The insignia of the rank of colonel, as seen on the right, is worn on the officer's left side (a mirror-image version is worn on the right side, such that the eagle always faces forward to the wearer's front; the left-side version is also worn centered on fatigue caps, helmets, Army ACU & ECWCS breasts, inter alia). By law, a colonel must have 22 years of service and a minimum of three years of service as a lieutenant colonel before being promoted.
The insignia for a colonel is a silver eagle which is a stylized representation of the eagle dominating the Great Seal of the United States (which is the coat of arms of the United States). As on the Great Seal, the eagle has a U.S. shield superimposed on its chest and is holding an olive branch and bundle of arrows in its talons. However, in simplification of the Great Seal image, the insignia lacks the scroll in the eagle's mouth and the rosette above its head. On the Great Seal, the olive branch is always clutched in the eagle's right-side talons, while the bundle of arrows is always clutched in the left-side talons. The head of the eagle faces towards the olive branch, rather than the arrows, advocating peace rather than war. As a result, the head of the eagle always faces towards the viewer's left.
However, when worn as a single insignia with no matching pair, such as on the patrol cap, garrison cap/flight cap, or the front of the Army ACU, there is a split between the services on which mirror image of the eagle should be worn. In the United States Army and United States Air Force, the eagle is always worn with "the head of the eagle to the wearer's right," with the olive branch clutched in the eagle's right hand talons (see Army Regulation 670-1, paragraph 28-6 (a)(1)). In the United States Marine Corps, United States Navy, United States Coast Guard and NOAA, the eagle is worn with "the head facing forward" on the wearer's right side of the garrison cover (see Marine Corps Order P1020.34G, Uniform Regulation, paragraph 4005d(1)). Since respective service's officer insignia is worn on the left side and the rank insignia is worn on the right hand side of the Marine, Navy, Coast Guard and NOAA garrison caps, the eagle is facing to the eagle's left with the olive branch clutched in the eagle's left hand talons, which is a mirror opposite to the wear of the single eagle for Army and Air Force officers.
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.