After the Battle of Antietam on September 17, General Robert E. Lee and the Confederate Army of Northern Virginia withdrew across the Potomac.

On September 19, elements of the Union V Corps dueled with Pendleton's artillery before pushing a short distance across the river at dusk.

On the morning of September 20, the Confederates counterattacked with A. P. Hill's Light Division, forcing the Union units back across the Potomac.

Lee then faced a Union advance by Major General John Pope's Army of Virginia, coming from a different direction.

Despite having suffered heavy casualties in the previous two campaigns, Lee led his army on an invasion of the Union, striking into the state of Maryland.

[2] In the resulting Battle of Antietam, Lee's outnumbered army repulsed Union assaults throughout the day.

Nearly 23,000 men were killed, wounded, or became missing in action during the fighting, making Antietam the bloodiest day in American history.

[4] Lee's army remained in position on September 18, and McClellan did not order an assault, despite having a significant number of fresh troops.

[6] After crossing the Potomac River, Lee left his artillery chief, Brigadier General William N. Pendleton, to hold a rear guard at Boteler's Ford.

[11] Pendleton had excellent terrain to defend at Boteler's Ford: the Virginia side of the Potomac had high cliffs that would be difficult to assault.

[13] At dusk, a 60-man raiding party from the 1st United States Sharpshooters crossed the ford under covering fire from the 4th Michigan Infantry Regiment.

[8] Lawton and Armistead's men, who were in poor condition and had low morale after suffering heavy losses at Antietam, quickly fled.

[17] While a Union force of about 2,000 men in total[1] only advanced a short distance from the ford before withdrawing with four captured guns and some prisoners, Pendleton's line routed.

[23] Confederate general Stonewall Jackson decided to attack and drive the Union forces back into Maryland.

[24] Having advanced about 1 mile (1.6 km) beyond the river,[18] Lovell's brigade detected the movement, and divisional commander George Sykes was informed, who authorized a withdrawal.

[1] The President of the United States, Abraham Lincoln, issued the Emancipation Proclamation after the repulse of the Confederate invasion of Maryland.

The Emancipation Proclamation underscored the key role that slavery played in the war, and between it and the fighting at Antietam, Confederate hopes of foreign intervention ended.