First, over the last few decades, there has been continuous development in methods and equipment for "emergency medicine" (ambulance crews and staff in emergency rooms.) This stuff had slowly been coming over to the military, but with the fighting in Iraq, it has nearly all been adopted by military medical personnel.
Second, there's the high intelligence and skill levels of the volunteer military. High enlistment standards have largely gone unnoticed by most people, but within the military, it's meant that combat troops, who are much brighter than at any time in the past, can handle more complex equipment and techniques. Getting the combat troops to learn these techniques is no problem, because for them, it could be a matter of life and death.
Third, medical teams, capable of performing complex surgery, are closer to the combat. These teams, like the medics and troops, have more powerful tools and techniques. This includes things like "telemedicine," where you do a videoconference with more expert doctors back in the U.S., to help save a patient.
The fighting in Iraq and Afghanistan has brought about a major change in how the United States deals with combat casualties. The result is that over 90 percent of the troops wounded, survive their wounds. That's the highest rate in history. There are several reasons for this. The main one is that medics, and the troops themselves, are being trained to deliver more complex, and effective first aid more quickly. Military doctors now talk of the "platinum 10 minutes," meaning that if you can keep the wounded soldier, especially the ones who are hurt real bad, alive for ten minutes, their chances of survival go way up. Medics have been equipped and trained to perform procedures previously done only by physicians, while troops are trained to do some procedures previously handled only by medics. This skill upgrade is made possible by a number of factors.
The "platinum 10 minutes" is part of a century old trend. During World War II, the "golden hour" standard of getting wounded troops to an operating table, was developed. Antibiotics were also developed at about the same time, along with the helicopter (whose first combat mission, in 1945 Burma, was to recover injured troops). So these new developments are not anything exotic.
Finally, the military medical community has a track record of success that the troops know about. So everyone realizes that if they pitch in, chances of survival are good, and they are.