American Civil War: Union soldiers find a copy of Robert E. Lee's battle plans in a field outside Frederick, Maryland. It is the prelude to the Battle of Antietam.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

The Union was the central government of the United States during the American Civil War. Its civilian and military forces resisted the Confederacy's attempt to secede following the election of Abraham Lincoln as president of the United States. Lincoln's administration asserted the permanency of the federal government and the continuity of the United States Constitution.

Robert Edward Lee was a Confederate general during the American Civil War, who was appointed the overall commander of the Confederate States Army toward the end of the war. He led the Army of Northern Virginia, the Confederacy's most powerful army, from 1862 until its surrender in 1865, earning a reputation as a one of the most skilled tacticians produced by the war.

Special Order 191, also known as the "Lost Dispatch" and the "Lost Order", was a general movement order issued by Confederate Army General Robert E. Lee on about September 9, 1862, during the Maryland Campaign of the American Civil War. A lost copy of this order was recovered on September 13 by Union Army troops in Frederick County, Maryland, and read by General George B. McClellan, who changed the direction of his movement to conform to Lee's movements as the document described.