Before answering this question, it's important to understand that it's quite difficult to say anything definitive about Tim O'Brien's The Things They Carried. Indeed, the book's confusing structure defies efforts to define its meaning at every turn. That said, I think one could say that it is, in fact, an antiwar story, at least for the most part.
Consider, first of all, the book's chaotic structure. This structure has several effects. First, it mirrors the chaotic state of warfare in which the characters are embroiled. Jumping back and forth in time, revisiting certain events more than once, and generally lacking any kind of linear structure, the "plot" (if there is one) of The Things They Carried mirrors the confusing, chaotic, illogical nature of the Vietnam War. Moreover, it mirrors the psychological states of most of the work's characters. Faced with war and death, the soldiers in O'Brien's work experience the deterioration of their own mental landscapes. Norman Bowker, for instance, carries the thumb of a dead Viet Cong child and then, after returning from the war, commits suicide. Additionally, Rat Kiley, unable to deal with the death of one of the platoon's members, brutally tortures a water buffalo. Clearly, the characters in O'Brien's fiction are suffering and psychologically traumatized, and the fragmented, confusing nature of the author's prose reflects this notion.
With these points in mind, I think it's possible to say The Things They Carried is an antiwar story. By focusing on the chaos and the trauma, and by mirroring these realities in a fragmented, chaotic narrative, O'Brien skillfully illustrates the calamity of war and death.