I've just had the thought that I have no idea how first aid was actually applied in combat during WWII. How effective were medics, what training did they receive and what sort of equipment would they make use of? Once first aid had been delivered how were casualty evacuations carried out and what capabilities did field hospitals have?
Is there a specific country that interests you? Each one had a different way of approaching combat medicine, and it could vary quite a bit by the climate, terrain, and specific circumstances of a campaign. In order to answer this without basically writing a book, we need to narrow it down.