Organized in May 1863 to meet the Union Army's growing manpower needs, the Invalid Corps--later renamed the Veteran Reserve Corps--was a unique military unit. With more than twenty-four regiments of troops, nearly all of them men disabled by illness or combat wounds, it was at one point twice as large as the entire pre-war United States Army. During four years of service its troops enforced the draft, guarded prisoners and vital outposts, protected...