Encyclopædia Britannica, Ninth Edition/Antietam

ANTIETAM, a small river of the United States, which rises in Pennsylvania, and flowing into Maryland, joins the Potomac about 50 miles from Washington. An indecisive battle between the Federals under M‘Clellan and the Confederates under Lee was fought on its banks on 16th and 17th September 1862. The victory, however, practically lay with the Federals, as the Confederates retreated out of Maryland on the night of the 18th September.