The U.S. Ambulance Corps was a unit of the Union Army during the American Civil War. The Ambulance Corps was initially formed as a unit only within the Army of the Potomac, due to the effort of several Army officials, notably Dr. Jonathan Letterman, medical director of the Army of the Potomac, and William Hammond, the US Surgeon-General. Until August 1862, the lack of trained ambulance drivers meant that the wounded had to wait a long time to receive medical care. This changed at the Battle of Antietam in Septmember 1862. The Ambulance Corps provided trained drivers and litter-bearers who provided much better service and care for the wounded. The corps also meant that ambulances had a more centralized organization. 
Due to public pressure, the Army created an Ambulance Corps for all units and theaters of operation, through the Ambulance Corps Act of March 11, 1864. The Confederate armies did not have any such service available within their military.
- The American Civil War: 365 Days, By Margaret Wagner, publ Abrams, New York, in association with the Library of Congress, page for Oct. 13 (all pages numbers for this book are indicated as dates).