3rd Missouri , US Reserve Corps Infantry ( 3 months , 1861 ) The 3rd United States Reserve Corps ( U . S . R . C . ) Infantry evolved from one of several unofficial pro - Unionist militia units formed semi - secretly in St . Louis in the early months of 1861 by Congressman Francis Preston Blair , Jr . and other Unionist activists . The organization that would become the Third U . S . R . C was largely composed of ethnic Germans , who were generally opposed to slavery and strongly supportive of the Unionist cause . 20 percent of the men of the regiment were " native " U . S . citizens . Although initially without any official standing , beginning on April 22 , 1861 the Unionist regiments Blair helped organize were sworn into Federal service at the St . Louis Arsenal by Captain John Schofield acting on the authority of President Lincoln . The first four regiments were considered to be the regiments of Missouri Volunteers called for under President Lincoln ' s April 15 , 1861 call for 75 , 000 volunteers . So many St . Louis Unionists mustered to volunteer , that after consultation with General - in - Chief Winfield Scott , and Secretary of War Simon Cameron , President Lincoln directed Captain Nathaniel Lyon to " enroll in the military service of the United States the loyal citizens of Saint Louis and vicinity , not exceeding with those heretofore enlisted , ten thousand in number , for the purpose of maintaining the authority of the United States ; [ and ] for the protection of the peaceable inhabitants of Missouri . " These additional regiments , in excess of the original Presidential call were designated " United States Reserve Corps " units , and were intended for local service . The Third U . S . R . C . Infantry was mustered into service on May 8 , 1861 under Colonel John McNeil . The new Missouri Volunteer regiments , subsequently elected ( then ) Captain Nathaniel Lyon as the brigadier general of the new brigade of Missouri volunteers . President Lincoln would later confirm Lyon ' s promotion from Captain to Brigadier general . Military Service . On May 10 , 1861 the Third U . S . R . C under Colonel McNeil participated in the arrest of the Missouri Volunteer Militia drilling at Camp Jackson at Lindell Grove on the western border of St . Louis City . As the Missouri militiamen were being march under guard back to the Arsenal near the riverfront , angry crowds confronted the Federal forces and the confused situation soon devolved into rioting and gunfire . Over 27 people were killed and the Camp Jackson Affair helped to polarize the state and send Missouri down the road to its own internal civil war . The 3rd U . S . R . C . served as part of the St . Louis garrison until July 1 , 1861 when three of its companies jointed Brigadier General Lyon ' s Southwest Expedition . On July 16 , six additional companies marched ton Callaway County , Missouri , where after a skirmish with members of the secessionist Missouri State Guard , they occupied Fulton . The regiment returned to St . Louis , where in September part of the unit ' s manpower was mustered out of service at the expiration of their enlistment . Those who chose to reenlist were reorganized as the 3rd U . S . R . C . Infantry ( 3 Years Service ) , under the command of Colonel Charles A . Fritz , who had been the regiment ' s original Lieutenant Colonel . In January 1862 , the 3rd U . S . R . C Infantry ( Three Years Service ) was consolidated with the Gasconade Battalion to form the 4th Missouri Volunteer Infantry