Bohlen was appointed Brigadier General, U.S. Volunteers, on April 28, 1862. He commanded the 3rd Brigade of Carl Schurz's Division in the Valley Campaign against Stonewall Jackson in the spring of 1862. His brigade participated in the Battle of Cross Keys, 8 June 1862, but did not fight at the Battle of Cedar Mountain, 9 August 1862, as Sigel's division was encamped at Culpeper Court House at the time of the battle.