The Maryland Campaign—or Antietam Campaign—occurs September 4–20, 1862, during the American Civil War.

Confederate Gen. Robert E. Lee's first invasion of the North is repulsed by the Army of the Potomac under Maj. Gen. George B. McClellan, who moves to intercept Lee and his Army of Northern Virginia and eventually attacks it near Sharpsburg, Maryland.

The resulting Battle of Antietam is the bloodiest single-day battle in American history.

Following his victory in the Northern Virginia Campaign, Lee moves north with fifty-five thousand men through the Shenandoah Valley starting on September 4, 1862.

His objective is to resupply his army outside of the war-torn Virginia theater and to damage Northern morale in anticipation of the November elections.

He undertakes the risky maneuver of splitting his army so that he can continue north into Maryland while simultaneously capturing the Federal garrison and arsenal at Harpers Ferry.

McClellan accidentally finds a copy of Lee's orders to his subordinate commanders and plans to isolate and defeat the separated portions of Lee's army.

While Confederate Maj. Gen. Stonewall Jackson surrounds, bombards, and captures Harpers Ferry (September 12–15), McClellan's army of one hundred and two thousand men attemptsto move quickly through the South Mountain passes that separate him from Lee.

The Battle of South Mountain on September 14 delays McClellan's advance and allows Lee sufficient time to concentrate most of his army at Sharpsburg.

The Battle of Antietam (or Sharpsburg) on September 17 is the bloodiest day in American military history with over twenty-two thousand casualties.

Lee, outnumbered two to one, moves his defensive forces to parry each offensive blow, but McClellan never deploys all of the reserves of his army to capitalize on localized successes and destroy the Confederates.

On September 18, Lee orders a withdrawal across the Potomac and on September 19–20, fights by Lee's rear guard at Shepherdstown ends the campaign.

Although Antietam was a tactical draw, it meant the strategy behind Lee's Maryland Campaign had failed

President Abraham Lincoln uses this Union victory as the justification for announcing his Emancipation Proclamation, which effectively ends any threat of European support for the Confederacy.