The 399th Regiment Distinctive Unit Insignia, also called a unit crest or a DUI, was first approved for the 399th Infantry Regiment, Organized Reserves on 21 April 1931 and was redesignated for the 399th Regiment, Army Reserve, on 18 August 1960. Its blue shield is a nod to the organization’s Infantry heritage (blue is the official branch color of Infantry). A rifle and powder-horn allude to the allocation of the unit to Fort Knox, Kentucky in the Army Reserve. "I Am Ready" is the Regimental motto.
You can find guidance on wear of the DUI in DA Pamphlet 670-1, Section 21-22, "Distinctive unit insignia" and 21–3(d) and (e), "Beret" and "Garrison Cap," respectively.
♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦
Constituted in the National Army as the 399th Infantry and assigned to the 100th Division (later 100th Infantry Division) on 23 July 1918, the 399th Regiment saw no action in World War I and was demobilized on 30 November 1918. Reconstituted in June 1921 in the Organized Reserves (Army Reserve after July 1952) as the 399th Infantry and assigned again to the 100th Division, the unit was organized in February 1922 with its Headquarters in Lexington, Kentucky.
The Regiment was called into active military service in November 1942 and fought in three campaigns in the European Theater of World War II, with its 1st and 3rd Battalions tapped for a Presidential Unit Citation for two different engagements (Raon L’Etape for the 1st, Fohlenberg for the 3rd). Inactivated from January to October of 1946, the Regiment was redesignated as the 399th Glider Infantry, an element of the 100th Airborne Division, on 15 October 1946; it activated on 28 January 1947 with its Headquarters at Louisville, Kentucky.
It was redesignated as the 399th Airborne Infantry in August 1950 but was reorganized and redesignated as the 399th Infantry on, an element of the 100th Infantry Division, on 12 May 1952. It assumed its current designation and new role as a training organization on 17 April 1959 when it was reorganized and redesignated as the 399th Regiment, an element of the 100th Division (Training), with Headquarters at Louisville, Kentucky.
It would see multiple reorganizations, usually affecting the number of Battalions the Regiment was allocated or activating a Regimental unit for active military service. In 2001, the Regiment was organized with four battalions, 1st through 4th, the most in its history—but in less than two decades they would almost all be inactivated. The 1st and 2nd Battalions cased their colors at the same time in 2007 when their parent unit, the 7th Brigade, 100th Division, was inactivated. The 3rd Battalion, 399th Regiment was inactivated nine years later on 3 December 2016, leaving only the 4th Battalion continuing its training mission as part of the U.S. Army Reserve’s 80th Training Command of The Army School System.
As of March Autumn 2023, the 399th Regiment has two Battalions assigned to the 104th Training Division, one in the 1st Brigade and one in 3rd Brigade. The 104th Training Division is a component of the Army Reserve's 108th Training Command.