In the Army of the Potomac, the Ambulance Corps soon demonstrated its worth as they fought in Virginia and then moved north into Maryland. It was only partially in place for the Battle of Antietam in September 1862, where, during twelve hours of combat, casualties from both sides rose to over 22,700. McClellan's casualties mounted to 25 percent of the soldiers who went into action. On the Union right wing, where Letterman's plan was in place, casualties were rapidly evacuated and all wounded within the Union lines were removed during the night. Casualties on the left, where the new evacuation plan was not in place, were not removed until the following night. Nevertheless, despite its success, the service received hostility from the military.