World War I

.^[3] The regiment was constituted 5 August 1917 in the National Army as the 340th Infantry and assigned to the 170th Infantry Brigade of the 85th Division. It was organized at Camp Custer, Michigan on during August and September 1917. In July 1943, the regiment was organized with 3,755 officers and enlisted men:

• Headquarters & Headquarters Company- 303
  • Supply Company- 140
  • Machine Gun Company- 178
  • Medical & Chaplain Detachment- 56
• Infantry Battalion (x3)- 1,026
  • Headquarters- 2
  • Rifle Company (x4)- 256^[4]

The Doughboys of the regiment deployed to France as part of the American Expeditionary Forces and were billeted in the city of Humbligny. The regiment didn't participate in any named campaigns during the war; its Infantrymen were used as individual replacements to the fighting Divisions. Pioneering American football player, physician, and local politician Bradbury Robinson commanded Company L during the war.^[5][6] After completing its war service in France the regiment demobilized at Camp Custer on 21 April 1919.^[7]

Post War Service

The Regiment was reconstituted on 24 June 1921 in the Organized Reserves with headquarters in the Saginaw, Michigan under TOE 29-7T as part of the 85th Division (later redesignated as the 85th Infantry Division) in the Sixth Corps Area.^[8] The 1st Battalion was located in Saginaw, the 2nd Battalion at Flint, Michigan and the 3rd Battalion was at Port Huron, Michigan. The entire regiment relocated on 28 July 1937 to Saginaw. During this period, the regiment conducted summer encampments in most years with the 2d Infantry Regiment at Camp Custer or some years at Camp Grayling. In 1929 the regiment conducted summer training with the 126th Infantry Regiment at Camp Grayling. The regiment conducted infantry CMTC training some years at Camp Custer, Fort Brady, Michigan, as an alternate form of summer training. Primary ROTC feeder schools were the Michigan State College of Agriculture and Applied Science.^[9] The regiment was relieved 31 March 1942 from assignment to the 85th Division under the wartime reorganization from the four-regiment square division to the three-regiment triangular division structure, allotted to the Army of the United States as an inactive unit, and finally disbanded on 4 August 1952.^[10]

Under the 85th Training Division

On May 5, 1959 the 340th Infantry was redesignated a subordinate element of the 85th Division (Training Support) as the 340th Regiment (Advanced Individual Training), and reorganized to consist of the Regimental Headquarters and Headquarters Company, Headquarters and Headquarters Companies 1st, 2d, and 3d Battalions, and Companies A, B, and C organized as Infantry with the below listed companies organized from other units:

The 340th Regiment was reorganized January 31, 1968 to consist of the 1st Battalion (Advanced Individual Training) and the 2nd and 3rd Battalions (Basic Combat Training). The Regimental Headquarters was inactivated. On May 1, 1971, the 1st Battalion (Advanced Individual Training) was redesignated as 1st Battalion (Combat Support Training) with no change to the 2nd and 3rd Battalions. The 3rd Battalion was inactivated on 1 October 1996 and reactivated on 1 October 2002. On 1 April 2007, the 1st and 2nd Battalions were assigned away from the 85th Division; on 15 June 2008 the 3rd Battalion was also reassigned.

Current Assignment

The 1st Battalion is an Army Reserve unit assigned to the 181st Infantry Brigade at Fort Snelling, Minnesota and provides Observer, Controller/ Trainers (OC/T) and Staff to various Mobilization Training Centers responsible for conducting post mobilization training to Reserve Component units preparing them for deployment to Overseas Contingency Operations.

The 2nd Battalion is an Army Reserve unit assigned to the 4th Cavalry Brigade and provides Observer, Controller/ Trainers (OC/T) and Staff to various Mobilization Training Centers responsible for conducting post mobilization training to Reserve Component units preparing them for deployment to Overseas Contingency Operations.
The 3rd Battalion is a Regular Army unit assigned to the 181st Infantry Brigade and stationed at Fort McCoy, Wisconsin with a mission to train Infantry units of the Army National Guard.

• 
  1st Battalion training the 26th Maneuver Enhancement Brigade in 2009
• 
  3rd Battalion supporting JRTC January 2016
• 
  3rd Battalion conduct sling load training for the 856th Quartermaster Company at Camp Blanding February 2016.