Who was Martha, and what did Jimmy Cross wonder about her?
In the eponymous story "The Things They Carried," Martha is a girl in New Jersey who plays volleyball, goes to college, reads Virginia Woolf, and writes Lt. Jimmy Cross letters. Cross wonders if she is still a virgin. In this way, Cross seems like some Knight on a crusade, and he expects his maiden to wait faithfully back at the castle. It's an unrealistic fantasy, but Martha gives Cross hope. Even though she doesn't love him and he only loves the idea of her, she serves as a willing audience, like the reader, who waits in judgment back home to decide if these men are worthy.
Lt. Cross burns Martha’s letters and photographs after one of his men die. Cross (initials J.C. for "Jesus Christ") is a Christ-figure, one who takes on the sins of all the others: namely guilt. He feels responsible for Lavender's death, blaming himself for thinking of Martha rather than his men.
Martha never appears in the story, but she (and the other American civilian women in the novel) represents home, warmth, protection, the earth-mother. The men in Alpha company dream of them and, through their letters, they try to make these women understand war. The women write back with only trivialities about their daily lives, so there is a disconnect between Martha and Cross. She will never understand "what men do." In fact, Cross says he loves her, but she cannot love any male who has killed, raped, burned villages.
Later Cross tells O'Brien, “Make me out to be a good guy, okay? Brave and handsome, all that stuff. Best platoon leader ever. And do me a favor don’t mention anything about—“? What isn’t O’Brien supposed to mention in his story about Cross? Killing? Rape? Whatever it is, O'Brien never tells us, or Martha. She will never understand "the things men do."