The United States rank of colonel is a direct successor to the same rank in the British Army. The first colonels in the U.S. were appointed from Colonial militias maintained as reserves to the British Army in the North American colonies. Upon the outbreak of the American Revolutionary War, colonial legislatures would grant commissions to men to raise a regiment and serve as its colonel. Thus, the first U.S. colonels were usually respected men with ties in local communities and active in politics.
With the post-war reduction of the U.S. Army, the rank of colonel disappeared, and was not re-introduced until 1802.
The rank of colonel was relatively rare in the early 19th century, partly because the U.S. Army was very small, and the rank was usually obtained only after long years of service. During the War of 1812 the Army grew rapidly and many colonels were appointed, but most of these colonels were discharged when their regiments were disbanded at the war's conclusion. A number of other colonels were appointed by brevet - an honorary promotion usually for distinguished service in combat.
The American Civil War saw a large influx of colonels as the rank was commonly held in both the Confederate army and Union Army by those who commanded a regiment. Since most regiments were state formations and were quickly raised, the colonels in command of the regiments were known by the title "Colonel of Volunteers," in contrast to Regular Army colonels who held permanent commissions.