How does the ambiguous nature of the term "carry" introduce the soldiers and themes of the novel?
In O'Brien's novel The Things They Carried, the description of items in this first chapter, “The Things They Carried,” moves from concrete to abstract things. It begins with military necessities (guns, helmets, radios, ammo, rations) and personal items (comic books, pictures, letters, drugs) and ends with emotions, memories, family, history, tragedy, heartbreak, guilt, love. In this way "carry" is a transitive verb: it requires direct objects, and O'Brien lists them, and their weights, to give the novel military verisimilitude and to give the men psychological baggage.
The heaviest thing they carry, ironically, can't be weighed at all: the “unweighed fear.” Jimmy Cross, a Christ-figure (initials J.C.), carries the guilt over Lavender's death. His mind was on Martha, girl who symbolizes the comfort of home, when Lavender was killed.
After his death, Lavender's ghost hangs over the novel like King Hamlet's Ghost: “ . . . they all carried ghosts.” And “they carried all the emotional baggage of men who might die.”
In the end, to atone for the death (again the symbolic Christ-like metaphors), Lt. Cross burns Martha’s letters and photographs? (p. 23) Or so we think.
At the beginning of the story, "The Things They Carried," O'Brien describes Lt. Jimmy Cross as carrying his flashlight and other tangible items as well as carrying the responsibility of leading his men. In that sentence, O'Brien uses a zeugma (use of one verb which relates differently--contextually and denotatively--to two or more words). While the zeugma certainly involves ambiguity, it also enables O'Brien to stress the physical, psychological, and emotional burdens that he bears.
For example, in "In the Field," a story which appears later in the collection, Jimmy Cross and his men search a swampy field for Kiowa's body, and Cross thinks about how he did not ask for the responsibility of leading the men. He thinks about just lying back in the field and allowing the muck to swallow him up, but he knows that he cannot do that because of the "things he carries."