Also known as a unit crest or DUI, a 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. More 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.
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“Fire For Effect,” the unit motto displayed on a scroll attached to the shield portion of the 166th Regiment Distinctive Unit Insignia, reveals the organization’s origins in the Artillery branch: It was originally constituted and organized as the 166th Field Artillery, Pennsylvania National Guard, on 18 June 1939 at Harrisburg, Pennsylvania.
It was assigned to the 22nd Cavalry Division in the National Guard, one of several Cavalry divisions raised in the Guard that eventually proved unwieldy because component units were too geographically dispersed to conduct effective training. This association with the Cavalry is displayed in the yellow portion of the insignia, while the canton (inset) with blue saltire refers to the military history, dating back to the Civil War, of some units (Battery A, 1st Battalion) used to form the Regiment. (This also accounts for the Regiment being credited with five World War I campaigns even though it was not organized until 1939).
The regiment was broken up in 1943, with its HQ and HQ Battery (HHB) redesignated as HHB, 166th Field Artillery Group (separate lineage from here) and its 1st and 2nd Battalions reorganized and redesignated as the 938th and 939th Field Artillery Battalions, respectively. These units eventually took part in seven World War II campaigns in the European-African-Middle Eastern Theater that were credited to the Regiment, with an Arrowhead device authorized for the Southern France campaign for taking part in an assault landing.
The 938th FA Battalion was redesignated as the 166th Field Artillery Battalion in 1946 and organized in 1947, and a dozen years later it would be consolidated with several other units to create the 166th Artillery, a parent regiment in the Combat Arms Regimental System (CARS), IN 1959. It would go on to be redesignated as the 166th Field Artillery Regiment in July 1975; it would retain this designation until 1997, when it was reorganized and redesignated as the 166th Regiment and assigned the mission of operating the Pennsylvania Army National Guard’s “military schoolhouse” at Fort Indiantown Gap.
Typically, these institutions are referred to by the Regiment’s numerical designation preceding “Regional Training Institute,” but this phrase is not used in any official capacity on the Pennsylvania Army National Guard Web site. The Regiment is responsible for multiple courses in the Career Management Fields of Infantry (MOS 11), Field Artillery (13), Cavalry/Armor (19), and Combat Medic, as well an Officer Candidate School, Warrant Officer Candidate School, Warrior Leader Course, and Noncommissioned Officer Educational System programs. These courses are offered to all three components—active Army, Army Reserve, and Army National Guard—of the United States Army and are hosted by a total of four battalions.