In the spring of 1862, Gen. Robert E. Lee defeated Maj. Gen. George B. McClellan’s Army of the Potomac on the Virginia Peninsula in the Seven Days campaign. Those victories relieved Richmond and shifted the focus of the war into Northern Virginia. There, at the end of August, Lee defeated Maj. Gen. John Pope’s Army of Virginia at Second Manassas. With no organized Union forces to bar his way, Lee envisioned a new campaign designed to bring relief to Virginia, draw new recruits to his army, and score a victory decisive enough to bring European recognition to the Confederacy. To accomplish all this, Lee wanted to take the war north. A Confederate victory on Union soil would also embolden opposition to President Abraham Lincoln’s party in the upcoming November congressional elections. Knowing his army was undersupplied, Lee told Confederate President Jefferson Davis “we cannot afford to be idle.” So, on September 4, Lee and his nearly 70,000-strong Army of Northern Virginia crossed the Potomac River unopposed and entered Maryland for the first time in the war. Little went right for Lee after that.
Panic gripped the national capital as Lee moved north, and Lincoln was once again forced to find an army commander. Reluctantly, he ordered McClellan to take command of the Federal forces defeated at Second Manassas and those from the Army of the Potomac defeated on the Peninsula. Although vain and egotistical, McClellan was an expert in organizing an army, which was just what was needed to meet the crisis. Within a week, McClellan had reinvigorated the demoralized elements of both armies and prepared to confront Lee’s new threat with 80,000 men. McClellan’s force included thousands of raw recruits, enlistees who had responded to Lincoln’s call for 300,000 volunteers earlier in the year. Perhaps one quarter of the new Union army had not yet fired their muskets at the enemy. Beginning September 4, McClellan’s six corps of infantry moved out of the defenses of Washington to intercept Lee.
Lee and his men crossed the Potomac using the river fords near Leesburg, just 40 miles upriver from Washington D.C. He had anticipated that the Federal garrisons at Harper’s Ferry and Martinsburg, some 20 and 40 miles further northwest in Virginia, would be called back to join McClellan’s army or be sent to defend Washington. Gathering his army around Frederick, Maryland on September 7, Lee was disheartened to learn that the Union troops south of the Potomac had not moved, and were now located in the rear of his invading force.
Undaunted, Lee sought to capitalize on his presence in Maryland, hoping that his occupation might convince slave-holders there to cast their lot with the government in Richmond. On September 8, he issued a proclamation to the citizens of the Old Line State, telling them his army was “prepared to assist you with the power of its arms,” and promised Marylanders the Confederate states would “welcome you to your natural position” among fellow slave owners. Lee, however, had overestimated Southern sympathies: the mountainous region his army occupied contained far fewer slave owners compared to large coastal plantations to the east. Few new recruits flocked to the Rebel flag.
While his proclamation fell on deaf ears, Lee was forced to address the threat to his communication and supply lines back to Virginia. He devised a bold plan to divide his army to capture the Union troops in his rear while still maintaining presence in Maryland. On September 9, he issued Special Order 191. The order sent Lieut. Gen. James Longstreet’s wing followed by Maj. Gen. Daniel H. Hill’s division westward toward Hagerstown. Hill would defend the passes of South Mountain while Longstreet pushed further west. Lieut. Gen. “Stonewall” Jackson’s wing, with Maj. Gen. Lafayette McLaws and John G. Walker, would capture Martinsburg and Harper’s Ferry by the 13th. If all went as planned, all four groups would then re-unite west of South Mountain.
Lee’s plans soon unraveled. On September 13, McClellan received a gift when a lost copy of Special Order 191 was brought to his headquarters. Although the intelligence was four days old, McClellan knew immediately that (1) Jackson was behind schedule, as Harper’s Ferry while surrounded had not been taken yet, and (2) that Lee’s army was divided and separated over miles of Maryland countryside. Seeking an opportunity to beat Lee piecemeal, McClellan ordered his cavalry to verify the positions of Lee’s forces. That evening, McClellan bragged to Lincoln about having “all the plans of the Rebels” and ordered his men to prepare to move up South Mountain where Hill’s unsuspecting Confederates guarded the mountain gaps.
McClellan engaged Hill’s division early on September 14. After an all-day fight, Hill’s men were withdrawn from the mountain gaps. That evening, defeated and with his army still scattered, Lee sought to return to Virginia, ending his bold gamble north of the Potomac. As he contemplated a return home via Sharpsburg and Shepherdstown, Lee received word from Jackson that the surrender of the Federal force at Harper’s Ferry was imminent. Changing his mind, Lee drew up plans to halt his retreat, bring most of Jackson’s men to join Longstreet, and to make a stand against McClellan’s army.
To hold against McClellan, Lee chose a 4-mile long position on the west bank of Antietam Creek. The deep, swift-moving waterway could only be crossed at three bridges, and rolling farm fields connected with roads provided artillery platforms and room to maneuver his infantry. Three miles in his rear was Lee’s escape route home: the Potomac River crossing at Boteler’s Ford near Shepherdstown.
On the morning of September 15, McClellan’s army arrived at Keedysville, four miles east of Sharpsburg and discovered Longstreet’s wing drawn up behind Antietam Creek. Jackson had not yet joined Longstreet, so an opportunity to defeat half of Lee’s men slipped from McClellan’s grasp. Early on the morning of September 16, a heavy fog enveloped the area. As McClellan waited for the fog to burn off, Jackson’s men began crossing the Potomac on their way to Sharpsburg. McClellan decided to attack Lee’s left across the upper, northernmost bridge. Around 4:00 p.m., Maj. Gen. Joseph Hooker’s First Corps crossed Antietam Creek and skirmished with the Confederates, setting the stage for the next day’s fight: the bloodiest in American history.
Intense fighting raged for 12 hours on September 17. That evening and the next day, both armies remained in their lines. Both sides cared for their wounded and counted their losses, about 23,000 men from both sides. On the night of September 18, Lee began to withdraw across the Potomac River back into Virginia using Boteler's Ford near Shepherdstown. For the next two days, small groups of Union infantry pushed across the river and attacked the Confederate rearguard. The action discouraged McClellan’s pursuit of the Confederates further into Virginia and marked the end of Lee's Maryland Campaign.
Although tactically a draw, McClellan’s strategic victory at Antietam gave Lincoln confidence to redefine the war. Lincoln issued the preliminary Emancipation Proclamation on September 22, freeing the slaves in the Confederate states. The document fundamentally changed the meaning of the conflict. No longer simply a rebellion, the North was now waging a crusade to end slavery. Lincoln sought to crush the Confederacy by destroying the basis of its economy and society. England and France, who had already abolished slavery, would no longer seriously consider formal recognition of a Confederacy that permitted it. The final Proclamation, effective January 1 1863, permitted Union armies to raise colored troops, and nearly 200,000 would ultimately serve. Although strictly a “fit and necessary war measure,” the Proclamation paved the way for the 13th Amendment in 1865 that freed all slaves forever.