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 rail lines and supply depots, and served as military police in cities all across the country. Members of the Corps escorted President...