The first field corps in the United States Army were legalized during the American Civil War by an Act of Congress on July 17 , 1862 .
Major General George B. McClellan had designated six corps organizations within his Army of the Potomac that spring .
The first field corps in the United States Army were legalized during the American Civil War by an Act of Congress on July 17 , 1862 , but Major General George B. McClellan had designated six corps organizations within his Army of the Potomac that spring .