In accordance with Gen. Robert E. Lee’s Special Orders No. 191, issued in Frederick, Md., on Sept. 9, his Army of Northern Virginia had been divided. Maj. Gen. Thomas “Stonewall” Jackson’s troops marched on the Union garrison at Harpers Ferry while Maj. Gen. James Longstreet’s command marched through Boonsborough to secure the passes in South Mountain, and prevent the Union Army under Maj. Gen. George McClellan from advancing on the rear of Lee’s Army of Northern Virginia.
On Saturday, Sept. 13, after the Confederates had departed Frederick, Gen. McClellan’s Federal troops moved into the town. Two enlisted men of the 27th Indiana Infantry, Sgt. John Bloss and Corp. Benton Mitchell, came across a packet of three cigars lying in the grass, wrapped in some papers. On seeing the contents of the papers, they immediately turned the documents over to their company commander, who in turn passed them up the line to Gen. McClellan. In McClellan’s hands were a copy of Gen. Lee’s Special Orders No. 191, apparently and inadvertently dropped by one of Maj. Gen. Daniel H. Hill’s staff; Hill commanded a division in Gen. Jackson’s command.
With the complete operational plans of the Confederate Army in his hands, the cautious Gen. McClellan feared it was a trap, deliberately placed where Union soldiers would find them. He marched his command towards South Mountain, hoping to catch the rear of Gen. Lee’s army and also to support the Union garrison at Harpers Ferry. Union troops engaged the Confederates holding Turner’s Gap, Fox’s Gap and Crampton’s Gap in intense fighting which lasted until after nightfall on Sept. 14. The Confederates were driven back, west towards Sharpsburg. The next day, Gen. Jackson’s command occupied the Maryland Heights, Loudoun Heights and Schoolhouse Ridge, overlooking Harpers Ferry, where the Federals under Col. Dixon Miles were garrisoned. There was some small fighting (one Confederate wrote home that it “was like shooting fish in a barrel”) and Col. Miles was mortally wounded. The 12,500 Union troops were surrendered to Gen. Jackson; it was the largest number of American troops surrendered until Gen. MacArthur’s troops surrendered to the Japanese in the Philippines in World War II.
Hearing that Federal troops were advancing on the Confederates near Sharpsburg, Md., on Sept. 16, Gen. Jackson ordered all of his troops except for Maj. Gen. Ambrose P. Hill’s “Light Division” to leave Harpers Ferry and march to Sharpsburg. Gen. Hill’s troops were left at Harpers Ferry to finalize the surrender proceedings. By dawn on Wednesday, Sept. 17, the Confederate Army occupied the ground on the west side of Antietam Creek and Union troops on the east side of the stream. The Union attacks against Gen. Jackson’s troops on the north end of the battleground began at sunrise; they were largely piecemeal and uncoordinated, with a large Federal force held in reserve.
Some of the most severe fighting of the war occurred in the East and West Woods, around the Dunkard Church, and in Daniel Miller’s cornfield. Casualties were enormous, with rows of Federal troops being mowed down by Confederate rifle and artillery fire. In the afternoon, Maj. Gen. Ambrose Burnside’s corps spent an incredible amount of time attempting to force a bridge crossing over the Antietam; he was held back by a brigade of Georgia troops on the high ground overlooking the creek. Responding to a summons from Gen. Jackson, Gen. A. P. Hill marched his “Light Division” quickly over the seventeen miles of hot, dusty roads from Harpers Ferry, and went into the attack against Gen. Burnside’s men, halting their advance around 4.30 p.m. The battle ended with neither side having an advantage. The bloodiest single day in American history, Wednesday, Sept. 17, 1862, was over. Out of 75,000 Union troops engaged, the Army of the Potomac sustained 12,469 casualties. Of some 40,000 Confederates engaged, the Army of Northern Virginia sustained 13,724 casualties. More than 26,000 Americans were killed, wounded, captured or missing that day, a record which has stood for 150 years.