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 September 1862 when his new system allowed the wounded men to be transferred quickly so there could be fewer deaths. The corps also meant that ambulances had a more centralized organization.