Having casualties on a battlefield is a certainty. Get that through your head that eventually without fail one of your troopers will be wounded or worse. It is inevitable. Even if you're the best trained and most disciplined team one of you will sneeze during an ambush and be caught with your pants down, someone will look up at just the right moment (for their team), you'll step on a twig you didn't see or someone will get blindly lucky. Regardless reducing those casualties is important to all of us. Superior firepower is the best medicine when combined with violence of action.
But say your man is hit. That poultice you made? It isn't worth a damn if he bleeds out. What if he has a TBI? Chemical burns? Or how about burns in general? Poisoning?
First and foremost stop thinking that a bit of moss and some mud will magically fix it. Because it won't. There's a reason we stopped putting leeches on people to subdue infections and stopped trying to 'rebalance their humors'. It's because science advanced.
What do you do when your buddy has been gut shot? Firstly: do NOT let him drink water. He can have small amounts; just enough to wet his lips and mouth but do not let him swallow it.
Using a pressure bandage, scoop his intestines off the ground with the clean side of the bandage. Do not attempt to put them back in. Doing so can twist the intestines and cause cuture and far greater problems down the line. Lay them on top of his body and wrap the bandages around his entire torso as tight as possible. You can't overtighten here. Get him to a hospital. Your poultice and your native American medicine isn't going to magically cure him.
What if he got shot in the arm or leg and has a severed artery? Tourniquet. Do so immediately as high on the limb as possible and as quickly as possible. Write the time and date on the tourniquet. Your belt is NOT a tourniquet. It does not get tight enough and will not be enough. Your shirt is a better improvised tourniquet but it should only be used when no alternative remains. Even then with a shirt he is likely to die. DO NOT REMOVE A TOURNIQUET! LET THE DOCTORS DO THAT!
What about a shot to the head? He's dead right?
Wrong. Over 5% of people shot in the head survive. Of them 60% are able to recover without changes to their lifestyle. Some even return to active service.
This method works for any head injury. If the brain is exposed do not press on it. Brains are not solid and have the consistency of jell-o; meaning they can easily be damaged just by trying to hold it.
Secondly, wrap bandages around his head and cover the brain with the sterile part. DO NOT TOUCH THE BRAIN! IT WILL LEAD TO INFECTION AND HE WILL DIE! Keep at least one of his eyes uncovered and his ears to prevent him from going into shock. Shock kills more people on the battlefield than anything else.
If the brain is not exposed the method is the same. Get him to a hospital the day before he was wounded. He will not survive without serious and intense surgery. Even then his chances are grim. Be prepared in this situation to write a letter home with a chaplain and NCO.
"Well my buddeh has a chest woond so he ded rite?"
WRONG! DEMOTION! GET OUT OF MY MILITIA!
If your comrade has a chest wound you need to act fast and decisively though. First, if his chest cavity is punctured you need to make a flutter bandage. When the chest cavity is punctured it disturbs the vacuum seal of the lungs and causes the cavity to fill with air; causing the lungs to collapse.
Using a piece of flexible plastic (literally a ziploc bag will work) place it over the wound and tape it on three sides. This is called a flutter bandage. When he breathes in the air in his chest will be pushed out through the bandage. This is temporary. Get him to a hospital. He has about 8 hours to live without help even with a flutter bandage. If he has an exit wound do the same thing but secure all sides of the bandage on the opposite side.
In the above if his lungs are punctured from the bullet... say a prayer. Fix him anyways. It is better to try and fail than to not try at all.
A shot to the arm or leg but no arterial bleeding? Bandage it tight and send him to the rear lines if he can walk. If not, carry him home to his family.
In the event of an amputated limb, tourniquet immediately. He will still probably die before you get to him though. It takes an average human being approximately 30 seconds to bleed out from a hit to the femoral artery. Approximately 5 from a hit to the jugular or cartoid artery.
Broken limbs are tricky but doable. Firstly do not try to straighten the limb. I was once riding along in an ambulance and saw an EMT try to do this. The guy he 'fixed' ended up with bone splinters in his lungs and died after they shredded his organs. That was from straightening a broken finger. Let the professionals do it.
However, find a stick or similar stiff object that is the same shape as the broken leg. DO NOT TRY TO SET THE BONES! THAT GUY DIED IN AGONY OVER A SIMPLE AND STUPID MISTAKE! Immobilize the limb entirely so that the injured limb cannot move. Better yet; tie it to his other limb or against his torso. Your job is not "fix muh budeh". Your job is to get him to a hospital alive. When you have the limb immobilized get him away from the battle. His fight is over for now.
Broken neck or spine? Immobilize it. DO NOT TRY TO TWIST HIM BACK INTO A PROPER SPINE ALIGNMENT! YOU CAN PARALYZE OR EVEN KILL THEM THIS WAY! Put him on a stretcher. Carry him home. His fight is over. Period. I broke my spine three different times in the military. I know from experience.
"Bu mu budee wuz shot in da ass"
Tell him to open a shrimping company. He'll make millions.
Seriously though, bandage it. Tightly. Same for an arm or a hand.
Let's do the ABCs of trauma care.
A : open the airway
B : stop the bleeding
C : control and treat for shock
D : Dress and bandage any wounds.
A and D are closely related. You may have to dress or bandage a wound to stop the bleeding. Obviously if he's breathing skip A. TREAT FOR SHOCK!!!
Talk to him. Tap him. Wave your hands in his face. DO NOT LET HIM FALL ASLEEP OR HE WILL DIE!
Oh right. A chest catheter. He's probably dead. Stick him in the third rib down from his collar bone. Either side. It's another way to relieve a sucking chest wound. Still, make the wound not a problem anymore.
This is probably a subject I feel very strongly about. Take what I said with a grain of salt. I have seen too many people die to stupid shit to be nice about this. MEDICAL TREATMENT IS IMPORTANT! YOUR POULTICE ISN'T WORTH A DAMN FOR TRAUMA!
Use your poultices on minor wounds where if something happens and shit goes sideways, it doesn't matter and can be fixed.
Burns and chemical burns... this is tricky. Pay attention. Firstly, we won't cover first degree burns because that's like a sunburn. Suck it up buttercup. Still, drink extra water. Those will dehydrate you quickly.
Second degree burns are basically blisters with maybe a little bit of flesh completely missing. These can be nasty. Treat for shock and bandage them to keep them clean. Run it under some water to reduce the heat inside the burn if you can but on a battlefield, send him home or get him back in the fight.
Third degree burns involve the complete destruction of human tissues and sometimes damage to the bone structure itself. These burns are catastrophic. Without help... he will die.
First, bandage his wounds and make sure he isn't still on fire. Secondly, move him away from the heat. Thirdly, put a blanket over him. Your skin is what retains your heat. Without it (and with third degree burns he will have huge bits of it missing) he will die of hypothermia even on a temperate day. On a hot day make sure you move him somewhere relatively cool to keep him from likewise overheating.
For a chemical burn...
He's dead. Or maimed. This is unavoidable. Most people aren't carrying an entire surgical theater on them plus replacement organs.
Crush some charcoal and put it into the burns. It will help neutralize the chemical. DO NOT USE WATER! IT MAY SPREAD THE CHEMICAL OR EVEN MAKE THE BURNS WORSE!!!! GET HIM TO A FUCKING HOSPITAL!!!!
Overall? Your job is not to be the uber badass and make sure no one dies with your magical healing powers and chanting rituals. Your job is to stabilize him and get him to a real doctor. Unless you have an entire hospital in your militia. In which case get him to them and why are your doctors in the field...?