Robert E. Lee led his Army of Northern Virginia across the Potomac River, invaded Maryland, and advanced to Frederick. He divided his army and sent various parts on separate missions.
Led by General George B. McClellan, the Army of the Potomac left Washington DC in pursuit of the invaders. By the time McClellan's army reached Frederick, the Confederates had moved on, leaving a copy of the Confederate General's battle plans wrapped around a bundle of cigars that Union soldiers accidentally discovered. See the complete story on Outlook By The Bay