The Potomac River flows past Washington DC. The first battle of the Civil War was fought not far from the national capital at Bull Run. The demoralization of Union forces after this battle left Washington in an undefended state that came close to disaster, but the Union benefited from the failure of the Confederacy to follow up.
Congress was quick to rectify its mistake. On July 25, 1861, not long after Congress had authorized the induction of volunteers, the Division of the Potomac was created and placed under the command of Gen. George B. McClellan. It`s immediate purpose was to defend the approaches to the Potomac and hence Washington.
The forces that McClellan inherited were "a collection of undisciplined, ill officered, and uninstructed men." McClellan started with a force of 37,000, of whom many were nearing the end of their enlistment period. Within four months, he had increased the number to 77,000 men available for active duty, and the count was growing.
Turning the volunteers into an effective fighting army was a challenge. McClellan was forbidden to take regular officers and put them in charge of volunteers, and the politically appointed volunteer officers were of low military quality. Nevertheless, McClellan eventually created a trained force that could hold its own against the Confederates.
He was nevertheless cautious, and his caution was supplemented with that of Congress, who held back troops for the defense of Washington against Stonewall Jackson`s smaller force. The Army of the Potomac fought its way to the James River after the Seven Days Battles, but Washington lost patience with McClellan and demoted him below Pope.
In 1864 and 1865, when Meade in command of the Army of the Potomac but receiving orders from General Grant, the Army was able to complete the work outlined and begun by McClellan in 1862.