Why did Tim O'Brien write this book in the first place?

The book is plainly anti-war. The fictional version of O’Brien, the narrator, writes that he could imagine himself fighting in World War II, so he is not entirely a pacifist. But fighting in Vietnam seemed wrong. The character lacks the moral courage to run away to Canada. In this sense The Things They Carried is a straightforward tragedy, with the main flaw of an otherwise likeable main character causing his downfall. All of the horrors of the book – killing, having friends killed, boredom punctuated by fear – follow from this decision.