I saw "Casualties of War" twice recently within a week, and I had an even stronger reaction to it the second viewing. I got teary-eyed both times I saw the film. I reacted so strongly because of the nightmare environment of the film. Of course, I know the film can't even touch what the real Vietnam was like, but all I know is the film had a nightmarish atmosphere. I think "Casualties of War" has what most De Palma films share: a protagonist who follows the rules and eventually finds out that sometimes the rules must be broken in order to survive the world.
"Casualties of War" also informs "Dressed to Kill" because the protagonist Peter Miller's father was killed in Vietnam; seeing "Casualties" makes me imagine Peter's father in the nightmarish environment of "Casualties." "Dressed to Kill" shows that there's another war going on on U.S. soil in the 1980s New York City, where disease derived from sex and the death that those things lead to.