The Missouri State Guard was a 28,000-strong state defense force established by the Missouri General Assembly on 15 May 1861 to protect the state after its secession from the union. The militia was formed to resist any expected US Army invasion during the American Civil War, and all able-bodied men from ages 18 to 45 were eligible to join the guard. In the winter of 1861, after repelling a Union invasion at Wilson's Creek with assistance from Sterling Price and the Confederate States Army, the State Guard began to be absorbed into the regular army as the army made winter camp, and it was merged into the Army of the West on 17 March 1862.