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.