In the Civil War, a very high percentage of casualties died of infection and shock. That was before germ theory and field hygene left a lot to be desired, as did surgery of the day. Surgeons amputated limbs in a matter of minutes, usually without any anesthesia and definitely without washing.

Things had improved by WWI. They knew about the importance of basic hygene (surgeons tried to use sterile technique) and the invention of the helmet meant that a lot of men who would've died of head injuries managed to survive, often with terrible facial wounds. They didn't have antibiotics, so infection still killed a lot of the wounded.

Things improved a lot more in WWII. They had better helmets and at the start of the war, they had sulfa drugs. By the last couple years of the war, the first real antibiotics were available. The death rate for the wounded dropped dramatically.

Things were still better in the Korean war. Helicopters could medevac the wounded to field hospitals very rapidly. The survival rate for the wounded reached something like 96%. Better helicopters and improved trauma care improved that ratio even more in Vietnam.

Today, a lot of the wounded are surviving wounds that would've killed them in previous wars, largely due to body armor. While far from perfect, it has save a lot of American lives. Many of the wounded have suffered the loss of a limb or a head injury around the edges of the armor, but they probably would've died without it.