Following the outbreak of the war, Porter was commissioned as colonel in the 15th US Infantry in May 1861. In August 1861, he was promoted to brigadier general and commanded a division of the Army of the Potomac from the fall of 1861 to the spring of 1862, and in July of that year achieved the rank of major general in the volunteers. During his service with the Army of the Potomac Porter became a close friend of General George Brinton McClellan, an alliance which proved damaging to him in the political climate that ensued.
