O'Brien briefly served in Vietnam and it's clearly an experience that has haunted him ever since; at one point in these stories he reflects that he's been able to use his writing to process his memories, a technique that's not been available to all his fellow soldiers. These linked stories feature the same cast of characters, including a version of O'Brien himself, and some of the same events, which he circles back around to view from different angles, changing some details, admitting inventing some elements and disguising others, so that the book itself parallels the confused, searing, obsessive experience of the vet's own painful memories, as well as a meditation of writing itself. How much is pure autobiography, how much is imagination? We can't know for sure.
The Things They Carried veers (again, like the experience of fighting the war itself, I suspect) between gallows humour, slapstick, poignant emotion and ghastly imagery (there is one story in particular I wish I hadn't read). But as a way of bearing witness to the unspeakable horrors, the bonds and deep emotions of war, The Things They Carried is probably as close as most of us are going to get.