The secret of constructing a good topic question for an essay is to consider the central themes of the story and then to ask a question based on one of those themes. I have included a link below to the enotes section on themes for this text to help you out, but you might want to think about the way in which this excellent short story is centrally concerns the relationship between fact and fiction and the difficulties of telling a "true" war story that are pointed towards by the way in which the speaker tells us a story only then towards the end to go back on what he said and tell us that it was all a fabrication. Consider in particular the following quote:
And in the end, of course, a true war story is never about war. It's about sunlight. It's about the special way that dawn spreads out on a river when you know you must cross the river and march into the mountains and do things you are afraid to do. It's about love and momory. It's about sorrow. It's about sisters who never write back and people who never listen.
Such a quote, after many that have told us what a true war story isn't, should beg the question, stemming from the title, "What is a true war story and how do we tell it?" This would be an excellent topic question for you to consider which would help you to write an essay based on this thought provoking story.