After the defeat at Bull Run, Major General George B. McClellan was placed in command of the new Union Army defending Washington, the Army of the Potomac. McDowell became a division commander in the Army of the Potomac. On March 14, 1862, President Lincoln issued an order forming the army into corps and McDowell got command of the I Corps as well as a promotion to major general of volunteers. When the army set off for the Virginia Peninsula in April, McDowell's command was detached for duty in the Rappahannock area out of concern over Stonewall Jackson's activities in the Shenandoah Valley (one division was later sent down to the Peninsula).