On September 7, 1862, the Maryland Campaign entered a pivotal phase. Fresh from victory at Second Manassas, Confederate Gen. Robert E. Lee crossed the Potomac with the Army of Northern Virginia, aiming to carry the war onto Northern soil, obtain supplies, relieve Virginia farms, encourage Maryland to join the Confederacy, and possibly sway European countries to recognize the Confederate States of America. By nightfall that day, most of Lee’s force—about 50,000 men, though reduced by straggling and illness—had concentrated around Frederick, Maryland, roughly 40 miles from Washington.
The same day, Union Maj. Gen. George B. McClellan, recently restored to command after John Pope’s defeat, at Second Manassas, began advancing the Army of the Potomac from the Washington defenses. His hastily assembled force, of some 80,000, included many raw recruits, and moved cautiously in multiple columns, to screen both the capital and Baltimore.
Lee’s occupation of Frederick threatened Union communications and cities. He soon issued Special Orders 191, dividing his army to seize the garrisons at Harpers Ferry and Martinsburg. These opening maneuvers, of September 7, set the campaign in motion, leading to the Battle of South Mountain on September 14 – and the Battle of Antietam on September 17. Antietam was referred to, in the South, as the Battle of Sharpsburg, and would be the bloodiest single day in American military history. (A bloody honor it still holds today.)