Times are approximate and based on those given in the after-action reports by unit commanders or in postwar reminiscences.
4-7 September 1862
The Army of Northern Virginia, commanded by General Robert E. Lee, crosses the Potomac River near Leesburg and marches to Frederick, Maryland.
The Army of the Potomac, commanded by Maj. Gen. George B. McClellan, leaves Washington in pursuit of Lee.
Lee issues Special Orders 191, detailing his plan to capture Union garrisons at Martinsburg and Harper's Ferry.
Lee and Maj. Gen. James Longstreet march to Hagerstown. Maj. Gen. D. H. Hill's division halts at Boonsboro as rear guard.
Maj. Gen. Thomas J. Jackson marches to capture the Union garrison at Martinsburg.
The divisions of Maj. Gens. Lafayette McLaws and Richard H. Anderson approach Harper's Ferry from the east.
Brig. Gen. John G. Walker's division crosses the Potomac River into Virginia and approaches Harper's Ferry from the south.
The Army of the Potomac begins arriving at Frederick.
Jackson reaches Martinsburg, but the Union garrison flees to Harper's Ferry.
McClellan arrives at Frederick.
A soldier in McClellan's army finds a copy of Lee's Special Orders 191. The document is given to McClellan, who plans to attack the Confederates the following day at South Mountain.
Jackson marches from Martinsburg to Harper's Ferry.
In the evening Lee receives information from Maj. Gen. James E. B. Stuart that McClellan's army has arrived at Frederick and that the enemy has a copy of Special Orders 191.
To delay an expected Union advance, Lee orders D. H. Hill to defend Turner's and Fox's Gaps on South Mountain. Longstreet is sent to support Hill. McLaws sends a portion of his command to defend Crampton's Gap.
The siege of Harper's Ferry begins.
The Battle of South Mountain occurs. Union forces take Crampton's Gap.
During the night Lee decides to withdraw his outnumbered forces from Turner's and Fox's Gaps and falls back to Sharpsburg.
Lee, along with Longstreet, the divisions of Brig. Gen. D. R. Jones and D. H. Hill, and part of Stuart's cavalry, arrives at Sharpsburg.
Harper's Ferry surrenders. Lee orders the Confederate troops there to march to Sharpsburg as soon as possible. Maj. Gen. Ambrose P. Hill's division remains at Harper's Ferry to parole prisoners and gather supplies.
The Army of the Potomac arrives near Sharpsburg. McClellan replaces Maj. Gen. Ambrose E. Burnside with Maj. Gen. Joseph Hooker in command of the army's right wing and puts Burnside in command of the left wing.
0730: Jackson's command reaches Sharpsburg, reuniting with D. H. Hill's division.
Walker's division arrives at Sharpsburg.
1530-1600: Hooker's I Corps crosses Antietam Creek north of Sharpsburg to turn Lee's left flank. A portion of Brig. Gen. George G. Meade's division, leading Hooker's corps, skirmishes with Brig. Gen. John B. Hood's division near the East Woods.
In response to Hooker's request for reinforcements, McClellan orders Maj. Gen. Edwin V. Sumner's XII Corps, commanded by Maj. Gen. Joseph K. F. Mansfield, from the center to the right wing.
Lee sends Jackson, with his divisions commanded by Brig. Gens. John R. Jones and A. R. Lawton, to support Hood. Walker's division remains in reserve south of the town.
1930: McClellan, after learning that Harper's Ferry has surrendered, orders Maj. Gen. William B. Franklin's VI Corps to join the army at Sharpsburg.
2400: Mansfield's XII Corps crosses Antietam Creek to join Hooker's I Corps.
0600: McLaws' and R. H. Anderson's divisions arrive at Sharpsburg.
Hooker's I Corps begins its attack south on Hagerstown Pike and Smoketown Road, initially meeting with great success at the Cornfield.
0700: Called to assist by Jackson, whose lines are collapsing, Hood's division launches a counterattack from the West Woods against Hooker's I Corps in the Cornfield.
0730: Acting on orders received the night before, A. P. Hill's division begins a forced march from Harper's Ferry to Sharpsburg.
Mansfield's XII Corps arrives in the East Woods. Brig. Gen. Alpheus S. Williams' division of the XII Corps drives Hood's divi-
sion from the Cornfield back into the West Woods. Mansfield is mortally wounded, and Williams assumes command of the division.
0800: Maj. Gen. John Sedgwick's and Brig. Gen. William H. French's divisions of Sumner's II Corps cross Antietam Creek to support Hooker's I Corps. Sumner leaves behind his third division, command by Maj. Gen. Israel B. Richardson, to guard artillery.
0900: Sedgwick's division attacks into the West Woods. French's division attacks at the Sunken Road against the division of D. H. Hill. Hill sends an urgent request to Lee for reinforcement.
One of Sedgwick's regiments, the 34th New York Infantry, becomes separated from the rest of the division and halts near Dunker Church, where it finds the 125th Pennsylvania Infantry of Williams' division, which had ended up there some time earlier.
Brig. Gen. Isaac P. Rodman's division of Burnside's IX Corps is sent to outflank the lower bridge by crossing Antietam Creek at Snavely's Ford.
Brig. Gen. George S. Greene's division of the XII Corps reaches the plateau east of the West Woods.
Hooker is slightly wounded, and Meade assumes command of the I Corps.
0930: Sedgwick's division is driven out of the West Woods by the division of Walker, sent from its reserve position south of Sharpsburg; the division of McLaws, newly arrived from Harper's Ferry; the brigade of Brig. Gen Jubal A. Early, from its station west of the West Woods; and the brigade of Col. G. T. Anderson of D. R. Jones' division. The Confederate advance into the Cornfield is stopped by Union artillery and Williams' division, which had remained in the Cornfield.
1000: D. H. Hill's division, defending the Sunken Road against French's attack, is reinforced by the division of R. H. Anderson, which had just arrived from Harper's Ferry.
Spearheaded by the 11th Connecticut Infantry, the IX Corps attack on Brig. Gen. Robert Toombs' brigade at the lower bridge begins.
1030: After crossing Antietam Creek, Richardson's division of the II Corps joins French's attack on the Sunken Road.
Greene's division occupies the West Woods.
1100: The 2d Maryland and 6th New Hampshire Infantries of the IX Corps unsuccessfully attempt to cross the lower bridge.
Greene's division withdraws from the West Woods and falls back to the East Woods. Walker's division reoccupies the West Woods.
The 27th North Carolina and 3d Arkansas Infantries, along with portions of other Confederate commands, attack the right flank of French's division. Maj. Gen. William F. Smith's division of the VI Corps drives the Confederates into the West Woods.
1230: D. H. Hill's Confederates withdraw from the Sunken Road. Richardson's division pursues them, but McClellan orders it to halt its advance, inadvertently saving Lee's center.
1300: The 51st New York and 51st Pennsylvania Infantries of the IX Corps cross the lower bridge. The remainder of the IX Corps begins to cross the bridge.
South of the lower bridge, Rodman's division crosses Antietam Creek at Snavely's Ford.
North of the lower bridge, elements of the 28th Ohio Infantry cross Antietam Creek.
With Union troops above, below, and across from him, Toombs withdraws to Sharpsburg.
1430: A. P. Hill's division begins arriving near Sharpsburg.
1500: Having finished crossing Antietam Creek, the IX Corps advances on Sharpsburg.
1600: A. P. Hill's division attacks the left flank of the IX Corps and successfully halts the Union advance toward the town.
1700: The IX Corps falls back to the lower bridge.
McClellan decides not to attack this day but instead issues orders to attack on 19 September.
Lee wishes to renew the fight, but he calculates that the odds are too much against him and withdraws his army back into Virginia during the night.
In the morning McClellan learns that the Confederates withdrew during the night. He chooses not to pursue them.
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