Once upon a time there were two Army Medical Department officers who revolutionized military medicine not only in the United States but also worldwide. One of them was Jonathan A. Letterman, Medical Director of the Army of the Potomac; his many reforms of battlefield medicine eventually formed what became known as the "Letterman System." The other was William A. Hammond, the Army Surgeon General during the middle years of the Civil War, who was the catalyst for the overall revolution in Army medicine that included Letterman's reforms.