The Army of the Potomac was the main army in the Eastern Theater for most of the Civil War, tasked with protecting the national capital of Washington and with capturing the Confederate capital of Richmond
It was created in July of 1861 from the wreckage of the army Irvin McDowell had led to Bull Run. The Departments of Annapolis, Pennsylvania and the Shenandoah were merged in, Giving McClellan command of most of the Eastern Theater, while tens of thousands of new recrtuits poured in from the northern states.
Lincoln forces the creation of army corps as McClellan launches the Penninsula Campaign. After its failure, the army returns to Washington to save the capital after Pope's Army of Virginia is defeated at Second Bull Run.
McClellan halts Lee's invasion of the North as he leads the Army of the Potomac to its bloodiest day at Antietam, but afterwards Lincoln cannot force him to move. McClellan is removed, and Ambrose Burnside reluctantly accepts command.