"For military medicine, in particular, the lessons that Letterman gave us are as true today as they were then," said retired Lt. Gen. Ronald Ray Blanck, the former surgeon general for the U.S. Army.
Before the war, medical supplies were handled by regular quartermaster wagons, Blanck said, meaning they had to compete with "beans and bullets."
The situation was so bad that, in some early Civil War battles, the wounded were left on the field for days, subject to the mercy of untrained troops and civilians.
In 1862, Letterman began to create an ambulance corps and three tiers of field hospitals: at the battlefield for simple wound dressing, nearby for emergency surgery and behind the battle lines for long-term care and recovery.
Dale Smith, a professor of military history at the Uniformed Services University in Bethesda, Md., said Letterman's innovations were so successful that Prussian and English observers wrote home to praise the system.
"There's never been any question that he changed military medicine internationally," Smith said.
But the Battle of Gettysburg was 150 years ago, and some have wondered how that could possibly be relevant for doctors in Iraq and Afghanistan, said George Wunderlich, director of the National Museum of Civil War Medicine in Frederick, Md.
Wunderlich recalled that about 10 years ago, a military member remarked that it was a shame the Civil War "has nothing to do with what we do today" with battlefield medicine.
But after Wunderlich told him how Civil War doctors resolved problems with transportation, training and even corruption, the man asked Wunderlich if those topics could be turned into a one-day course.
Another man who complained that the Civil War training sessions were "unrealistic" called Wunderlich later after responding to Hurricane Katrina, where moving supplies was slow and difficult and even some cell towers were down.
"He says, 'I'm so sorry. I'm in 1862 down here and now I get it,'" Wunderlich recalled.
Now, more than 5,500 military members and emergency responders have attended history courses run by the Museum of Civil War Medicine. The classes are designed to get people to think about how decisions get made in combat or crisis, and some are taught on battlefields at Gettysburg and Antietam. The courses include topics such as courage and innovation; artillery and its effects; evacuation; and stress and fatigue.
"Our job is to use history to save the lives of people" today, Wunderlich said.
Some of the lessons are subtle. For example, instead of just inspecting hospitals and his staff, Letterman sat beside Union General George McClellan during pre-battle meetings to better predict where to station ambulances and doctors.
"These are the kinds of things that come out from thinking about history," Blanck said. "The battles are won or lost on the creativity of the medical officer and the support of the commander."
Wunderlich said the museum also works to dispel many myths about Civil War medicine. The battles and wounds were certainly horrible, but anesthesia using chloroform or ether was involved in more than 95 percent of all major operations, he said.
And while doctors didn't yet understand exactly what germs were, they had noticed that patients did better when certain folklore was practiced. So while military camps were known for being filthy, hospitals followed strict rules for washing bed sheets and letting in plenty of fresh air and sunlight.
"They didn't know why, but they knew it worked and they put it to use," Wunderlich said.
But the biggest benefits of Civil War medicine may have come in the decades after the war, Wunderlich said. The young doctors and medics who had witnessed so much horror and saved so many lives went on to become leaders in many communities, pushing for public health reforms in major cities.
"Those people never stopped practicing medicine," Wunderlich said. "The benefit to the public was immediate."