Preparing for the invasion of Normandy wasn’t just a matter of training troops to take the objectiveory, it’s easy to assume this was a given. If I were storming the beaches, I’d want to know that if I got hit, the brass had a plan to get me out of there safely as opposed to leaving me to explore Nazi Germany’s idea of hospitality. As it turns out, the Allies had a plan for retrieving the injured, but it was far from trivial.
The widespread use of helicopters to evacuate wounded troops wasn’t made practical until the Korean War.
(USAF)
On the battlefield, a medic (or corpsman) would move to aid a casualty as quickly as possible. He’d assess the condition and the troop would then be moved back, either on foot or by jeep, to the battalion aid station. From there, if needed, a troop would be moved further back from the front for more intensive care.
Now, in World War II, using helicopters for medical evacuations wasn’t possible. The first practical helicopters were flying, but they still didn’t have the lift capacity needed — even still, there were ways to get troops back reasonably quickly.