For O'Brien, something isn't true unless it feels true. Whether or not it actually happened is beside the point; something can even have happened and not be true. While we feel betrayed when we realize that Rat and Kiowa and Mitchell Sanders are all fictional creations, O'Brien's trying to tell us that even though they're made up, there's a lot of truth to their characters. True stories and true characters don't fit neatly into a narrative or into character roles. The best way for O'Brien to communicate his Vietnam experience in The Things They Carried is to give us the "story-truth," not the "happening-truth."
O'Brien's use of story-truth reflects the shifting nature of reality in wartime.
Story-truth is an unsuccessful attempt to connect with the reader on the subject of Vietnam. Rather than connect the reader to O'Brien's experience, it alienates, breaching the trust between reader and writer.