When President Abraham Lincoln was inaugurated in March 1861, Hill was among the U.S. Army officers who resigned their commissions and joined the Confederacy. He was given command of the 13th Virginia Infantry with the rank of colonel. At the First Battle of Bull Run, his regiment was held in reserve. In February 1862 he was promoted to brigadier general. In the Peninsula Campaign, he fought against his former West Point roommate, Union General George McClellan. Proving himself as an aggressive leader, he was promoted again on May 26, 1862. This made him the Confederate Army's youngest major general. Hill called his division "the Light Division" even though it was one of the Confederate Army's larger divisions. This was probably because of how fast Hill could move his troops. He was known as a fearless general and was often seen in battle at the front lines.