His system of organization enabled thousands of wounded men to be recovered and treated during the American Civil War.

[1] His studies were directed by a private tutor until he entered Jefferson College, where he became a member of Beta Theta Pi.

His younger brother, William Henry Letterman, co-founded the Phi Kappa Psi fraternity with Charles Page Thomas Moore in Canonsburg after tending to sick classmates in Fall 1850.

In other words, Letterman instituted standing operating procedures for the intake and subsequent treatment of war casualties and was the first person to apply management principles to battlefield medicine.

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.'

"The expediency of the order I, of course, do not pretend to question, but its effect was to deprive this department of the appliances necessary for the proper care of the wounded".

[12] He appreciated that command had to make hard decisions when allocating transport vehicles for the best overall benefit of the Army's operations and soldiers.

[13] Unfortunately for Letterman's career, his mentor and superior officer, William A. Hammond, was undergoing censure as a result of his decision to ban the use of calomel, a mercury derivative, in May 1863.

After a brief period of serving as Inspector of Hospitals in the Department of the Susquehanna, Letterman resigned from the army in December 1864 well before the end of the American Civil War.

Railroad magnate Thomas A. Scott knew of Letterman's organizational abilities and administrative skill and offered him a job as general superintendent of a company exploring for oil in California.