Video: 'Ken.Burns.The.Civil.War.3of9.Forever.Free.' (Sep. 3-4, 1862, at 40:19)

(Thursday, September 4, 1862, during the Maryland Campaign, part of the American Civil War) — The Maryland campaign (or Antietam campaign) began today as advance elements of Confederate General Robert E. Lee’s Army of Northern Virginia crossed into Maryland from Loudoun County, taking the Civil War into Union territory for the first time.

His objective was to resupply his army outside of the war-torn Virginia theater and to damage Northern morale in anticipation of the November elections. He undertook the risky maneuver of splitting his army so that he could continue north into Maryland while simultaneously capturing the Federal garrison and arsenal at Harpers Ferry.

The main body of the army would advance into Frederick, Maryland, on Sept. 7, 1862.

The 55,000-man army had been reinforced by troops who had been defending Richmond, but they merely made up for the 9,000 men lost at Bull Run and Chantilly.

Lee’s first invasion of the North would be repulsed by the Army of the Potomac under Maj. Gen. George B. McClellan, who moved to intercept Lee and his Army of Northern Virginia and eventually attacked it near Sharpsburg, Maryland.

McClellan accidentally found a copy of Lee’s orders to his subordinate commanders and planned to isolate and defeat the separated portions of Lee’s army.

The resulting Battle of Antietam on Sept. 17, 1862, would become the bloodiest single-day battle in American history.

Although Antietam was a tactical draw, it meant the strategy behind Lee’s Maryland campaign had failed.

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