Being in combat is one of the craziest experiences a person can have. Bullets are zipping by your melon and impacting the wall behind you, eyes wide and on the alert as the incoming rounds blanket your position. Sounds crazy. Because it is.
War is hell.
Well-trained military minds know, winning the battle is the most important aspect of winning the war. In combat, the rules are different than in any other situation you'll probably find yourself. All available fingers need to be pulling triggers.
So if allied forces take a mass casualty, the guy who is hurt the worst isn't necessarily the one who gets treated first.
US Marine in Afghanistan returning fire (Source: Youtube/Screenshot)
In the civilian world, there are typically more assets and resources to treat just about everyone and every ailment or injury in the book.
By contrast, fighting an enemy in a third world country, Navy Corpsmen and medics only carry a small inventory of medical gear strapped onto their persons.
HM2 Lamonte Hammond and HM3 Simon Trujillo treat a Marine who was wounded during a firefight in the Nawa district of Helmand province, Afghanistan. (Photo by Cpl. Artur Shvartsberg)
During combat, the rules on who receives care first changes in a matter of moments. If a squad is under heavy attack and a few trigger pullers get hurt, then the unit is down a few bodies.
After the field medic takes care of their wounds, let's say subject "A" sustained a "GSW" or gunshot wound to the chest, they are now out of the fight. If subject "B" took a bullet to their leg, they're still considered in the fight because it's not life-threatening.
So during wartime rules, subject "B" is supposed to be treated first to allow them the chance to get back on their weapon system and return to the fight. Hopefully subject "A" will be okay and pull through.