Letterman didn't personally invent ambulances, the triage of casualties, or field medical health care facilities, but what he did do was to organize them on a systematic basis in the Army of the Potomac. After he resigned his commission in 1863 the U.S. Congress passed a law that put his reforms into effect in the Army.

His system is now the model for military medical services worldwide and also for how civilian ambulances and emergency rooms operate. His main change was to centralize medical personnel and assets that were then assigned to regiments and form them into an Army-wide medical service to rapidly evacuate casualties and treat the worst ones first, those who were believed to have had a chance of surviving.

The concept of the "Golden Hour" from the U.S. Army AMEDD during World War II is a refinement of his concept -- the idea that if a seriously injured person can be brought to medical professionals within an hour of his wounding he will most likely survive.