I guess the more horrifying the better.
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Not sure it counts, but the one that sticks most to me would be the black and white movie with the only color being a girl in a RED dress.
Oscar winning Schindler's list. If the red dress was the only as anti-war part of it, it would certainly still count very strongly as a cautionary. I haven't thought of that one, thanks for coming up with it.
The beach landing scene in Saving Private Ryan in the sense of who in their right mind would want to trade places with the men in the landing craft and then have to struggle to actually get onto the beach and then advance up the beach while under heavy fire.
Yes, that whole opening sequence by Spielberg is a lesson in the horrors of war!