The Lee Headquarters Maryland historical marker is on the west side of Urbana Pike (State Route 355) 100 yards south of the entrance to the Monocacy Visitor Center. (39.37644° N, 77.397046° W; map)
This marker is not from the Battle of Monocacy, but dates to the Antietam campaign almost two years previously. It shows how the border state of Maryland was repeatedly fought over during the war.
This area was the headquarters for Lee’s Army of Northern Virginia during the early days of his first invasion of the North. It was here that the infamous “Lost Order” was written, which fell into the hands of Union General George McClellan and led directly to the Battle of South Mountain as well as the Battle of Antietam, the bloodiest single day of fighting in American history.
From the marker:
Generals Robert E. Lee,
Longstreet Sept. 6-9, 1862.
Here was written the famous
Lost order No. 191 and the
proclamation to the people
See more about Lee’s Special Orders Number 191, including the full text