During the American Civil War, Northern armies were frequently named for major rivers (Army of the Potomac, Army of the Tennessee, Army of the Mississippi), while Southern armies for states or geographic regions (Army of Northern Virginia, Army of Tennessee, Army of Mississippi). Several names also exist for the forces on each side; the opposing forces named battles differently as well. The Union forces frequently also named battles for bodies of water that were prominent on or near the battlefield, but Confederates most often used the name of the nearest town. As a result, many battles have two or more names that have had varying uses, but with some notable exceptions, one name has eventually tended to take precedence.

Source: Wikipedia