If your story is about the events after the war, start your book there. The reader doesn't have to know everything about the characters the moment you introduce them. If you introduce John as a traumatized war veteran, don't expect anyone to sympathize--or, better yet, empathize--with him. Why should I care about this person if all I know about him is that he is supposed to be traumatized by war? You can't just tell those things, you have to show it.
Begin the book at the point the real story begins, and use dialogue, character interactions, their fears, insecurities, speech patterns, etc., to show who they are. You should never have to tell who a character is if you know what you're doing. Have a character fight a bout of claustrophobia on a subway car and freak out, because maybe he was a POW and locked in a small, water logged cage with other dead and/or dying soldiers for several years...something like that.
You can leave most things about the past implied, and gradually reveal these things over the course of the story. This way, by the time we have a real grasp on what happened to these people before, the characters are developed enough that they feel real, and as a result, we care about their plight.
Use the powers of implication. The mind can create images much more gruesome and horrifying than anything you might physically see. You keep that element of fear of the unknown by leaving things implied.
Don't dictate what emotions your reader should be feeling. You will only patronize and alienate them. Show who your characters are, and let them grow over the course of the story, and readers will empathize with them, if not sympathize.