Army of the Potomac

The Army of the Potomac was the Union's largest army in the War of Secession. When it was organized in the winter of 1861, it was the largest army ever seen in the history of the world. However, its commander, George McClellan, was extremely reluctant to commit it to action and mostly stayed in the defenses of Washington, D.C.. McClellan eventually campaigned against Richmond by marching up the Peninsula in April 1862, but McClellan's timidity in the face of resistance of a much smaller army caused the Army of the Potomac to be driven away from Richmond.

In September 1862, it was the Army of the Potomac's turn to defend its capital when Confederate General Robert E. Lee invaded the US. Due partly to poor Union intelligence and partly to McClellan's own incompetence, Lee was able to steal a march on the Army of the Potomac and threaten Philadelphia. In desperation, and against the advice of his subordinates such as Ambrose Burnside, McClellan offered battle on the unfavorable terrain of Camp Hill, Pennsylvania. The Army of the Potomac was destroyed, Lee took Philadelphia, and the War of Secession was lost.