Accused of plotting to capture the federal St. Louis Arsenal (now the location of the Budweiser Brewery) as part of the Camp Jackson Affair, the 1st Missouri Volunteer Militia, with its two companies of St. Louis Greys, was captured by Union troops and marched to the arsenal on 10 May 1861.

Governor Jackson was voted out of office and replaced, a motion carried by the Unionist majority that now existed in the Missouri Convention.

They mustered into service at Jefferson Barracks and mobilized to Chickamauga Park, Georgia on 21 May 1898 but were never sent to Cuba or Puerto Rico due to lack of funding from the state.

After taking part in the Punitive Expedition in 1916 with service near Laredo, Texas, the 1st Missouri returned home.

Ordered to Camp Doniphan, now part of Fort Sill, Oklahoma, the regiment began training for the Great War.

During World War I, the 138th Infantry Regiment first took over operations in the Vosges Mountains in southern France and drew first blood for the 35th Division during a trench raid at Hilsenfirst.

The regiment fought alongside Colonel George S. Patton's tank brigade to capture the villages of Cheppy and Exermont.

After the war's end, the regiment sailed home on the USS Aeolus and arrived at the port of Newport News, Virginia, on 28 April 1919.

The 138th Infantry conducted annual summer training most years at Camp Clark, near Nevada, Missouri, 1921–39.

The 138th Infantry was inducted into active federal service on 23 December 1940, and moved to Camp Joseph T. Robinson, Little Rock, Arkansas, where it arrived on 5 January 1941.

[4] The regiment served on mainland Alaska and in the Aleutian Islands, assisting in construction projects, defending bases and manning outposts.

The 1-138th Infantry began building, forming, and equipping actions on 1 September 2008 with Federal Recognition granted in January 2012.

The 1st Battalion, 138th Infantry, deployed to conduct state emergency duty during the winter storms of 2010 and most recently, to protect persons, property, and civil liberties in Ferguson, Missouri in 2014.

The battalion deployed to Qatar in 2017 in support of Operation Enduring Freedom (Spartan Shield) to provide force protection of U.S. military assets and was there during the turbulent beginning of the GCC-Qatar Crisis.

The battalion conducted security operations and was tasked with Quick Reaction Force responsibilities at various outposts in the region.

The 3rd Battalion, 138th Infantry Regiment (3-138th) was officially reactivated as of 1 September 2022 and is currently headquartered in Kansas City, Missouri.