A year and a half into the Civil War, Union victory was far from assured. Confederate forces were fighting successfully in the Eastern Theater (comprising operations mainly in Virginia). After his victory at the Second Battle of Manassas (Bull Run), Confederate Gen. Robert E. Lee decided to move his army out of war-torn Virginia. On September 4, 1862, he led his over 40,000 Confederates across the Potomac River and through the lush Maryland countryside to Frederick.

Lee's Maryland Campaign—his first foray onto Union soil—was the most significant in a series of loosely coordinated Confederate incursions along a 1,000-mile front. Lee intended to keep moving north into Pennsylvania, but his line of supply and communication into Virginia was threatened by the 12,500-man Union garrison at Harpers Ferry, Va. (now West Virginia). Lee therefore divided his army to neutralize this threat. Part of Gen. James Longstreet's command went to Hagerstown, Md., close to Pennsylvania. Three columns led by Gen. Thomas J. "Stonewall" Jackson surrounded Harpers Ferry and held Crampton Gap on South Mountain. A third force, Gen. D.H. Hill's command, guarded the South Mountain gaps near Boonsboro, Md.

On September 12, Union Gen. George B. McClellan led the Army of the Potomac into Frederick, Md., just as the last Confederate soldiers were departing. Over the next few days a chain of events would draw all of these men together for the bloodiest one-day battle of the Civil War.

On September 13 a Union soldier found a copy of Lee's Special Order 191, his plan of operations for the campaign. This "Lost Order," as it has become known, was taken to McClellan, who realized that this was the time to strike Lee's divided forces. On the morning of September 14, Union soldiers engaged Confederates guarding the gaps on South Mountain. The day-long battle ended with the Confederates being forced from the gaps.

Lee considered returning to Virginia, but on September 15, after learning that Harpers Ferry had fallen, he reevaluated his plans. He would make a stand at Sharpsburg, Md., a quiet, 100-year-old farming community of some 1,200 residents.

That night we lay in line of battle behind a small brick church called the Dunkers Church, situated on the Hagerstown Turnpike, with arms, and ready to move at any moment.

—William Snakenberg, Private, 14th Louisiana