Ar'my of the Poto'mac
1. Union forces, trained and organized by Gen. George B. McClellan, that guarded Washington, D.C., against a Confederate invasion across the Potomac and fought battles in the eastern sector during the Civil War.
2. Confederate forces from the Alexandria, Potomac, and Shenandoah districts from mid–1861 to mid–1862: later known as Army of Northern Virginia.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.