A Union soldier, when wounded in such early battles as Shiloh or the First Battle of Bull Run, likely lingered helpless until the fighting stopped. If he lived that long, he was collected by civilian contractors who drove ambulance wagons to field hospitals staffed with volunteer surgeons who might never before have sawed off a leg or performed other necessary procedures.

When surgeon Jonathan Letterman was promoted to medical director of the Army of the Potomac 150 years ago, in 1862, he set about mastering the chaos that bedeviled the military’s way of dealing with casualties and medical supplies. In an order he wrote that year, Letterman created an ambulance corps that ferried injured fighters immediately from the front to the nearest field hospital. His order not only specified the staffing of ambulances and hospitals; it also contained extensive lists of stock for medical corps wagons: 10 yards of muslin, 2 gutta-percha cloths (drop cloths), 5 yards of ichthyocolla plaster (fish glue used to cover wounds), 2 scarificators (for bloodletting), a dozen probangs (sponge-tipped flexible rods used for applying medications to or clearing the upper airway or esophagus), 2 thumb lancets, 12 cupping tins (used to bring blood to the surface of a wound) and 6 penis syringes (for the treatment of venereal disease).

Crucially, says George Wunderlich, executive director of the National Museum of Civil War Medicine in Frederick, Md., Letterman recognized the importance of experience. Surgeons in his hospitals would no longer be civilian volunteers. He established a hierarchy of well-trained military physicians in which the most skilled prepared the next tier.

The Letterman system likely helped the North successfully execute its war plan. By the First World War, every major power was using some form of his system, says Dale Smith, professor in the Department of Medical History at the Uniformed Services University of the Health Sciences in Bethesda. And in New York City, Edward Dalton brought his former colleague’s ideas to Bellevue Hospital, where he staffed ambulance carts with interns in what became the prototype of the modern emergency medical system.

Today’s military still bears the stamp of Letterman’s gift for organization, Smith says. “We’ve replaced wagons and trains with helicopters and planes, but we’re using a Letterman system in Afghanistan today—a system of forward care and evacuations to higher echelons for more definitive care.”