The 1st Missouri Volunteers evolved from one of several unofficial pro-Unionist Home Guards militia formed in St. Louis in the early months of 1861 by Congressman Francis Preston Blair, Jr. and other Unionist activists. The militia that would become the First Missouri was largely composed of ethnic Germans, although Companies K and I had significant numbers of native born American citizens and Irish-Americans. Although initially without any official standing, beginning on April 22, 1861 the four "underground" militia 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.