The primary Confederate force in the Eastern theater was the Army of Northern Virginia. The Army originated as the (Confederate) Army of the Potomac, which was organized on June 20, 1861, from all operational forces in Northern Virginia. On July 20 and 21, the Army of the Shenandoah and forces from the District of Harpers Ferry were added. Units from the Army of the Northwest were merged into the Army of the Potomac between March 14 and May 17, 1862. The Army of the Potomac was renamed Army of Northern Virginia on March 14. The Army of the Peninsula was merged into it on April 12, 1862.