Confederate Gen. Robert E. Lee, victorious at the Second Battle of Bull Run, or Manassas, Va., begins sending his Army of Northern Virginia northward toward Maryland in the first week of September 1862.

His bold plan: to strike a heavy blow directly at the North even as the federal government is reeling from defeat at Bull Run and a failed attempt earlier in 1862 to capture Richmond, Va., seat of the Confederacy.

The Confederates number about 70,000 overall but are ragtag, often hungry and wearing ill-fitting uniforms.

Moving from Leesburg, Va., they are intent on entering Maryland in the shadow of its western mountains. On Sept. 5, 1862, the first advance forces splash across the Potomac River into Maryland. Just ahead is one of the most fearsome appointments of the war: Antietam.

The battle of Antietam in Maryland, in mid- September, will constitute the bloodiest single day of combat on American soil. Lee’s intent is to bring war to the North by rolling into Union-held Maryland, a slave-holding state pocked by divided sympathies. The rebel incursion prompts a massive federal force to respond to the threat.

A Sept. 8, 1862, newspaper dispatch reports from Rockville, Md. — outside Washington — that “To-day matters here are assuming a more warlike appearance.” It reported that Union Maj. Gen. George B. McClellan himself had been seen backed by a daunting force of cavalry, artillery and infantry moving into Maryland “in great numbers, and they are still coming.” The report added: “McClellan’s presence leads many to suppose he is to assume offensive action.”

On Sept. 17, 1862, the two opposing armies will clash at Antietam at a cost of more than 23,000 dead, wounded or missing — one of the great battles of the war.