It'd be a bad idea to challenge Tim O'Brien to a round of Truth-Or-Dare because he'd find a way to pick Truth, launch into a story, recant it, then make you think he really chose Dare, but in the end, you'll be pretty sure he actually told you the Truth after all. Maybe... 
 That's kind of the point about this account of his time Vietnam as an infantry soldier that warns us that war stories are tricky. The ones that sound true are probably lies and the ones that seem outlandish probably have a healthy dose of truth in them. By telling us some fact and some fiction, then revealing which is which (Allegedly.), O'Brien shows that sometimes a well told lie based on fact has more power than a real story accurately told. 
 Taken together, O'Brien's stories make it clear that he spent the decades after the war mulling over the various things he took away from it. This isn't the memoir of a guy who obtained some kind of closure by writing it, it's the story of the fear, doubt and confusion he still wrestled with decades later. In order to convey that experience, he had to tell the reader some war stories and let us decide just how true they were.