Constituted 1 July 1916 as the 21st Field Artillery in the Regular Army and organized at Camp Wilson, Texas on 1 June 1917, the 21st Field Artillery Regiment would serve under several designations (21st Field Artillery Battalion, 21st Artillery, 21st Field Artillery Regiment) over the course of the next 98 years before the inactivation of its last active battalion, the 1st, on 12 June 2014.
During that time, the Regiment as whole would see action in World War I (two campaigns), World War II (five campaigns), the Vietnam War (sixteen out of seventeen total campaigns), Southwest Asia (AKA First Gulf War), and the War on Terrorism (campaigns yet to be determined). During its service in Vietnam, the Regiment was awarded a Presidential Unit Citation, a Valorous Unit Award, and a Meritorious Unit Commendation; it would go on to receive another Meritorious Unit Commendation for its action in Southwest Asia.
In June 2014, the Regiment's sole active Battalion, the 1st (1-21st FAR), was inactivated at Fort Hood, Texas, which was renamed Fort Cavazos in the Spring of 2023.
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The 21st Field Artillery Regiment Distinctive Unit Insignia, frequently called a unit crest or a DUI, was originally approved on 27 November 1939. It features a bend dexter (horizontal band running from the viewer’s upper left to the lower right) colored in scarlet to denote Artillery. A canton (small inset box) in the upper right contains the insignia from the crest of the coat of arms of the 3rd Field Artillery Regiment in recognition of the transfers from that regiment who formed the 21st. A collared eagle is one of the supporters of the arms of St. Mihiel, and it symbolizes the unit’s actions at the Battle of St. Mihiel, one of the most vital engagements in which the unit fought during World War I.
The Distinctive Unit Insignia is worn by all Soldiers (except General Officers) in units that have been authorized to be issued the device. It is worn centered on the shoulder loops of the Army Green Service Uniform (AGSU) and the blue Army Service Uniform (ASU, Enlisted only) with the base of the insignia toward the outside shoulder seam. DUIs are not worn on the Dress variations of either uniform, however.
Full guidance on wear of the DUI is found in DA Pamphlet 670-1,
Section 21-22, "Distinctive unit insignia" and 21–3(d) and (e),
"Beret" and
"Garrison Cap," respectively.