The greatest casualties for the Army of the Potomac were suffered at the three-day Battle of Gettysburg in July 1863, but it also showed the efficiency of Letterman's system, since although the mortality rate of the Army of the Potomac had been 33% during 1862's Peninsular Campaign, the mortality rate after this bloody three-day battle was only 2%. No official report of the battle mentioned Letterman's contribution. To deal with more than 14,000 Union wounded, along with 6,800 Confederate wounded who were left behind, a vast medical encampment was created northeast of Gettysburg off the York Pike on the George Wolf farm, named "Camp Letterman." Of the Battle of Gettysburg and his system for treating casualties, Jonathan Letterman on October 3, 1863, reported to Brig. Gen. S. Williams, A.A.G., Army of the Potomac, "Surgeon John McNulty, medical director of that corps, reports that 'it is with extreme satisfaction that I can assure you that it enabled me to remove the wounded from the field, shelter, feed them, and dress their wounds within six hours after the battle ended, and to have every capital operation performed within twenty-four hours after the injury was received.' I can, I think, safely say that such would have been the result in other corps had the same facilities been allowed—a result not to have been surpassed, if equaled, in any battle of magnitude that has ever taken place.'" Maj. McNulty, the medical director of the XII Corps (Union Army), either "had not received" or more likely had outright disobeyed orders to leave behind medical baggage wagons and, therefore, unlike many other Union medical commanders at Gettysburg had sufficient medical equipment and supplies to adequately implement Letterman's systems at Gettysburg for the XII Corps.