Robertson joined the Confederate Army in August 1861. He became a colonel in the Fourth Virginia Cavalry in November 1861, followed by a promotion to brigadier general in June 1862. He served initially under Stuart in northern Virginia and on the Peninsula before being transferred to the Valley. He fought at the battles of Cedar Mountain and Second Manassas. In October 1862, he was transferred to North Carolina. He returned North during the Gettysburg campaign, but due to poor performance, he spent his last year in the army serving in South Carolina during the approach of Sherman.
