Monocacy National Battlefield - http://www.nps.gov/mono

The Battle that Saved the Capital - nearly a year to the day after the decisive battle at Gettysburg, a Confederate (Southern) army advanced on a nearly defenseless Washington, DC, but was met on July 9, 1864 by a small Union (Northern) army, which it defeated, but by this delay, was unable to press on with its goal of taking the Capital city. The Greater Washington, DC area has a plethora of American Civil War battlefield sites.