The setting of this story is almost a character itself. How and why?
In The Things They Carried, the country of Vietnam literally swallows men alive: the shit field takes Kiowa and forever haunts Alpha Company. In other instances, men and women literally ingest the country (they drink the water), and this act of communion transforms and indoctrinates them into the horrors of jungle warfare.
In "Sweetheart of the Song Tra Bong," O'Brien calls it "the habits of the bush" (98). It is Mary Anne's physical environment (the country itself) which erodes her femininity and equalizes her gender. Her baptism in the river is the catalyst for Mary Anne's adaptation from innocent female into war-like male. Mary Anne survives swimming in the Song Tra Bong, whereas the innocent soldier, Kiowa, is swallowed by it. Morty Phillips swallows a mouthful of it and dies, and Bowker, the professional soldier, kills himself because of its stench. The Song Tra Bong is both a rite of passage and a sirens' song, and once baptized by it, one longs to return to it (to bury Kiowa's moccasins) and be tortured by it (Bowker's suicide). And lest we not forget that O'Brien could have been saved from the whole Vietnam experience by a swim across a different river.
After her swim, Mary Anne refuses to wear make-up, jewelry, or filed nails. She cuts her hair short and wraps it in a green bandana. After “hygiene became a matter of small consequence" she follows the "natural progression of learning how to use a weapon" (98). Her body too prepares for combat; her tall, big-boned frame and long legs "seemed foreign somehow"..."too stiff in places, too firm where the softness use to be" (99). In effect, Mary Anne becomes sick, like Morty Phillips, because she "swallowed bad water on that swim...[s]wallowed a VC virus or something" (195). That something is the essence of the country, ma: the spirit of war, the staple rice seedling, the ghosts of Vietnam, and the graves marked by a thousand years of rebellion.