What is that white powder medics use in WWII movies?
What does it do to the wounds?
Just a WAG, but maybe styptic powder (to help stop bleeding on smaller wounds)?
My guess would be that it was sulfa powder, put on the wounds to prevent infection.
The precursor to antibiotics, sulphanilamide was pretty new then and was widely acclaimed as a wonder drug - which of course it was - so maybe it could have been that.
If so, it was to fight infection
Interesting. If it is for infection, why do they do it so urgently?
In Saving Private Ryan the medic got shot up and they were frantically pouring powder all over him… surely a soldier’s wounds wouldn’t get infected that quickly and it would be more important to put bandages on and try to get medical help… because when he reachs a hospital they would clean him up (removie the powder) and operate, right?
Sulfanilamide (or sulphanilamide outside the US)… as issued to WWII troops, pilots, and sailors, it was not in a pure form. It contained a cauterizing agent as well as actual sulfanilamide; the urgency is because the quicker it was applied the quicker the bleeding could be stopped.
Well, if we are right and it is sulphanilamide powder, for fighting infection
AND
If the films portray these things correctly and the soldiers did throw the powder around with great urgency
THEN
I’d say it was because the mechanism of fighting infection would not have been clear to the average Joe Bloggs soldier at that time.
Back then, the notion that some of this white powder would stop you getting gangrene or save your life would have to have been a pretty amazing notion.
We, as the average folk now know so much more about many things than the average folk did sixty odd years ago.
Science and technology will have made other great leaps in the next sixty years and the average folk then will look at the things we do now and some may query why we did it in the way we do.
http://www.sproe.com/s-sulfa.htm
Note that the above is from a site dedicated to the film Saving Private Ryan, and maybe should be taken with a grain of salt.
I can guarantee you that white powder is NOT salt. 
The sulfa powder needs to be in the wound before the wound is closed. The wound needs to be closed (i.e., bandaged) before the soldier bleeds to death. So there is great urgency, even with pure sulfa; if Dutchboy is correct about the cauterizing agent, the urgency is even more justified.
I betcha it was powdered alum.
my father still uses an alum stick to stop any small wounds from bleeding when he’s just shaved.
"Interesting. If it is for infection, why do they do it so urgently? "
To stop from getting shot seems like a pretty safe bet.
But you’re not supposed to take shots at medics in the first place.
And?
Not all bullets hit the targets at which they are aimed, but they will all hit something eventually. If the medic is in front of the bullet, the medic will be hit.
In combat, a soldier shooting at various colored uniforms in the heat of battle may not note the armband or helmet badge on the guy with the red crosses on the arms or helmet (especially if the medic is carrying something that obscures his badges or is turned so that his body obscures them).
Not every soldier in the heat of battle cares whether or not he is shooting at a medic.
The white powder was, indeed, sulfa.
Note that belly wounds are the most likely to cause infection problems. The insides of your intestines contains bugs that do a lot of damage in a short period of time once freed to roam around. So a belly wound would require immediate steps to stop sepsis.
In general, with any wound “an ounce of prevention is worth a pound of cure”. The sooner sulfa is put on, the better.
At some point in WWII, german soldiers actually started shooting at medics. Medics were getting so good at getting people off the battlefield and patching them up that if they shot one medic, it was like shooting four regular infantry. This was on the history channel.
Thanks all!
I was just going to chime in with what Ficer67 said.
Besides, since most bullets are addressed “to whom it may concern”, I would think that getting to safety would be a priority - red cross armband or not.
Huh? Intestines are full of bugs?
Bugs as in bacteria.
E. coli is safe (and, IIRC, neccessary) in the intestines, nasty if it gets elsewhere.