One of the main themes of the novel is the allure of war. This trope, common in war literature, is made more complex here as O'Brien adds the layers of a Conrad-esque "heart of darkness" fascination in the character of Mary Anne.
The seductive allure of war is inextricably linked to the tendencies of human nature in O'Brien's novel. War, more specifically the act of killing, acts as a catalyst for some individuals, causing them to become primal versions of themselves, to become less human, to become killing machines. O'Brien revisits this idea numerous times throughout the text, adding subtle variations on the theme as he introduces different characters that struggle with the same core issue. O'Brien initially creates this tension by offering the counterpoint of O'Brien's daily work duty of declotting slaughtered pigs with his anxiety about his imminent service as a soldier in Vietnam. O'Brien merges the ideas of killing with animals, a symbolic linkage he revisits by describing the soldiers of Alpha Company as animal-like, "humping" their packs and "saddling up" their gear.
O'Brien struggles to hold onto the obverse of this animalism, this barbarism, which is a sort of hyper-civility. He succeeds in doing this by continually offering a highly self-conscious and self-aware cultural criticism that frequently draws on the archetypal works that are the foundation of western civilization like Plato's Republic.
Contrary to the protagonist "O'Brien's" experiential insulation from Vietnamese culture, which is a kind of "uncivilized other" according to the terms of U.S. rhetoric that largely defined the war, Mary Anne Bell is a character who deliberately strove for cultural immersion. For "O'Brien," the landscape and the Vietnamese occupying that landscape, such as the elderly Vietnamese men who watch him revisit the spot where Kiowa perished, are mostly incidental. Mary Anne actively sought out the ways of the Vietnamese, not just to observe from a distance, but to participate in if possible. Mary Anne, who should have behaved according to accepted Western norms, becomes so much a part of the landscape of Vietnam that she becomes "unnatural" to Mark and Rat. For example, the humming they hear coming from the Greenies' hut is freaky and unnatural, somehow not human, but it is Mary Anne's humming. And particularly as a female, she should be "domesticated" and behave in accordance with the readers' expectations of a young woman in a decade prior to the women's liberation movement. Instead she is seduced by the foreign landscape of Vietnam — one which "O'Brien" resists and barely describes — and is reduced to her animal-like primal self, a killing machine. Finally, opposite to "O'Brien," Mary Anne shows no resistance to the landscape, and has the agility and prowess to slip into the jungle like an adept, predatory jungle animal ready for the hunt.
O'Brien relies on symbolism Joseph Conrad created in Heart of Darkness to connect the landscape of Vietnam to the landscape of immorality that Mary Anne succumbs to and "O'Brien" resists. Mary Anne becomes a part of what O'Brien/"O'Brien" most vehemently opposes and what O'Brien/"O'Brien" most fears: the struggle between the light and dark forces of human nature and the predominance of the darker forces. Just as the character of Mary Anne echoes Conrad's character, Kurtz, "O'Brien" is a cousin to Conrad's character, Marlow. Like Marlow, O'Brien struggles against his imagination and the fantastic cultural stories that feed it, in "O'Brien's" case, the stories of World War II he learned from movies and stories of his father's generation. Ultimately, O'Brien shields himself from a fate similar to Mary Anne's through the way he employs stories, just as he did during the summer when he worked at the meatpacking plant, by forcing him to look at the struggle between dark and light within himself.